[RFC 0/1] drm/pl111: Initial drm/kms driver for pl111

[RFC 0/1] drm/pl111: Initial drm/kms driver for pl111

[tom.cooksey]

From: Tom Cooksey <tom.cooksey@arm.com>

Please find below the current state of our pl111 DRM/KMS driver. This
is lightly tested on a Versatile Express using X running the
xf86-video-armsoc DDX driver[i] with the patches applied to drm-next
as of ~last week. To actually see anything on the DVI output, you
must also apply Pawel Moll's VExpress DVI mux driver[ii] to select
the video signal from the ca9x4 core tile.

[i] <https://git.linaro.org/gitweb?p=arm/xorg/driver/xf86-video-armsoc.git;a=summary>
[ii] <https://patchwork.kernel.org/patch/1765981/>


Known issues:
 * It uses KDS. We intend to switch to whatever implicit per-buffer
   synchronisation mechanism gets merged, once something is merged.
 * It abuses flags parameter of DRM_IOCTL_MODE_CREATE_DUMB to also
   allocate buffers for the GPU. Still not sure how to resolve this
   as we don't use DRM for our GPU driver.
 * Doesn't handle page flip event sequence numbers and timestamps
 * The v_sync handling needs work in general - a work queue is a
   little overkill
 * Doesn't support the import half of PRIME properly, only export
 * Need to validate src rectangle size in
   pl111_drm_cursor_plane_update()
 * Only supports 640x480 mode, which is hard-coded. We intend to
   rebase on top of CDF once it is merged, which hopefully will
   handle a lot of the EDID parsing & mode setting for us (once
   Pawel's CDF patches for VExpress also land).

I appreciate that's a fairly hefty list of known issues already!
However, we're waiting for both CDF & dma_buf sync mechanisms to land
before we can address some of those. So in the mean-time, I thought
someone might be interested in taking a look at what we have so far,
which is why I'm posting this now. Needless to say the code will need
to be refactored a fair bit, however I'm keen to get and additional
feedback anyone cares to give.


Cheers,

Tom

Tom Cooksey (1):
  drm/pl111: Initial drm/kms driver for pl111 display controller

 drivers/gpu/drm/Kconfig                     |    2 +
 drivers/gpu/drm/Makefile                    |    1 +
 drivers/gpu/drm/pl111/Kbuild                |   14 +
 drivers/gpu/drm/pl111/Kconfig               |    9 +
 drivers/gpu/drm/pl111/pl111_clcd_ext.h      |   78 ++++
 drivers/gpu/drm/pl111/pl111_drm.h           |  227 ++++++++++++
 drivers/gpu/drm/pl111/pl111_drm_connector.c |  166 +++++++++
 drivers/gpu/drm/pl111/pl111_drm_crtc.c      |  432 ++++++++++++++++++++++
 drivers/gpu/drm/pl111/pl111_drm_cursor.c    |   97 +++++
 drivers/gpu/drm/pl111/pl111_drm_device.c    |  319 +++++++++++++++++
 drivers/gpu/drm/pl111/pl111_drm_dma_buf.c   |  339 ++++++++++++++++++
 drivers/gpu/drm/pl111/pl111_drm_encoder.c   |  106 ++++++
 drivers/gpu/drm/pl111/pl111_drm_fb.c        |  152 ++++++++
 drivers/gpu/drm/pl111/pl111_drm_funcs.h     |  127 +++++++
 drivers/gpu/drm/pl111/pl111_drm_gem.c       |  287 +++++++++++++++
 drivers/gpu/drm/pl111/pl111_drm_pl111.c     |  513 +++++++++++++++++++++++++++
 drivers/gpu/drm/pl111/pl111_drm_platform.c  |  150 ++++++++
 drivers/gpu/drm/pl111/pl111_drm_suspend.c   |   35 ++
 drivers/gpu/drm/pl111/pl111_drm_vma.c       |  214 +++++++++++
 19 files changed, 3268 insertions(+)
 create mode 100644 drivers/gpu/drm/pl111/Kbuild
 create mode 100644 drivers/gpu/drm/pl111/Kconfig
 create mode 100644 drivers/gpu/drm/pl111/pl111_clcd_ext.h
 create mode 100644 drivers/gpu/drm/pl111/pl111_drm.h
 create mode 100644 drivers/gpu/drm/pl111/pl111_drm_connector.c
 create mode 100644 drivers/gpu/drm/pl111/pl111_drm_crtc.c
 create mode 100644 drivers/gpu/drm/pl111/pl111_drm_cursor.c
 create mode 100644 drivers/gpu/drm/pl111/pl111_drm_device.c
 create mode 100644 drivers/gpu/drm/pl111/pl111_drm_dma_buf.c
 create mode 100644 drivers/gpu/drm/pl111/pl111_drm_encoder.c
 create mode 100644 drivers/gpu/drm/pl111/pl111_drm_fb.c
 create mode 100644 drivers/gpu/drm/pl111/pl111_drm_funcs.h
 create mode 100644 drivers/gpu/drm/pl111/pl111_drm_gem.c
 create mode 100644 drivers/gpu/drm/pl111/pl111_drm_pl111.c
 create mode 100644 drivers/gpu/drm/pl111/pl111_drm_platform.c
 create mode 100644 drivers/gpu/drm/pl111/pl111_drm_suspend.c
 create mode 100644 drivers/gpu/drm/pl111/pl111_drm_vma.c

-- 
1.7.9.5

Re: [RFC 0/1] drm/pl111: Initial drm/kms driver for pl111

[Rob Clark]

On Thu, Jul 25, 2013 at 1:17 PM,  <tom.cooksey@arm.com> wrote:
> From: Tom Cooksey <tom.cooksey@arm.com>
>
> Please find below the current state of our pl111 DRM/KMS driver. This
> is lightly tested on a Versatile Express using X running the
> xf86-video-armsoc DDX driver[i] with the patches applied to drm-next
> as of ~last week. To actually see anything on the DVI output, you
> must also apply Pawel Moll's VExpress DVI mux driver[ii] to select
> the video signal from the ca9x4 core tile.
>
> [i] <https://git.linaro.org/gitweb?p=arm/xorg/driver/xf86-video-armsoc.git;a=summary>
> [ii] <https://patchwork.kernel.org/patch/1765981/>
>
>
> Known issues:
>  * It uses KDS. We intend to switch to whatever implicit per-buffer
>    synchronisation mechanism gets merged, once something is merged.
>  * It abuses flags parameter of DRM_IOCTL_MODE_CREATE_DUMB to also
>    allocate buffers for the GPU. Still not sure how to resolve this
>    as we don't use DRM for our GPU driver.

any thoughts/plans about a DRM GPU driver?  Ideally long term (esp.
once the dma-fence stuff is in place), we'd have gpu-specific drm
(gpu-only, no kms) driver, and SoC/display specific drm/kms driver,
using prime/dmabuf to share between the two.

>  * Doesn't handle page flip event sequence numbers and timestamps
>  * The v_sync handling needs work in general - a work queue is a
>    little overkill
>  * Doesn't support the import half of PRIME properly, only export
>  * Need to validate src rectangle size in
>    pl111_drm_cursor_plane_update()
>  * Only supports 640x480 mode, which is hard-coded. We intend to
>    rebase on top of CDF once it is merged, which hopefully will
>    handle a lot of the EDID parsing & mode setting for us (once
>    Pawel's CDF patches for VExpress also land).

note that drm core already handles EDID parsing quite nicely.. I'm not
entirely sure why CDF would be needed for this?

>
> I appreciate that's a fairly hefty list of known issues already!
> However, we're waiting for both CDF & dma_buf sync mechanisms to land
> before we can address some of those. So in the mean-time, I thought
> someone might be interested in taking a look at what we have so far,
> which is why I'm posting this now. Needless to say the code will need
> to be refactored a fair bit, however I'm keen to get and additional
> feedback anyone cares to give.
>

What is the dependency on CDF?  Is there an external encoder/bridge
that could be shared between platforms?

btw, I think that having some share-able KMS objects is a good idea.
I'm not entirely sure that the directions that the current CDF
proposals are headed is necessarily the right way forward.  I'd prefer
to see small/incremental evolution of KMS (ie. add drm_bridge and
drm_panel, and refactor the existing encoder-slave).  Keeping it
inside drm means that we can evolve it more easily, and avoid layers
of glue code for no good reason.

BR,
-R


>
> Cheers,
>
> Tom
>
> Tom Cooksey (1):
>   drm/pl111: Initial drm/kms driver for pl111 display controller
>
>  drivers/gpu/drm/Kconfig                     |    2 +
>  drivers/gpu/drm/Makefile                    |    1 +
>  drivers/gpu/drm/pl111/Kbuild                |   14 +
>  drivers/gpu/drm/pl111/Kconfig               |    9 +
>  drivers/gpu/drm/pl111/pl111_clcd_ext.h      |   78 ++++
>  drivers/gpu/drm/pl111/pl111_drm.h           |  227 ++++++++++++
>  drivers/gpu/drm/pl111/pl111_drm_connector.c |  166 +++++++++
>  drivers/gpu/drm/pl111/pl111_drm_crtc.c      |  432 ++++++++++++++++++++++
>  drivers/gpu/drm/pl111/pl111_drm_cursor.c    |   97 +++++
>  drivers/gpu/drm/pl111/pl111_drm_device.c    |  319 +++++++++++++++++
>  drivers/gpu/drm/pl111/pl111_drm_dma_buf.c   |  339 ++++++++++++++++++
>  drivers/gpu/drm/pl111/pl111_drm_encoder.c   |  106 ++++++
>  drivers/gpu/drm/pl111/pl111_drm_fb.c        |  152 ++++++++
>  drivers/gpu/drm/pl111/pl111_drm_funcs.h     |  127 +++++++
>  drivers/gpu/drm/pl111/pl111_drm_gem.c       |  287 +++++++++++++++
>  drivers/gpu/drm/pl111/pl111_drm_pl111.c     |  513 +++++++++++++++++++++++++++
>  drivers/gpu/drm/pl111/pl111_drm_platform.c  |  150 ++++++++
>  drivers/gpu/drm/pl111/pl111_drm_suspend.c   |   35 ++
>  drivers/gpu/drm/pl111/pl111_drm_vma.c       |  214 +++++++++++
>  19 files changed, 3268 insertions(+)
>  create mode 100644 drivers/gpu/drm/pl111/Kbuild
>  create mode 100644 drivers/gpu/drm/pl111/Kconfig
>  create mode 100644 drivers/gpu/drm/pl111/pl111_clcd_ext.h
>  create mode 100644 drivers/gpu/drm/pl111/pl111_drm.h
>  create mode 100644 drivers/gpu/drm/pl111/pl111_drm_connector.c
>  create mode 100644 drivers/gpu/drm/pl111/pl111_drm_crtc.c
>  create mode 100644 drivers/gpu/drm/pl111/pl111_drm_cursor.c
>  create mode 100644 drivers/gpu/drm/pl111/pl111_drm_device.c
>  create mode 100644 drivers/gpu/drm/pl111/pl111_drm_dma_buf.c
>  create mode 100644 drivers/gpu/drm/pl111/pl111_drm_encoder.c
>  create mode 100644 drivers/gpu/drm/pl111/pl111_drm_fb.c
>  create mode 100644 drivers/gpu/drm/pl111/pl111_drm_funcs.h
>  create mode 100644 drivers/gpu/drm/pl111/pl111_drm_gem.c
>  create mode 100644 drivers/gpu/drm/pl111/pl111_drm_pl111.c
>  create mode 100644 drivers/gpu/drm/pl111/pl111_drm_platform.c
>  create mode 100644 drivers/gpu/drm/pl111/pl111_drm_suspend.c
>  create mode 100644 drivers/gpu/drm/pl111/pl111_drm_vma.c
>
> --
> 1.7.9.5
>
>
>
> _______________________________________________
> linux-arm-kernel mailing list
> linux-arm-kernel@lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

Re: [RFC 0/1] drm/pl111: Initial drm/kms driver for pl111

[Pawel Moll]

On Thu, 2013-07-25 at 19:21 +0100, Rob Clark wrote:
> >  * Only supports 640x480 mode, which is hard-coded. We intend to
> >    rebase on top of CDF once it is merged, which hopefully will
> >    handle a lot of the EDID parsing & mode setting for us (once
> >    Pawel's CDF patches for VExpress also land).
> 
> note that drm core already handles EDID parsing quite nicely.. I'm not
> entirely sure why CDF would be needed for this?
>
> What is the dependency on CDF?  Is there an external encoder/bridge
> that could be shared between platforms?

By all means. The platform we have here at ARM - Versatile Express - has
a pretty generic idea of "multimedia bus", carrying a set of RGB pixel
data and digital audio. There are three possible source of those: an
FPGA on the motherboard and two daughterboards which can take random
combination of FPGAs or test chips. In other words: the pixel data can
be generated by anything. And some of those things can be driven by
existing fbdev drivers. Now Tom is trying to make the most modern driver
for the oldest of the things ;-)

So that's about sources. All of them are connected to yet another FPGA
which acts as a muxer (switch if you wish). And from there things are
fed to SiI9022 HDMI/DVI formatter which is pretty common in the
"embedded world". In other words: loads of other fbdev-driven display
controllers are driving them as well (googling for sii9022 reveals at
least 3 different more or less custom drivers for it). And this chip is
also a reason Tom mentioned EDID. In order to get (just access, not even
parse) it we must jump through hoops - the chip acts as a I2C master
itself and must be kindly asked to switch into a sort of I2C bridge
mode.

So yes, the display pipeline is exactly straight-forward...

> btw, I think that having some share-able KMS objects is a good idea.
> I'm not entirely sure that the directions that the current CDF
> proposals are headed is necessarily the right way forward.  I'd prefer
> to see small/incremental evolution of KMS (ie. add drm_bridge and
> drm_panel, and refactor the existing encoder-slave).  Keeping it
> inside drm means that we can evolve it more easily, and avoid layers
> of glue code for no good reason.

To keep the story short - I'm a great enthusiast of CDF because it
caters for both DRM *and* fbdev. Today. I'm looking forward to the day
when the last fbdev driver is kicked off the kernel, but it doesn't look
like happening soon. At the same time I couldn't care less about naming,
so if you prefer to call it drm_panel (but keep the API abstract enough
so non-DRM drivers can use them as well) - it's fine for me :-) There
are already generic mode/timing structures and DT bidnings available
(with helpers for the interesting parties), so this doesn't seem like a
unreasonable request?

Cheers!

Pawel

Re: [RFC 0/1] drm/pl111: Initial drm/kms driver for pl111

[Rob Clark]

On Fri, Jul 26, 2013 at 10:06 AM, Pawel Moll <pawel.moll@arm.com> wrote:
> On Thu, 2013-07-25 at 19:21 +0100, Rob Clark wrote:
>> >  * Only supports 640x480 mode, which is hard-coded. We intend to
>> >    rebase on top of CDF once it is merged, which hopefully will
>> >    handle a lot of the EDID parsing & mode setting for us (once
>> >    Pawel's CDF patches for VExpress also land).
>>
>> note that drm core already handles EDID parsing quite nicely.. I'm not
>> entirely sure why CDF would be needed for this?
>>
>> What is the dependency on CDF?  Is there an external encoder/bridge
>> that could be shared between platforms?
>
> By all means. The platform we have here at ARM - Versatile Express - has
> a pretty generic idea of "multimedia bus", carrying a set of RGB pixel
> data and digital audio. There are three possible source of those: an
> FPGA on the motherboard and two daughterboards which can take random
> combination of FPGAs or test chips. In other words: the pixel data can
> be generated by anything. And some of those things can be driven by
> existing fbdev drivers. Now Tom is trying to make the most modern driver
> for the oldest of the things ;-)
>
> So that's about sources. All of them are connected to yet another FPGA
> which acts as a muxer (switch if you wish). And from there things are
> fed to SiI9022 HDMI/DVI formatter which is pretty common in the
> "embedded world". In other words: loads of other fbdev-driven display
> controllers are driving them as well (googling for sii9022 reveals at
> least 3 different more or less custom drivers for it). And this chip is
> also a reason Tom mentioned EDID. In order to get (just access, not even
> parse) it we must jump through hoops - the chip acts as a I2C master
> itself and must be kindly asked to switch into a sort of I2C bridge
> mode.

btw, you might want to have a look at some of the existing i2c slave
encoders, since that sounds a lot like what the si9022 is.  There is
already drivers/gpu/drm/i2c/sil164_drv.c for some other SI part.  And
tda998x is another more recently added hdmi encoder/formatter (the
edid probing code in there might serve as a reasonable source of
inspiration)

> So yes, the display pipeline is exactly straight-forward...
>
>> btw, I think that having some share-able KMS objects is a good idea.
>> I'm not entirely sure that the directions that the current CDF
>> proposals are headed is necessarily the right way forward.  I'd prefer
>> to see small/incremental evolution of KMS (ie. add drm_bridge and
>> drm_panel, and refactor the existing encoder-slave).  Keeping it
>> inside drm means that we can evolve it more easily, and avoid layers
>> of glue code for no good reason.
>
> To keep the story short - I'm a great enthusiast of CDF because it
> caters for both DRM *and* fbdev. Today. I'm looking forward to the day
> when the last fbdev driver is kicked off the kernel, but it doesn't look
> like happening soon. At the same time I couldn't care less about naming,
> so if you prefer to call it drm_panel (but keep the API abstract enough
> so non-DRM drivers can use them as well) - it's fine for me :-) There
> are already generic mode/timing structures and DT bidnings available
> (with helpers for the interesting parties), so this doesn't seem like a
> unreasonable request?

Well, if you have something complex enough to benefit from CDF, then
you probably ought to be looking at drm/kms.  If someone wants to
somehow come up with some glue to re-use shareable drm/kms components
from fbdev, well.. I guess I don't really care about what goes in
fbdev.  My main concern is that we have more than enough work to do,
and have more than enough complexity.  If we end up w/ bunch of glue
to tie things in to drm properties, how drm helpers sequence things
during modeset, locking, etc.. I just don't see that turning out well.
 And also, just on the logistical side, having something shared across
subsystems makes changes more of a pain during the merge window.  Not
to mention backports to stable kernels, distro kernels, etc.  It all
just seems like it will be a lot of unnecessary headache.

> Cheers!
>
> Pawel
>
>

Re: [RFC 0/1] drm/pl111: Initial drm/kms driver for pl111

[Pawel Moll]

On Fri, 2013-07-26 at 15:21 +0100, Rob Clark wrote:
> Well, if you have something complex enough to benefit from CDF, then
> you probably ought to be looking at drm/kms. 

That's what we're doing - hope you appreciate Tom's effort of re-writing
a driver for something that pre-dates DRM/KMS by generations ;-)

> I guess I don't really care about what goes in fbdev.  

Well, I've guessed that ;-)

> My main concern is that we have more than enough work to do,
> and have more than enough complexity.  If we end up w/ bunch of glue
> to tie things in to drm properties, how drm helpers sequence things
> during modeset, locking, etc.. I just don't see that turning out well.
>  And also, just on the logistical side, having something shared across
> subsystems makes changes more of a pain during the merge window.  Not
> to mention backports to stable kernels, distro kernels, etc.  It all
> just seems like it will be a lot of unnecessary headache.

I won't argue that, however it seems to me that CDF is such a small
variable in the equation that it would disappear in the noise. Surely
you have (or will have) standard DRM interface to drive the display
pipeline where CDF would be one of many back-ends? In the end you'll
have to solve the same problem - how to represent a random pipeline in a
generic way so different components can be plugged one into another.
Anyway, I'm not planning to stick my nose between DRM and CDF, really.
I'm just hoping for a solution that will fit my needs :-)

Pawel

Re: [RFC 0/1] drm/pl111: Initial drm/kms driver for pl111

[Russell King - ARM Linux]

On Thu, Jul 25, 2013 at 02:21:59PM -0400, Rob Clark wrote:
> On Thu, Jul 25, 2013 at 1:17 PM,  <tom.cooksey@arm.com> wrote:
> > Known issues:
> >  * It uses KDS. We intend to switch to whatever implicit per-buffer
> >    synchronisation mechanism gets merged, once something is merged.
> >  * It abuses flags parameter of DRM_IOCTL_MODE_CREATE_DUMB to also
> >    allocate buffers for the GPU. Still not sure how to resolve this
> >    as we don't use DRM for our GPU driver.
> 
> any thoughts/plans about a DRM GPU driver?  Ideally long term (esp.
> once the dma-fence stuff is in place), we'd have gpu-specific drm
> (gpu-only, no kms) driver, and SoC/display specific drm/kms driver,
> using prime/dmabuf to share between the two.

The PL111 primecell is just a scanout device.  It has no GPU embedded
in it.  It's just like the Armada/Dove DRM stuff - if a GPU is provided
it's an entirely separate piece of IP, and will likely remain a separate
non-DRM driver.

In the case of PL111, PL111 is not manufacturer specific so the GPU can
be any standalone GPU.

Re: [RFC 0/1] drm/pl111: Initial drm/kms driver for pl111

[Rob Clark]

On Fri, Jul 26, 2013 at 10:14 AM, Russell King - ARM Linux
<linux@arm.linux.org.uk> wrote:
> On Thu, Jul 25, 2013 at 02:21:59PM -0400, Rob Clark wrote:
>> On Thu, Jul 25, 2013 at 1:17 PM,  <tom.cooksey@arm.com> wrote:
>> > Known issues:
>> >  * It uses KDS. We intend to switch to whatever implicit per-buffer
>> >    synchronisation mechanism gets merged, once something is merged.
>> >  * It abuses flags parameter of DRM_IOCTL_MODE_CREATE_DUMB to also
>> >    allocate buffers for the GPU. Still not sure how to resolve this
>> >    as we don't use DRM for our GPU driver.
>>
>> any thoughts/plans about a DRM GPU driver?  Ideally long term (esp.
>> once the dma-fence stuff is in place), we'd have gpu-specific drm
>> (gpu-only, no kms) driver, and SoC/display specific drm/kms driver,
>> using prime/dmabuf to share between the two.
>
> The PL111 primecell is just a scanout device.  It has no GPU embedded
> in it.  It's just like the Armada/Dove DRM stuff - if a GPU is provided
> it's an entirely separate piece of IP, and will likely remain a separate
> non-DRM driver.
>
> In the case of PL111, PL111 is not manufacturer specific so the GPU can
> be any standalone GPU.

Oh, yeah, I know PL111 is different block independent from GPU.  I
wasn't advocating adding mali support in this driver.  But instead
creating a mali DRM driver which had GEM and gpu bits, but not KMS.
Then buffer sharing w/ prime/dmabuf just like you would, for example
on a laptop w/ both integrated graphics and discrete GPU.

BR,
-R

Re: [RFC 0/1] drm/pl111: Initial drm/kms driver for pl111

[Daniel Vetter]

On Fri, Jul 26, 2013 at 04:58:55PM +0100, Tom Cooksey wrote:
> Hi Rob,
> 
> > >  * It abuses flags parameter of DRM_IOCTL_MODE_CREATE_DUMB to also
> > >    allocate buffers for the GPU. Still not sure how to resolve this
> > >    as we don't use DRM for our GPU driver.
> > 
> > any thoughts/plans about a DRM GPU driver?  Ideally long term (esp.
> > once the dma-fence stuff is in place), we'd have gpu-specific drm
> > (gpu-only, no kms) driver, and SoC/display specific drm/kms driver,
> > using prime/dmabuf to share between the two.
> 
> The "extra" buffers we were allocating from armsoc DDX were really
> being allocated through DRM/GEM so we could get an flink name
> for them and pass a reference to them back to our GPU driver on
> the client side. If it weren't for our need to access those
> extra off-screen buffers with the GPU we wouldn't need to
> allocate them with DRM at all. So, given they are really "GPU"
> buffers, it does absolutely make sense to allocate them in a
> different driver to the display driver.
> 
> However, to avoid unnecessary memcpys & related cache
> maintenance ops, we'd also like the GPU to render into buffers
> which are scanned out by the display controller. So let's say
> we continue using DRM_IOCTL_MODE_CREATE_DUMB to allocate scan
> out buffers with the display's DRM driver but a custom ioctl
> on the GPU's DRM driver to allocate non scanout, off-screen
> buffers. Sounds great, but I don't think that really works
> with DRI2. If we used two drivers to allocate buffers, which
> of those drivers do we return in DRI2ConnectReply? Even if we
> solve that somehow, GEM flink names are name-spaced to a
> single device node (AFAIK). So when we do a DRI2GetBuffers,
> how does the EGL in the client know which DRM device owns GEM
> flink name "1234"? We'd need some pretty dirty hacks.

I don't know the details, but having different gem driver nodes is exactly
what prime support in X allows. The X server then passes the buffer object
between the different ddx drivers using dma-buf sharing.

So if we have two drm drivers, one a kms-only scanout driver and one a
gem-only mali driver, then the userspace gl mali driver would only ever
talk to the mali drm node. So flink for DRI2 would use that node
exclusively. The scanout drm node would then be treated like e.g. an usb
display-link device which is also display-only.

> So then we looked at allocating _all_ buffers with the GPU's
> DRM driver. That solves the DRI2 single-device-name and single
> name-space issue. It also means the GPU would _never_ render
> into buffers allocated through DRM_IOCTL_MODE_CREATE_DUMB.
> One thing I wasn't sure about is if there was an objection
> to using PRIME to export scanout buffers allocated with
> DRM_IOCTL_MODE_CREATE_DUMB and then importing them into a GPU
> driver to be rendered into? Is that a concern?

Imo sharing dumb buffers should be ok. The "dumb" concept is only really
relevant when the display and render part are integrated into one IP block
and driver.

> Anyway, that latter case also gets quite difficult. The "GPU"
> DRM driver would need to know the constraints of the display
> controller when allocating buffers intended to be scanned out.
> For example, pl111 typically isn't behind an IOMMU and so
> requires physically contiguous memory. We'd have to teach the
> GPU's DRM driver about the constraints of the display HW. Not
> exactly a clean driver model. :-(

Well the current dma-buf sharing code essentially only really works on x86
where everyone has a decent graphics tt and no cache flushing is required.

For ARM I expect that we need to have a common dma-buf backing storage
layer which walks all attached devices on the dma-buf, checks allocation
constraints and then picks a suitable pool to allocate the buffer.

Since dma-bufs should only be allocated once they're getting used (and not
when establishing the sharing) that should work out. But with the current
dma apis exposed to drivers that's not possible really. Essentially we
need ion but please not expose the heaps explicitly to userspace.  The
kernel already knows all this, so could take care of this for userspace
without breaking the current dma api abstractions we have.

Of course if you start to share buffers between different userspace drives
they need to talk to each another about what stride/pixel layout/tiling is
suitable. Since I'm a kernel guy I'll punt on this problem ;-)

> I'm still a little stuck on how to proceed, so any ideas
> would greatly appreciated! My current train of thought is
> having a kind of SoC-specific DRM driver which allocates
> buffers for both display and GPU within a single GEM
> namespace. That SoC-specific DRM driver could then know the
> constraints of both the GPU and the display HW. We could then
> use PRIME to export buffers allocated with the SoC DRM driver
> and import them into the GPU and/or display DRM driver.

That's pretty much how ION works and I'm not in favour of leaking
allocation constraints to userspace like that ...

> Note: While it doesn't use the DRM framework, the Mali T6xx
> kernel driver has supported importing buffers through dma_buf
> for some time. I've even written an EGL extension :-):
> 
> <http://www.khronos.org/registry/egl/extensions/EXT/EGL_EXT_image_dma_buf_im
> port.txt>
> 
> 
> > I'm not entirely sure that the directions that the current CDF
> > proposals are headed is necessarily the right way forward.  I'd prefer
> > to see small/incremental evolution of KMS (ie. add drm_bridge and
> > drm_panel, and refactor the existing encoder-slave).  Keeping it
> > inside drm means that we can evolve it more easily, and avoid layers
> > of glue code for no good reason.
> 
> I think CDF could allow vendors to re-use code they've written
> for their Android driver stack in DRM drivers more easily. Though
> I guess ideally KMS would evolve to a point where it could be used
> by an Android driver stack. I.e. Support explicit fences.

Just fyi with intel we have some DSI/MIPI support patches floating around.
Our plan is to just merge them and then once we have a 2nd drm driver with
DSI support grow some common infrastructure out of this.

The same approach seems to work neatly for hdmi (infoframes) and dp
(although the dp helpers can be seriously extended).

Cheers, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch

Re: [RFC 0/1] drm/pl111: Initial drm/kms driver for pl111

[Rob Clark]

On Fri, Jul 26, 2013 at 11:58 AM, Tom Cooksey <tom.cooksey@arm.com> wrote:
> Hi Rob,
>
>> >  * It abuses flags parameter of DRM_IOCTL_MODE_CREATE_DUMB to also
>> >    allocate buffers for the GPU. Still not sure how to resolve this
>> >    as we don't use DRM for our GPU driver.
>>
>> any thoughts/plans about a DRM GPU driver?  Ideally long term (esp.
>> once the dma-fence stuff is in place), we'd have gpu-specific drm
>> (gpu-only, no kms) driver, and SoC/display specific drm/kms driver,
>> using prime/dmabuf to share between the two.
>
> The "extra" buffers we were allocating from armsoc DDX were really
> being allocated through DRM/GEM so we could get an flink name
> for them and pass a reference to them back to our GPU driver on
> the client side. If it weren't for our need to access those
> extra off-screen buffers with the GPU we wouldn't need to
> allocate them with DRM at all. So, given they are really "GPU"
> buffers, it does absolutely make sense to allocate them in a
> different driver to the display driver.
>
> However, to avoid unnecessary memcpys & related cache
> maintenance ops, we'd also like the GPU to render into buffers
> which are scanned out by the display controller. So let's say
> we continue using DRM_IOCTL_MODE_CREATE_DUMB to allocate scan
> out buffers with the display's DRM driver but a custom ioctl
> on the GPU's DRM driver to allocate non scanout, off-screen
> buffers. Sounds great, but I don't think that really works
> with DRI2. If we used two drivers to allocate buffers, which
> of those drivers do we return in DRI2ConnectReply? Even if we
> solve that somehow, GEM flink names are name-spaced to a
> single device node (AFAIK). So when we do a DRI2GetBuffers,
> how does the EGL in the client know which DRM device owns GEM
> flink name "1234"? We'd need some pretty dirty hacks.

You would return the name of the display driver allocating the
buffers.  On the client side you can use generic ioctls to go from
flink -> handle -> dmabuf.  So the client side would end up opening
both the display drm device and the gpu, but without needing to know
too much about the display.

(Probably in (for example) mesa there needs to be a bit of work to
handle this better, but I think that would be needed as well for
sharing between, say, nouveau and udl displaylink driver.. which is
really the same scenario.)

> So then we looked at allocating _all_ buffers with the GPU's
> DRM driver. That solves the DRI2 single-device-name and single
> name-space issue. It also means the GPU would _never_ render
> into buffers allocated through DRM_IOCTL_MODE_CREATE_DUMB.

Well, I think we can differentiate between shared buffers and internal
buffers (textures, vertex upload, etc, etc).

For example, in mesa/gallium driver there are two paths to getting a
buffer...  ->resource_create() and ->resource_from_handle(), the
latter is the path you go for dri2 buffers shared w/ x11.  The former
is buffers that are just internal to gpu (if !(bind &
PIPE_BIND_SHARED)).  I guess you must have something similar in mali
driver.

> One thing I wasn't sure about is if there was an objection
> to using PRIME to export scanout buffers allocated with
> DRM_IOCTL_MODE_CREATE_DUMB and then importing them into a GPU
> driver to be rendered into? Is that a concern?

well.. it wasn't quite the original intention of the 'dumb' ioctls.
But I guess in the end if you hand the gpu a buffer with a
layout/format that it can't grok, then gpu does a staging texture plus
copy.  If you had, for example, some setup w/ gpu that could only
render to tiled format, plus a display that could scanout that format,
then your DDX driver would need to allocate the dri2 buffers with
something other than dumb ioctl.  (But at that point, you probably
need to implement your own EXA so you could hook in your own custom
upload-to/download-from screen fxns for sw fallbacks, so you're not
talking about using generic DDX anyways.)

> Anyway, that latter case also gets quite difficult. The "GPU"
> DRM driver would need to know the constraints of the display
> controller when allocating buffers intended to be scanned out.
> For example, pl111 typically isn't behind an IOMMU and so
> requires physically contiguous memory. We'd have to teach the
> GPU's DRM driver about the constraints of the display HW. Not
> exactly a clean driver model. :-(
>
> I'm still a little stuck on how to proceed, so any ideas
> would greatly appreciated! My current train of thought is
> having a kind of SoC-specific DRM driver which allocates
> buffers for both display and GPU within a single GEM
> namespace. That SoC-specific DRM driver could then know the
> constraints of both the GPU and the display HW. We could then
> use PRIME to export buffers allocated with the SoC DRM driver
> and import them into the GPU and/or display DRM driver.

Usually if the display drm driver is allocating the buffers that might
be scanned out, it just needs to have minimal knowledge of the GPU
(pitch alignment constraints).  I don't think we need a 3rd device
just to allocate buffers.

Really I don't think the separate display drm and gpu drm device is
much different from desktop udl/displaylink case.  Or desktop
integrated-gpu + external gpu.

> Note: While it doesn't use the DRM framework, the Mali T6xx
> kernel driver has supported importing buffers through dma_buf
> for some time. I've even written an EGL extension :-):
>
> <http://www.khronos.org/registry/egl/extensions/EXT/EGL_EXT_image_dma_buf_im
> port.txt>
>
>
>> I'm not entirely sure that the directions that the current CDF
>> proposals are headed is necessarily the right way forward.  I'd prefer
>> to see small/incremental evolution of KMS (ie. add drm_bridge and
>> drm_panel, and refactor the existing encoder-slave).  Keeping it
>> inside drm means that we can evolve it more easily, and avoid layers
>> of glue code for no good reason.
>
> I think CDF could allow vendors to re-use code they've written
> for their Android driver stack in DRM drivers more easily. Though
> I guess ideally KMS would evolve to a point where it could be used
> by an Android driver stack. I.e. Support explicit fences.
>

yeah, the best would be evolving DRM/KMS to the point that it meets
android requirements.  Because I really don't think android has any
special requirements compared to, say, a wayland compositor.  (Other
than the way they do synchronization.  But that doesn't really *need*
to be different, as far as I can tell.)  It would certainly be easier
if android dev's actually participated in upstream linux graphics, and
talked about what they needed, and maybe even contributed a patch or
two.  But that is a whole different topic.

BR,
-R

>
> Cheers,
>
> Tom
>
>
>
>
>

Re: [RFC 0/1] drm/pl111: Initial drm/kms driver for pl111

[Rob Clark]

On Mon, Aug 5, 2013 at 1:10 PM, Tom Cooksey <tom.cooksey@arm.com> wrote:
> Hi Rob,
>
> +linux-media, +linaro-mm-sig for discussion of video/camera
> buffer constraints...
>
>
>> On Fri, Jul 26, 2013 at 11:58 AM, Tom Cooksey <tom.cooksey@arm.com>
>> wrote:
>> >> >  * It abuses flags parameter of DRM_IOCTL_MODE_CREATE_DUMB to also
>> >> >    allocate buffers for the GPU. Still not sure how to resolve
>> >> >    this as we don't use DRM for our GPU driver.
>> >>
>> >> any thoughts/plans about a DRM GPU driver?  Ideally long term (esp.
>> >> once the dma-fence stuff is in place), we'd have gpu-specific drm
>> >> (gpu-only, no kms) driver, and SoC/display specific drm/kms driver,
>> >> using prime/dmabuf to share between the two.
>> >
>> > The "extra" buffers we were allocating from armsoc DDX were really
>> > being allocated through DRM/GEM so we could get an flink name
>> > for them and pass a reference to them back to our GPU driver on
>> > the client side. If it weren't for our need to access those
>> > extra off-screen buffers with the GPU we wouldn't need to
>> > allocate them with DRM at all. So, given they are really "GPU"
>> > buffers, it does absolutely make sense to allocate them in a
>> > different driver to the display driver.
>> >
>> > However, to avoid unnecessary memcpys & related cache
>> > maintenance ops, we'd also like the GPU to render into buffers
>> > which are scanned out by the display controller. So let's say
>> > we continue using DRM_IOCTL_MODE_CREATE_DUMB to allocate scan
>> > out buffers with the display's DRM driver but a custom ioctl
>> > on the GPU's DRM driver to allocate non scanout, off-screen
>> > buffers. Sounds great, but I don't think that really works
>> > with DRI2. If we used two drivers to allocate buffers, which
>> > of those drivers do we return in DRI2ConnectReply? Even if we
>> > solve that somehow, GEM flink names are name-spaced to a
>> > single device node (AFAIK). So when we do a DRI2GetBuffers,
>> > how does the EGL in the client know which DRM device owns GEM
>> > flink name "1234"? We'd need some pretty dirty hacks.
>>
>> You would return the name of the display driver allocating the
>> buffers.  On the client side you can use generic ioctls to go from
>> flink -> handle -> dmabuf.  So the client side would end up opening
>> both the display drm device and the gpu, but without needing to know
>> too much about the display.
>
> I think the bit I was missing was that a GEM bo for a buffer imported
> using dma_buf/PRIME can still be flink'd. So the display controller's
> DRM driver allocates scan-out buffers via the DUMB buffer allocate
> ioctl. Those scan-out buffers than then be exported from the
> dispaly's DRM driver and imported into the GPU's DRM driver using
> PRIME. Once imported into the GPU's driver, we can use flink to get a
> name for that buffer within the GPU DRM driver's name-space to return
> to the DRI2 client. That same namespace is also what DRI2 back-buffers
> are allocated from, so I think that could work... Except...
>

(and.. the general direction is that things will move more to just use
dmabuf directly, ie. wayland or dri3)

>
>> > Anyway, that latter case also gets quite difficult. The "GPU"
>> > DRM driver would need to know the constraints of the display
>> > controller when allocating buffers intended to be scanned out.
>> > For example, pl111 typically isn't behind an IOMMU and so
>> > requires physically contiguous memory. We'd have to teach the
>> > GPU's DRM driver about the constraints of the display HW. Not
>> > exactly a clean driver model. :-(
>> >
>> > I'm still a little stuck on how to proceed, so any ideas
>> > would greatly appreciated! My current train of thought is
>> > having a kind of SoC-specific DRM driver which allocates
>> > buffers for both display and GPU within a single GEM
>> > namespace. That SoC-specific DRM driver could then know the
>> > constraints of both the GPU and the display HW. We could then
>> > use PRIME to export buffers allocated with the SoC DRM driver
>> > and import them into the GPU and/or display DRM driver.
>>
>> Usually if the display drm driver is allocating the buffers that might
>> be scanned out, it just needs to have minimal knowledge of the GPU
>> (pitch alignment constraints).  I don't think we need a 3rd device
>> just to allocate buffers.
>
> While Mali can render to pretty much any buffer, there is a mild
> performance improvement to be had if the buffer stride is aligned to
> the AXI bus's max burst length when drawing to the buffer.

I suspect the display controllers might frequently benefit if the
pitch is aligned to AXI burst length too..

> So in some respects, there is a constraint on how buffers which will
> be drawn to using the GPU are allocated. I don't really like the idea
> of teaching the display controller DRM driver about the GPU buffer
> constraints, even if they are fairly trivial like this. If the same
> display HW IP is being used on several SoCs, it seems wrong somehow
> to enforce those GPU constraints if some of those SoCs don't have a
> GPU.

Well, I suppose you could get min_pitch_alignment from devicetree, or
something like this..

In the end, the easy solution is just to make the display allocate to
the worst-case pitch alignment.  In the early days of dma-buf
discussions, we kicked around the idea of negotiating or
programatically describing the constraints, but that didn't really
seem like a bounded problem.

> We may also then have additional constraints when sharing buffers
> between the display HW and video decode or even camera ISP HW.
> Programmatically describing buffer allocation constraints is very
> difficult and I'm not sure you can actually do it - there's some
> pretty complex constraints out there! E.g. I believe there's a
> platform where Y and UV planes of the reference frame need to be in
> separate DRAM banks for real-time 1080p decode, or something like
> that?

yes, this was discussed.  This is different from pitch/format/size
constraints.. it is really just a placement constraint (ie. where do
the physical pages go).  IIRC the conclusion was to use a dummy
devices with it's own CMA pool for attaching the Y vs UV buffers.

> Anyway, I guess my point is that even if we solve how to allocate
> buffers which will be shared between the GPU and display HW such that
> both sets of constraints are satisfied, that may not be the end of
> the story.
>

that was part of the reason to punt this problem to userspace ;-)

In practice, the kernel drivers doesn't usually know too much about
the dimensions/format/etc.. that is really userspace level knowledge.
There are a few exceptions when the kernel needs to know how to setup
GTT/etc for tiled buffers, but normally this sort of information is up
at the next level up (userspace, and drm_framebuffer in case of
scanout).  Userspace media frameworks like GStreamer already have a
concept of format/caps negotiation.  For non-display<->gpu sharing, I
think this is probably where this sort of constraint negotiation
should be handled.

BR,
-R

>
> Cheers,
>
> Tom
>
>
>
>
>

Re: [RFC 0/1] drm/pl111: Initial drm/kms driver for pl111

[Rob Clark]

On Tue, Aug 6, 2013 at 7:31 AM, Tom Cooksey <tom.cooksey@arm.com> wrote:
>
>> > So in some respects, there is a constraint on how buffers which will
>> > be drawn to using the GPU are allocated. I don't really like the idea
>> > of teaching the display controller DRM driver about the GPU buffer
>> > constraints, even if they are fairly trivial like this. If the same
>> > display HW IP is being used on several SoCs, it seems wrong somehow
>> > to enforce those GPU constraints if some of those SoCs don't have a
>> > GPU.
>>
>> Well, I suppose you could get min_pitch_alignment from devicetree, or
>> something like this..
>>
>> In the end, the easy solution is just to make the display allocate to
>> the worst-case pitch alignment.  In the early days of dma-buf
>> discussions, we kicked around the idea of negotiating or
>> programatically describing the constraints, but that didn't really
>> seem like a bounded problem.
>
> Yeah - I was around for some of those discussions and agree it's not
> really an easy problem to solve.
>
>
>
>> > We may also then have additional constraints when sharing buffers
>> > between the display HW and video decode or even camera ISP HW.
>> > Programmatically describing buffer allocation constraints is very
>> > difficult and I'm not sure you can actually do it - there's some
>> > pretty complex constraints out there! E.g. I believe there's a
>> > platform where Y and UV planes of the reference frame need to be in
>> > separate DRAM banks for real-time 1080p decode, or something like
>> > that?
>>
>> yes, this was discussed.  This is different from pitch/format/size
>> constraints.. it is really just a placement constraint (ie. where do
>> the physical pages go).  IIRC the conclusion was to use a dummy
>> devices with it's own CMA pool for attaching the Y vs UV buffers.
>>
>> > Anyway, I guess my point is that even if we solve how to allocate
>> > buffers which will be shared between the GPU and display HW such that
>> > both sets of constraints are satisfied, that may not be the end of
>> > the story.
>> >
>>
>> that was part of the reason to punt this problem to userspace ;-)
>>
>> In practice, the kernel drivers doesn't usually know too much about
>> the dimensions/format/etc.. that is really userspace level knowledge.
>> There are a few exceptions when the kernel needs to know how to setup
>> GTT/etc for tiled buffers, but normally this sort of information is up
>> at the next level up (userspace, and drm_framebuffer in case of
>> scanout).  Userspace media frameworks like GStreamer already have a
>> concept of format/caps negotiation.  For non-display<->gpu sharing, I
>> think this is probably where this sort of constraint negotiation
>> should be handled.
>
> I agree that user-space will know which devices will access the buffer
> and thus can figure out at least a common pixel format. Though I'm not
> so sure userspace can figure out more low-level details like alignment
> and placement in physical memory, etc.

well, let's divide things up into two categories:

1) the arrangement and format of pixels.. ie. what userspace would
need to know if it mmap's a buffer.  This includes pixel format,
stride, etc.  This should be negotiated in userspace, it would be
crazy to try to do this in the kernel.

2) the physical placement of the pages.  Ie. whether it is contiguous
or not.  Which bank the pages in the buffer are placed in, etc.  This
is not visible to userspace.  This is the purpose of the attach step,
so you know all the devices involved in sharing up front before
allocating the backing pages.  (Or in the worst case, if you have a
"late attacher" you at least know when no device is doing dma access
to a buffer and can reallocate and move the buffer.)  A long time
back, I had a patch that added a field or two to 'struct
device_dma_parameters' so that it could be known if a device required
contiguous buffers.. looks like that never got merged, so I'd need to
dig that back up and resend it.  But the idea was to have the 'struct
device' encapsulate all the information that would be needed to
do-the-right-thing when it comes to placement.

> Anyway, assuming user-space can figure out how a buffer should be
> stored in memory, how does it indicate this to a kernel driver and
> actually allocate it? Which ioctl on which device does user-space
> call, with what parameters? Are you suggesting using something like
> ION which exposes the low-level details of how buffers are laid out in
> physical memory to userspace? If not, what?

no, userspace should not need to know this.  And having a central
driver that knows this for all the other drivers in the system doesn't
really solve anything and isn't really scalable.  At best you might
want, in some cases, a flag you can pass when allocating.  For
example, some of the drivers have a 'SCANOUT' flag that can be passed
when allocating a GEM buffer, as a hint to the kernel that 'if this hw
requires contig memory for scanout, allocate this buffer contig'.  But
really, when it comes to sharing buffers between devices, we want this
sort of information in dev->dma_params of the importing device(s).

BR,
-R

Re: [RFC 0/1] drm/pl111: Initial drm/kms driver for pl111

[Rob Clark]

On Tue, Aug 6, 2013 at 10:03 AM, Tom Cooksey <tom.cooksey@arm.com> wrote:
> Hi Rob,
>
>> >> > We may also then have additional constraints when sharing buffers
>> >> > between the display HW and video decode or even camera ISP HW.
>> >> > Programmatically describing buffer allocation constraints is very
>> >> > difficult and I'm not sure you can actually do it - there's some
>> >> > pretty complex constraints out there! E.g. I believe there's a
>> >> > platform where Y and UV planes of the reference frame need to be
>> >> > in separate DRAM banks for real-time 1080p decode, or something
>> >> > like that?
>> >>
>> >> yes, this was discussed.  This is different from pitch/format/size
>> >> constraints.. it is really just a placement constraint (ie. where
>> >> do the physical pages go).  IIRC the conclusion was to use a dummy
>> >> devices with it's own CMA pool for attaching the Y vs UV buffers.
>> >>
>> >> > Anyway, I guess my point is that even if we solve how to allocate
>> >> > buffers which will be shared between the GPU and display HW such
>> >> > that both sets of constraints are satisfied, that may not be the
>> >> > end of the story.
>> >> >
>> >>
>> >> that was part of the reason to punt this problem to userspace ;-)
>> >>
>> >> In practice, the kernel drivers doesn't usually know too much about
>> >> the dimensions/format/etc.. that is really userspace level
>> >> knowledge. There are a few exceptions when the kernel needs to know
>> >> how to setup GTT/etc for tiled buffers, but normally this sort of
>> >> information is up at the next level up (userspace, and
>> >> drm_framebuffer in case of scanout).  Userspace media frameworks
>> >> like GStreamer already have a concept of format/caps negotiation.
>> >> For non-display<->gpu sharing, I think this is probably where this
>> >> sort of constraint negotiation should be handled.
>> >
>> > I agree that user-space will know which devices will access the
>> > buffer and thus can figure out at least a common pixel format.
>> > Though I'm not so sure userspace can figure out more low-level
>> > details like alignment and placement in physical memory, etc.
>> >
>>
>> well, let's divide things up into two categories:
>>
>> 1) the arrangement and format of pixels.. ie. what userspace would
>> need to know if it mmap's a buffer.  This includes pixel format,
>> stride, etc.  This should be negotiated in userspace, it would be
>> crazy to try to do this in the kernel.
>
> Absolutely. Pixel format has to be negotiated by user-space as in
> most cases, user-space can map the buffer and thus will need to
> know how to interpret the data.
>
>
>
>> 2) the physical placement of the pages.  Ie. whether it is contiguous
>> or not.  Which bank the pages in the buffer are placed in, etc.  This
>> is not visible to userspace.
>
> Seems sensible to me.
>
>
>> ... This is the purpose of the attach step,
>> so you know all the devices involved in sharing up front before
>> allocating the backing pages. (Or in the worst case, if you have a
>> "late attacher" you at least know when no device is doing dma access
>> to a buffer and can reallocate and move the buffer.)  A long time
>> back, I had a patch that added a field or two to 'struct
>> device_dma_parameters' so that it could be known if a device required
>> contiguous buffers.. looks like that never got merged, so I'd need to
>> dig that back up and resend it.  But the idea was to have the 'struct
>> device' encapsulate all the information that would be needed to
>> do-the-right-thing when it comes to placement.
>
> As I understand it, it's up to the exporting device to allocate the
> memory backing the dma_buf buffer. I guess the latest possible point
> you can allocate the backing pages is when map_dma_buf is first
> called? At that point the exporter can iterate over the current set
> of attachments, programmatically determine the all the constraints of
> all the attached drivers and attempt to allocate the backing pages
> in such a way as to satisfy all those constraints?

yes, this is the idea..  possibly some room for some helpers to help
out with this, but that is all under the hood from userspace
perspective

> Didn't you say that programmatically describing device placement
> constraints was an unbounded problem? I guess we would have to
> accept that it's not possible to describe all possible constraints
> and instead find a way to describe the common ones?

well, the point I'm trying to make, is by dividing your constraints
into two groups, one that impacts and is handled by userspace, and one
that is in the kernel (ie. where the pages go), you cut down the
number of permutations that the kernel has to care about considerably.
 And kernel already cares about, for example, what range of addresses
that a device can dma to/from.  I think really the only thing missing
is the max # of sglist entries (contiguous or not)

> One problem with this is it duplicates a lot of logic in each
> driver which can export a dma_buf buffer. Each exporter will need to
> do pretty much the same thing: iterate over all the attachments,
> determine of all the constraints (assuming that can be done) and
> allocate pages such that the lowest-common-denominator is satisfied.
>
> Perhaps rather than duplicating that logic in every driver, we could
> Instead move allocation of the backing pages into dma_buf itself?
>

I tend to think it is better to add helpers as we see common patterns
emerge, which drivers can opt-in to using.  I don't think that we
should move allocation into dma_buf itself, but it would perhaps be
useful to have dma_alloc_*() variants that could allocate for multiple
devices.  That would help for simple stuff, although I'd suspect
eventually a GPU driver will move away from that.  (Since you probably
want to play tricks w/ pools of pages that are pre-zero'd and in the
correct cache state, use spare cycles on the gpu or dma engine to
pre-zero uncached pages, and games like that.)

>
>> > Anyway, assuming user-space can figure out how a buffer should be
>> > stored in memory, how does it indicate this to a kernel driver and
>> > actually allocate it? Which ioctl on which device does user-space
>> > call, with what parameters? Are you suggesting using something like
>> > ION which exposes the low-level details of how buffers are laid out
>> in
>> > physical memory to userspace? If not, what?
>>
>> no, userspace should not need to know this.  And having a central
>> driver that knows this for all the other drivers in the system doesn't
>> really solve anything and isn't really scalable.  At best you might
>> want, in some cases, a flag you can pass when allocating.  For
>> example, some of the drivers have a 'SCANOUT' flag that can be passed
>> when allocating a GEM buffer, as a hint to the kernel that 'if this hw
>> requires contig memory for scanout, allocate this buffer contig'.  But
>> really, when it comes to sharing buffers between devices, we want this
>> sort of information in dev->dma_params of the importing device(s).
>
> If you had a single driver which knew the constraints of all devices
> on that particular SoC and the interface allowed user-space to specify
> which devices a buffer is intended to be used with, I guess it could
> pretty trivially allocate pages which satisfy those constraints? It

keep in mind, even a number of SoC's come with pcie these days.  You
already have things like

  https://developer.nvidia.com/content/kayla-platform

You probably want to get out of the SoC mindset, otherwise you are
going to make bad assumptions that come back to bite you later on.

> wouldn't need a way to programmatically describe the constraints
> either: As you say, if userspace sets the "SCANOUT" flag, it would
> just "know" that on this SoC, that buffer needs to be physically
> contiguous for example.

not really.. it just knows it wants to scanout the buffer, and tells
this as a hint to the kernel.

For example, on omapdrm, the SCANOUT flag does nothing on omap4+
(where phys contig is not required for scanout), but causes CMA
(dma_alloc_*()) to be used on omap3.  Userspace doesn't care.  It just
knows that it wants to be able to scanout that particular buffer.

> Though It would effectively mean you'd need an "allocation" driver per
> SoC, which as you say may not be scalable?

Right.. and not actually even possible in the general sense (see SoC +
external pcie gfx card)

BR,
-R

>
>
> Cheers,
>
> Tom
>
>
>
>
>

Re: [RFC 0/1] drm/pl111: Initial drm/kms driver for pl111

[Rob Clark]

On Tue, Aug 6, 2013 at 1:38 PM, Tom Cooksey <tom.cooksey@arm.com> wrote:
>
>> >> ... This is the purpose of the attach step,
>> >> so you know all the devices involved in sharing up front before
>> >> allocating the backing pages. (Or in the worst case, if you have a
>> >> "late attacher" you at least know when no device is doing dma access
>> >> to a buffer and can reallocate and move the buffer.)  A long time
>> >> back, I had a patch that added a field or two to 'struct
>> >> device_dma_parameters' so that it could be known if a device
>> >> required contiguous buffers.. looks like that never got merged, so
>> >> I'd need to dig that back up and resend it.  But the idea was to
>> >> have the 'struct device' encapsulate all the information that would
>> >> be needed to do-the-right-thing when it comes to placement.
>> >
>> > As I understand it, it's up to the exporting device to allocate the
>> > memory backing the dma_buf buffer. I guess the latest possible point
>> > you can allocate the backing pages is when map_dma_buf is first
>> > called? At that point the exporter can iterate over the current set
>> > of attachments, programmatically determine the all the constraints of
>> > all the attached drivers and attempt to allocate the backing pages
>> > in such a way as to satisfy all those constraints?
>>
>> yes, this is the idea..  possibly some room for some helpers to help
>> out with this, but that is all under the hood from userspace
>> perspective
>>
>> > Didn't you say that programmatically describing device placement
>> > constraints was an unbounded problem? I guess we would have to
>> > accept that it's not possible to describe all possible constraints
>> > and instead find a way to describe the common ones?
>>
>> well, the point I'm trying to make, is by dividing your constraints
>> into two groups, one that impacts and is handled by userspace, and one
>> that is in the kernel (ie. where the pages go), you cut down the
>> number of permutations that the kernel has to care about considerably.
>>  And kernel already cares about, for example, what range of addresses
>> that a device can dma to/from.  I think really the only thing missing
>> is the max # of sglist entries (contiguous or not)
>
> I think it's more than physically contiguous or not.
>
> For example, it can be more efficient to use large page sizes on
> devices with IOMMUs to reduce TLB traffic. I think the size and even
> the availability of large pages varies between different IOMMUs.

sure.. but I suppose if we can spiff out dma_params to express "I need
contiguous", perhaps we can add some way to express "I prefer
as-contiguous-as-possible".. either way, this is about where the pages
are placed, and not about the layout of pixels within the page, so
should be in kernel.  It's something that is missing, but I believe
that it belongs in dma_params and hidden behind dma_alloc_*() for
simple drivers.

> There's also the issue of buffer stride alignment. As I say, if the
> buffer is to be written by a tile-based GPU like Mali, it's more
> efficient if the buffer's stride is aligned to the max AXI bus burst
> length. Though I guess a buffer stride only makes sense as a concept
> when interpreting the data as a linear-layout 2D image, so perhaps
> belongs in user-space along with format negotiation?
>

Yeah.. this isn't about where the pages go, but about the arrangement
within a page.

And, well, except for hw that supports the same tiling (or
compressed-fb) in display+gpu, you probably aren't sharing tiled
buffers.

>
>> > One problem with this is it duplicates a lot of logic in each
>> > driver which can export a dma_buf buffer. Each exporter will need to
>> > do pretty much the same thing: iterate over all the attachments,
>> > determine of all the constraints (assuming that can be done) and
>> > allocate pages such that the lowest-common-denominator is satisfied.
>> >
>> > Perhaps rather than duplicating that logic in every driver, we could
>> > Instead move allocation of the backing pages into dma_buf itself?
>> >
>>
>> I tend to think it is better to add helpers as we see common patterns
>> emerge, which drivers can opt-in to using.  I don't think that we
>> should move allocation into dma_buf itself, but it would perhaps be
>> useful to have dma_alloc_*() variants that could allocate for multiple
>> devices.
>
> A helper could work I guess, though I quite like the idea of having
> dma_alloc_*() variants which take a list of devices to allocate memory
> for.
>
>
>> That would help for simple stuff, although I'd suspect
>> eventually a GPU driver will move away from that.  (Since you probably
>> want to play tricks w/ pools of pages that are pre-zero'd and in the
>> correct cache state, use spare cycles on the gpu or dma engine to
>> pre-zero uncached pages, and games like that.)
>
> So presumably you're talking about a GPU driver being the exporter
> here? If so, how could the GPU driver do these kind of tricks on
> memory shared with another device?
>

Yes, that is gpu-as-exporter.  If someone else is allocating buffers,
it is up to them to do these tricks or not.  Probably there is a
pretty good chance that if you aren't a GPU you don't need those sort
of tricks for fast allocation of transient upload buffers, staging
textures, temporary pixmaps, etc.  Ie. I don't really think a v4l
camera or video decoder would benefit from that sort of optimization.

>
>> >> > Anyway, assuming user-space can figure out how a buffer should be
>> >> > stored in memory, how does it indicate this to a kernel driver and
>> >> > actually allocate it? Which ioctl on which device does user-space
>> >> > call, with what parameters? Are you suggesting using something
>> >> > like ION which exposes the low-level details of how buffers are
>> > >> laid out in physical memory to userspace? If not, what?
>> >> >
>> >>
>> >> no, userspace should not need to know this.  And having a central
>> >> driver that knows this for all the other drivers in the system
>> >> doesn't really solve anything and isn't really scalable.  At best
>> >> you might want, in some cases, a flag you can pass when allocating.
>> >> For example, some of the drivers have a 'SCANOUT' flag that can be
>> >> passed when allocating a GEM buffer, as a hint to the kernel that
>> >> 'if this hw requires contig memory for scanout, allocate this
>> >> buffer contig'. But really, when it comes to sharing buffers
>> >> between devices, we want this sort of information in dev->dma_params
>> >> of the importing device(s).
>> >
>> > If you had a single driver which knew the constraints of all devices
>> > on that particular SoC and the interface allowed user-space to
>> > specify which devices a buffer is intended to be used with, I guess
>> > it could pretty trivially allocate pages which satisfy those
> constraints?
>>
>> keep in mind, even a number of SoC's come with pcie these days.  You
>> already have things like
>>
>>   https://developer.nvidia.com/content/kayla-platform
>>
>> You probably want to get out of the SoC mindset, otherwise you are
>> going to make bad assumptions that come back to bite you later on.
>
> Sure - there are always going to be PC-like devices where the
> hardware configuration isn't fixed like it is on a traditional SoC.
> But I'd rather have a simple solution which works on traditional SoCs
> than no solution at all. Today our solution is to over-load the dumb
> buffer alloc functions of the display's DRM driver - For now I'm just
> looking for the next step up from that! ;-)
>

True.. the original intention, which is perhaps a bit desktop-centric,
really was for there to be a userspace component talking to the drm
driver for allocation, ie. xf86-video-foo and/or
src/gallium/drivers/foo (for example) ;-)

Which means for x11 having a SoC vendor specific xf86-video-foo for
x11..  or vendor specific gbm implementation for wayland.  (Although
at least in the latter case it is a pretty small piece of code.)  But
that is probably what you are trying to avoid.

At any rate, for both xorg and wayland/gbm, you know when a buffer is
going to be a scanout buffer.  What I'd recommend is define a small
userspace API that your customers (the SoC vendors) implement to
allocate a scanout buffer and hand you back a dmabuf fd.  That could
be used both for x11 and for gbm.  Inputs should be requested
width/height and format.  And outputs pitch plus dmabuf fd.

(Actually you might even just want to use gbm as your starting point.
You could probably just use gbm from xf86-video-armsoc for allocation,
to have one thing that works for both wayland and x11.  Scanout and
cursor buffers should go to vendor/SoC specific fxn, rest can be
allocated from mali kernel driver.)

>
>> > wouldn't need a way to programmatically describe the constraints
>> > either: As you say, if userspace sets the "SCANOUT" flag, it would
>> > just "know" that on this SoC, that buffer needs to be physically
>> > contiguous for example.
>>
>> not really.. it just knows it wants to scanout the buffer, and tells
>> this as a hint to the kernel.
>>
>> For example, on omapdrm, the SCANOUT flag does nothing on omap4+
>> (where phys contig is not required for scanout), but causes CMA
>> (dma_alloc_*()) to be used on omap3.  Userspace doesn't care.  It just
>> knows that it wants to be able to scanout that particular buffer.
>
> I think that's the idea? The omap3's allocator driver would use
> contiguous memory when it detects the SCANOUT flag whereas the omap4
> allocator driver wouldn't have to. No complex negotiation of
> constraints - it just "knows".
>

well, it is same allocating driver in both cases (although maybe that
is unimportant).  The "it" that just knows it wants to scanout is
userspace.  The "it" that just knows that scanout translates to
contiguous (or not) is the kernel.  Perhaps we are saying the same
thing ;-)

BR,
-R

>
> Cheers,
>
> Tom
>
>
>
>

Re: [Linaro-mm-sig] [RFC 0/1] drm/pl111: Initial drm/kms driver for pl111

[Alex Deucher]

On Wed, Aug 7, 2013 at 1:33 PM, Tom Cooksey <tom.cooksey@arm.com> wrote:
>
>> >> > Didn't you say that programmatically describing device placement
>> >> > constraints was an unbounded problem? I guess we would have to
>> >> > accept that it's not possible to describe all possible constraints
>> >> > and instead find a way to describe the common ones?
>> >>
>> >> well, the point I'm trying to make, is by dividing your constraints
>> >> into two groups, one that impacts and is handled by userspace, and
>> >> one that is in the kernel (ie. where the pages go), you cut down
>> >> the number of permutations that the kernel has to care about
>> >>  considerably. And kernel already cares about, for example, what
>> >> range of addresses that a device can dma to/from.  I think really
>> >> the only thing missing is the max # of sglist entries (contiguous
>> >> or not)
>> >
>> > I think it's more than physically contiguous or not.
>> >
>> > For example, it can be more efficient to use large page sizes on
>> > devices with IOMMUs to reduce TLB traffic. I think the size and even
>> > the availability of large pages varies between different IOMMUs.
>>
>> sure.. but I suppose if we can spiff out dma_params to express "I need
>> contiguous", perhaps we can add some way to express "I prefer
>> as-contiguous-as-possible".. either way, this is about where the pages
>> are placed, and not about the layout of pixels within the page, so
>> should be in kernel.  It's something that is missing, but I believe
>> that it belongs in dma_params and hidden behind dma_alloc_*() for
>> simple drivers.
>
> Thinking about it, isn't this more a property of the IOMMU? I mean,
> are there any cases where an IOMMU had a large page mode but you
> wouldn't want to use it? So when allocating the memory, you'd have to
> take into account not just the constraints of the devices themselves,
> but also of any IOMMUs any of the device sit behind?
>
>
>> > There's also the issue of buffer stride alignment. As I say, if the
>> > buffer is to be written by a tile-based GPU like Mali, it's more
>> > efficient if the buffer's stride is aligned to the max AXI bus burst
>> > length. Though I guess a buffer stride only makes sense as a concept
>> > when interpreting the data as a linear-layout 2D image, so perhaps
>> > belongs in user-space along with format negotiation?
>> >
>>
>> Yeah.. this isn't about where the pages go, but about the arrangement
>> within a page.
>>
>> And, well, except for hw that supports the same tiling (or
>> compressed-fb) in display+gpu, you probably aren't sharing tiled
>> buffers.
>
> You'd only want to share a buffer between devices if those devices can
> understand the same pixel format. That pixel format can't be device-
> specific or opaque, it has to be explicit. I think drm_fourcc.h is
> what defines all the possible pixel formats. This is the enum I used
> in EGL_EXT_image_dma_buf_import at least. So if we get to the point
> where multiple devices can understand a tiled or compressed format, I
> assume we could just add that format to drm_fourcc.h and possibly
> v4l2's v4l2_mbus_pixelcode enum in v4l2-mediabus.h.
>
> For user-space to negotiate a common pixel format and now stride
> alignment, I guess it will obviously need a way to query what pixel
> formats a device supports and what its stride alignment requirements
> are.
>
> I don't know v4l2 very well, but it certainly seems the pixel format
> can be queried using V4L2_SUBDEV_FORMAT_TRY when attempting to set
> a particular format. I couldn't however find a way to retrieve a list
> of supported formats - it seems the mechanism is to try out each
> format in turn to determine if it is supported. Is that right?
>
> There doesn't however seem a way to query what stride constraints a
> V4l2 device might have. Does HW abstracted by v4l2 typically have
> such constraints? If so, how can we query them such that a buffer
> allocated by a DRM driver can be imported into v4l2 and used with
> that HW?
>
> Turning to DRM/KMS, it seems the supported formats of a plane can be
> queried using drm_mode_get_plane. However, there doesn't seem to be a
> way to query the supported formats of a crtc? If display HW only
> supports scanning out from a single buffer (like pl111 does), I think
> it won't have any planes and a fb can only be set on the crtc. In
> which case, how should user-space query which pixel formats that crtc
> supports?
>
> Assuming user-space can query the supported formats and find a common
> one, it will need to allocate a buffer. Looks like
> drm_mode_create_dumb can do that, but it only takes a bpp parameter,
> there's no format parameter. I assume then that user-space defines
> the format and tells the DRM driver which format the buffer is in
> when creating the fb with drm_mode_fb_cmd2, which does take a format
> parameter? Is that right?
>
> As with v4l2, DRM doesn't appear to have a way to query the stride
> constraints? Assuming there is a way to query the stride constraints,
> there also isn't a way to specify them when creating a buffer with
> DRM, though perhaps the existing pitch parameter of
> drm_mode_create_dumb could be used to allow user-space to pass in a
> minimum stride as well as receive the allocated stride?
>
>
>> >> > One problem with this is it duplicates a lot of logic in each
>> >> > driver which can export a dma_buf buffer. Each exporter will
>> >> > need to do pretty much the same thing: iterate over all the
>> >> > attachments, determine of all the constraints (assuming that
>> >> > can be done) and allocate pages such that the lowest-common-
>> >> > denominator is satisfied. Perhaps rather than duplicating that
>> >> > logic in every driver, we could instead move allocation of the
>> >> > backing pages into dma_buf itself?
>> >>
>> >> I tend to think it is better to add helpers as we see common
>>
>> >> patterns emerge, which drivers can opt-in to using.  I don't
>> >> think that we should move allocation into dma_buf itself, but
>> >> it would perhaps be useful to have dma_alloc_*() variants that
>> >> could allocate for multiple devices.
>> >
>> > A helper could work I guess, though I quite like the idea of
>> > having dma_alloc_*() variants which take a list of devices to
>> > allocate memory for.
>> >
>> >
>> >> That would help for simple stuff, although I'd suspect
>> >> eventually a GPU driver will move away from that.  (Since you
>> >> probably want to play tricks w/ pools of pages that are
>> >> pre-zero'd and in the correct cache state, use spare cycles on
>> >> the gpu or dma engine to pre-zero uncached pages, and games
>> >> like that.)
>> >
>> > So presumably you're talking about a GPU driver being the exporter
>> > here? If so, how could the GPU driver do these kind of tricks on
>> > memory shared with another device?
>>
>> Yes, that is gpu-as-exporter.  If someone else is allocating buffers,
>> it is up to them to do these tricks or not.  Probably there is a
>> pretty good chance that if you aren't a GPU you don't need those sort
>> of tricks for fast allocation of transient upload buffers, staging
>> textures, temporary pixmaps, etc.  Ie. I don't really think a v4l
>> camera or video decoder would benefit from that sort of optimization.
>
> Right - but none of those are really buffers you'd want to export with
> dma_buf to share with another device are they? In which case, why not
> just have dma_buf figure out the constraints and allocate the memory?
>
> If a driver needs to allocate memory in a special way for a particular
> device, I can't really imagine how it would be able to share that
> buffer with another device using dma_buf? I guess a driver is likely
> to need some magic voodoo to configure access to the buffer for its
> device, but surely that would be done by the dma_mapping framework
> when dma_buf_map happens?
>
>
>
>> >> You probably want to get out of the SoC mindset, otherwise you are
>> >> going to make bad assumptions that come back to bite you later on.
>> >
>> > Sure - there are always going to be PC-like devices where the
>> > hardware configuration isn't fixed like it is on a traditional SoC.
>> > But I'd rather have a simple solution which works on traditional SoCs
>> > than no solution at all. Today our solution is to over-load the dumb
>> > buffer alloc functions of the display's DRM driver - For now I'm just
>> > looking for the next step up from that! ;-)
>>
>> True.. the original intention, which is perhaps a bit desktop-centric,
>> really was for there to be a userspace component talking to the drm
>> driver for allocation, ie. xf86-video-foo and/or
>> src/gallium/drivers/foo (for example) ;-)
>>
>> Which means for x11 having a SoC vendor specific xf86-video-foo for
>> x11..  or vendor specific gbm implementation for wayland.  (Although
>> at least in the latter case it is a pretty small piece of code.)  But
>> that is probably what you are trying to avoid.
>
> I've been trying to get my head around how PRIME relates to DDX
> drivers. As I understand it (which is likely wrong), you have a laptop
> with both an Intel & an nVidia GPU. You have both the i915 & nouveau
> kernel drivers loaded. What I'm not sure about is which GPU's display
> controller is actually hooked up to the physical connector? Perhaps
> there is a MUX like there is on Versatile Express?
>
> What I also don't understand is what DDX driver is loaded? Is it
> xf86-video-intel, xf86-video-nouveau or both? I get the impression
> that there's a "master" DDX which implements 2D operations but can
> import buffers using PRIME from the other driver and draw to them.
> Or is it more that it's able to export rendered buffers to the
> "slave" DRM for scanout? Either way, it's pretty similar to an ARM
> SoC setup which has the GPU and the display as two totally
> independent devices.
>

In the early days, there was a MUX to switch the displays between the
two GPUs, but most modern systems are MUX-less and the dGPU is either
connected to no displays or in some cases the local panel is attached
to the integrated GPU and the external displays are connected to the
dGPU.

In the MUX-less case, the dGPU can be used to render, and then the
result is copied to the integrated GPU for display.

Alex

Re: [RFC 0/1] drm/pl111: Initial drm/kms driver for pl111

[Rob Clark]

On Wed, Aug 7, 2013 at 1:33 PM, Tom Cooksey <tom.cooksey@arm.com> wrote:
>
>> >> > Didn't you say that programmatically describing device placement
>> >> > constraints was an unbounded problem? I guess we would have to
>> >> > accept that it's not possible to describe all possible constraints
>> >> > and instead find a way to describe the common ones?
>> >>
>> >> well, the point I'm trying to make, is by dividing your constraints
>> >> into two groups, one that impacts and is handled by userspace, and
>> >> one that is in the kernel (ie. where the pages go), you cut down
>> >> the number of permutations that the kernel has to care about
>> >>  considerably. And kernel already cares about, for example, what
>> >> range of addresses that a device can dma to/from.  I think really
>> >> the only thing missing is the max # of sglist entries (contiguous
>> >> or not)
>> >
>> > I think it's more than physically contiguous or not.
>> >
>> > For example, it can be more efficient to use large page sizes on
>> > devices with IOMMUs to reduce TLB traffic. I think the size and even
>> > the availability of large pages varies between different IOMMUs.
>>
>> sure.. but I suppose if we can spiff out dma_params to express "I need
>> contiguous", perhaps we can add some way to express "I prefer
>> as-contiguous-as-possible".. either way, this is about where the pages
>> are placed, and not about the layout of pixels within the page, so
>> should be in kernel.  It's something that is missing, but I believe
>> that it belongs in dma_params and hidden behind dma_alloc_*() for
>> simple drivers.
>
> Thinking about it, isn't this more a property of the IOMMU? I mean,
> are there any cases where an IOMMU had a large page mode but you
> wouldn't want to use it? So when allocating the memory, you'd have to
> take into account not just the constraints of the devices themselves,
> but also of any IOMMUs any of the device sit behind?
>

perhaps yes.  But the device is associated w/ the iommu it is attached
to, so this shouldn't be a problem

>
>> > There's also the issue of buffer stride alignment. As I say, if the
>> > buffer is to be written by a tile-based GPU like Mali, it's more
>> > efficient if the buffer's stride is aligned to the max AXI bus burst
>> > length. Though I guess a buffer stride only makes sense as a concept
>> > when interpreting the data as a linear-layout 2D image, so perhaps
>> > belongs in user-space along with format negotiation?
>> >
>>
>> Yeah.. this isn't about where the pages go, but about the arrangement
>> within a page.
>>
>> And, well, except for hw that supports the same tiling (or
>> compressed-fb) in display+gpu, you probably aren't sharing tiled
>> buffers.
>
> You'd only want to share a buffer between devices if those devices can
> understand the same pixel format. That pixel format can't be device-
> specific or opaque, it has to be explicit. I think drm_fourcc.h is
> what defines all the possible pixel formats. This is the enum I used
> in EGL_EXT_image_dma_buf_import at least. So if we get to the point
> where multiple devices can understand a tiled or compressed format, I
> assume we could just add that format to drm_fourcc.h and possibly
> v4l2's v4l2_mbus_pixelcode enum in v4l2-mediabus.h.
>
> For user-space to negotiate a common pixel format and now stride
> alignment, I guess it will obviously need a way to query what pixel
> formats a device supports and what its stride alignment requirements
> are.
>
> I don't know v4l2 very well, but it certainly seems the pixel format
> can be queried using V4L2_SUBDEV_FORMAT_TRY when attempting to set
> a particular format. I couldn't however find a way to retrieve a list
> of supported formats - it seems the mechanism is to try out each
> format in turn to determine if it is supported. Is that right?

it is exposed for drm plane's.  What is missing is to expose the
primary-plane associated with the crtc.

> There doesn't however seem a way to query what stride constraints a
> V4l2 device might have. Does HW abstracted by v4l2 typically have
> such constraints? If so, how can we query them such that a buffer
> allocated by a DRM driver can be imported into v4l2 and used with
> that HW?
>
> Turning to DRM/KMS, it seems the supported formats of a plane can be
> queried using drm_mode_get_plane. However, there doesn't seem to be a
> way to query the supported formats of a crtc? If display HW only
> supports scanning out from a single buffer (like pl111 does), I think
> it won't have any planes and a fb can only be set on the crtc. In
> which case, how should user-space query which pixel formats that crtc
> supports?
>
> Assuming user-space can query the supported formats and find a common
> one, it will need to allocate a buffer. Looks like
> drm_mode_create_dumb can do that, but it only takes a bpp parameter,
> there's no format parameter. I assume then that user-space defines
> the format and tells the DRM driver which format the buffer is in
> when creating the fb with drm_mode_fb_cmd2, which does take a format
> parameter? Is that right?

Right, the gem object has no inherent format, it is just some bytes.
The format/width/height/pitch are all attributes of the fb.

> As with v4l2, DRM doesn't appear to have a way to query the stride
> constraints? Assuming there is a way to query the stride constraints,
> there also isn't a way to specify them when creating a buffer with
> DRM, though perhaps the existing pitch parameter of
> drm_mode_create_dumb could be used to allow user-space to pass in a
> minimum stride as well as receive the allocated stride?
>

well, you really shouldn't be using create_dumb..  you should have a
userspace piece that is specific to the drm driver, and knows how to
use that driver's gem allocate ioctl.

>
>> >> > One problem with this is it duplicates a lot of logic in each
>> >> > driver which can export a dma_buf buffer. Each exporter will
>> >> > need to do pretty much the same thing: iterate over all the
>> >> > attachments, determine of all the constraints (assuming that
>> >> > can be done) and allocate pages such that the lowest-common-
>> >> > denominator is satisfied. Perhaps rather than duplicating that
>> >> > logic in every driver, we could instead move allocation of the
>> >> > backing pages into dma_buf itself?
>> >>
>> >> I tend to think it is better to add helpers as we see common
>>
>> >> patterns emerge, which drivers can opt-in to using.  I don't
>> >> think that we should move allocation into dma_buf itself, but
>> >> it would perhaps be useful to have dma_alloc_*() variants that
>> >> could allocate for multiple devices.
>> >
>> > A helper could work I guess, though I quite like the idea of
>> > having dma_alloc_*() variants which take a list of devices to
>> > allocate memory for.
>> >
>> >
>> >> That would help for simple stuff, although I'd suspect
>> >> eventually a GPU driver will move away from that.  (Since you
>> >> probably want to play tricks w/ pools of pages that are
>> >> pre-zero'd and in the correct cache state, use spare cycles on
>> >> the gpu or dma engine to pre-zero uncached pages, and games
>> >> like that.)
>> >
>> > So presumably you're talking about a GPU driver being the exporter
>> > here? If so, how could the GPU driver do these kind of tricks on
>> > memory shared with another device?
>>
>> Yes, that is gpu-as-exporter.  If someone else is allocating buffers,
>> it is up to them to do these tricks or not.  Probably there is a
>> pretty good chance that if you aren't a GPU you don't need those sort
>> of tricks for fast allocation of transient upload buffers, staging
>> textures, temporary pixmaps, etc.  Ie. I don't really think a v4l
>> camera or video decoder would benefit from that sort of optimization.
>
> Right - but none of those are really buffers you'd want to export with
> dma_buf to share with another device are they? In which case, why not
> just have dma_buf figure out the constraints and allocate the memory?

maybe not.. but (a) you don't necessarily know at creation time if it
is going to be exported (maybe you know if it is definitely not going
to be exported, but the converse is not true), and (b) there isn't
really any reason to special case the allocation in the driver because
it is going to be exported.

helpers that can be used by simple drivers, yes.  Forcing the way the
buffer is allocated, for sure not.  Currently, for example, there is
no issue to export a buffer allocated from stolen-mem.  If we put the
page allocation in dma-buf, this would not be possible.  That is just
one quick example off the top of my head, I'm sure there are plenty
more.  But we definitely do not want the allocate in dma_buf itself.

> If a driver needs to allocate memory in a special way for a particular
> device, I can't really imagine how it would be able to share that
> buffer with another device using dma_buf? I guess a driver is likely
> to need some magic voodoo to configure access to the buffer for its
> device, but surely that would be done by the dma_mapping framework
> when dma_buf_map happens?
>

if, what it has to configure actually manages to fit in the
dma-mapping framework

anyways, where the pages come from has nothing to do with whether a
buffer can be shared or not

>
>
>> >> You probably want to get out of the SoC mindset, otherwise you are
>> >> going to make bad assumptions that come back to bite you later on.
>> >
>> > Sure - there are always going to be PC-like devices where the
>> > hardware configuration isn't fixed like it is on a traditional SoC.
>> > But I'd rather have a simple solution which works on traditional SoCs
>> > than no solution at all. Today our solution is to over-load the dumb
>> > buffer alloc functions of the display's DRM driver - For now I'm just
>> > looking for the next step up from that! ;-)
>>
>> True.. the original intention, which is perhaps a bit desktop-centric,
>> really was for there to be a userspace component talking to the drm
>> driver for allocation, ie. xf86-video-foo and/or
>> src/gallium/drivers/foo (for example) ;-)
>>
>> Which means for x11 having a SoC vendor specific xf86-video-foo for
>> x11..  or vendor specific gbm implementation for wayland.  (Although
>> at least in the latter case it is a pretty small piece of code.)  But
>> that is probably what you are trying to avoid.
>
> I've been trying to get my head around how PRIME relates to DDX
> drivers. As I understand it (which is likely wrong), you have a laptop
> with both an Intel & an nVidia GPU. You have both the i915 & nouveau
> kernel drivers loaded. What I'm not sure about is which GPU's display
> controller is actually hooked up to the physical connector? Perhaps
> there is a MUX like there is on Versatile Express?

afaiu it can be a, b, or c (ie. either gpu can have the display or
there can be a mux)..

> What I also don't understand is what DDX driver is loaded? Is it
> xf86-video-intel, xf86-video-nouveau or both? I get the impression
> that there's a "master" DDX which implements 2D operations but can
> import buffers using PRIME from the other driver and draw to them.
> Or is it more that it's able to export rendered buffers to the
> "slave" DRM for scanout? Either way, it's pretty similar to an ARM
> SoC setup which has the GPU and the display as two totally
> independent devices.
>
>
>
>> At any rate, for both xorg and wayland/gbm, you know when a buffer is
>> going to be a scanout buffer.  What I'd recommend is define a small
>> userspace API that your customers (the SoC vendors) implement to
>> allocate a scanout buffer and hand you back a dmabuf fd.  That could
>> be used both for x11 and for gbm.  Inputs should be requested
>> width/height and format.  And outputs pitch plus dmabuf fd.
>>
>> (Actually you might even just want to use gbm as your starting point.
>> You could probably just use gbm from xf86-video-armsoc for allocation,
>> to have one thing that works for both wayland and x11.  Scanout and
>> cursor buffers should go to vendor/SoC specific fxn, rest can be
>> allocated from mali kernel driver.)
>
> What does that buy us over just using drm_mode_create_dumb on the
> display's DRM driver?
>

well, for example, if there was actually some hw w/ omap's dss + mali,
you could actually have mali render transparently to tiled buffers
which could be scanned out rotated.  Which would not be possible w/
dumb buffers.

>
>
>> >> > wouldn't need a way to programmatically describe the constraints
>> >> > either: As you say, if userspace sets the "SCANOUT" flag, it would
>> >> > just "know" that on this SoC, that buffer needs to be physically
>> >> > contiguous for example.
>> >>
>> >> not really.. it just knows it wants to scanout the buffer, and tells
>> >> this as a hint to the kernel.
>> >>
>> >> For example, on omapdrm, the SCANOUT flag does nothing on omap4+
>> >> (where phys contig is not required for scanout), but causes CMA
>> >> (dma_alloc_*()) to be used on omap3.  Userspace doesn't care.
>> >> It just knows that it wants to be able to scanout that particular
>> >> buffer.
>> >
>> > I think that's the idea? The omap3's allocator driver would use
>> > contiguous memory when it detects the SCANOUT flag whereas the omap4
>> > allocator driver wouldn't have to. No complex negotiation of
>> > constraints - it just "knows".
>> >
>>
>> well, it is same allocating driver in both cases (although maybe that
>> is unimportant).  The "it" that just knows it wants to scanout is
>> userspace.  The "it" that just knows that scanout translates to
>> contiguous (or not) is the kernel.  Perhaps we are saying the same
>> thing ;-)
>
> Yeah - I think we are... so what's the issue with having a per-SoC
> allocation driver again?
>

In a way the display driver is a per-SoC allocator.  But not
necessarily the *central* allocator for everything.  Ie. no need for
display driver to allocate vertex buffers for a separate gpu driver,
and that sort of thing.

BR,
-R

>
>
> Cheers,
>
> Tom
>
>
>
>
>

Re: [RFC 0/1] drm/pl111: Initial drm/kms driver for pl111

[Rob Clark]

On Fri, Aug 9, 2013 at 12:15 PM, Tom Cooksey <tom.cooksey@arm.com> wrote:
>
>> > Turning to DRM/KMS, it seems the supported formats of a plane can be
>> > queried using drm_mode_get_plane. However, there doesn't seem to be a
>> > way to query the supported formats of a crtc? If display HW only
>> > supports scanning out from a single buffer (like pl111 does), I think
>> > it won't have any planes and a fb can only be set on the crtc. In
>> > which case, how should user-space query which pixel formats that crtc
>> > supports?
>>
>> it is exposed for drm plane's.  What is missing is to expose the
>> primary-plane associated with the crtc.
>
> Cool - so a patch which adds a way to query the what formats a crtc
> supports would be welcome?

well, I kinda think we want something that exposes the "primary plane"
of the crtc.. I'm thinking something roughly like:

---------
diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
index 53db7ce..c7ffca8 100644
--- a/include/uapi/drm/drm_mode.h
+++ b/include/uapi/drm/drm_mode.h
@@ -157,6 +157,12 @@ struct drm_mode_get_plane {
 struct drm_mode_get_plane_res {
 	__u64 plane_id_ptr;
 	__u32 count_planes;
+	/* The primary planes are in matching order to crtc_id_ptr in
+	 * drm_mode_card_res (and same length).  For crtc_id[n], it's
+	 * primary plane is given by primary_plane_id[n].
+	 */
+	__u32 count_primary_planes;
+	__u64 primary_plane_id_ptr;
 };

 #define DRM_MODE_ENCODER_NONE	0
---------

then use the existing GETPLANE ioctl to query the capabilities

> What about a way to query the stride alignment constraints?
>
> Presumably using the drm_mode_get_property mechanism would be the
> right way to implement that?
>

I suppose you could try that.. typically that is in userspace,
however.  It seems like get_property would get messy quickly (ie. is
it a pitch alignment constraint, or stride alignment?  What if this is
different for different formats (in particular tiled)? etc)

>
>> > As with v4l2, DRM doesn't appear to have a way to query the stride
>> > constraints? Assuming there is a way to query the stride constraints,
>> > there also isn't a way to specify them when creating a buffer with
>> > DRM, though perhaps the existing pitch parameter of
>> > drm_mode_create_dumb could be used to allow user-space to pass in a
>> > minimum stride as well as receive the allocated stride?
>> >
>>
>> well, you really shouldn't be using create_dumb..  you should have a
>> userspace piece that is specific to the drm driver, and knows how to
>> use that driver's gem allocate ioctl.
>
> Sorry, why does this need a driver-specific allocation function? It's
> just a display controller driver and I just want to allocate a scan-
> out buffer - all I'm asking is for the display controller driver to
> use a minimum stride alignment so I can export the buffer and use
> another device to fill it with data.

Sure.. but userspace has more information readily available to make a
better choice.  For example, for omapdrm I'd do things differently
depending on whether I wanted to scan out that buffer (or a portion of
it) rotated.  This is something I know in the DDX driver, but not in
the kernel.  And it is quite likely something that is driver specific.
 Sure, we could add that to a generic "allocate me a buffer" ioctl.
But that doesn't really seem better, and it becomes a problem as soon
as we come across some hw that needs to know something different.  In
userspace, you have a lot more flexibility, since you don't really
need to commit to an API for life.

And to bring back the GStreamer argument (since that seems a fitting
example when you start talking about sharing buffers between many
devices, for example camera+codec+display), it would already be
negotiating format between v4l2src + fooencoder + displaysink.. the
pitch/stride is part of that format information.  If the display isn't
the one with the strictest requirements, we don't want the display
driver deciding what pitch to use.

> The whole point is to be able to allocate the buffer in such a way
> that another device can access it. So the driver _can't_ use a
> special, device specific format, nor can it allocate it from a
> private memory pool because doing so would preclude it from being
> shared with another device.
>
> That other device doesn't need to be a GPU wither, it could just as
> easily be a camera/ISP or video decoder.
>
>
>
>> >> > So presumably you're talking about a GPU driver being the exporter
>> >> > here? If so, how could the GPU driver do these kind of tricks on
>> >> > memory shared with another device?
>> >>
>> >> Yes, that is gpu-as-exporter.  If someone else is allocating
>> >> buffers, it is up to them to do these tricks or not.  Probably
>> >> there is a pretty good chance that if you aren't a GPU you don't
>> >> need those sort of tricks for fast allocation of transient upload
>> >> buffers, staging textures, temporary pixmaps, etc.  Ie. I don't
>> >> really think a v4l camera or video decoder would benefit from that
>> >> sort of optimization.
>> >
>> > Right - but none of those are really buffers you'd want to export
>>
>> > with dma_buf to share with another device are they? In which case,
>> > why not just have dma_buf figure out the constraints and allocate
>> > the memory?
>>
>> maybe not.. but (a) you don't necessarily know at creation time if it
>> is going to be exported (maybe you know if it is definitely not going
>> to be exported, but the converse is not true),
>
> I can't actually think of an example where you would not know if a
> buffer was going to be exported or not at allocation time? Do you have
> a case in mind?

yeah, dri2.. when the front buffer is allocated it is just a regular
pixmap.  If you swap/flip it becomes the back buffer and now shared
;-)
And pixmaps are allocated w/ enough frequency that it is the sort of
thing you might want to optimize.

And even when you know it will be shared, you don't know with who.

> Regardless, you'd certainly have to know if a buffer will be exported
> pretty quickly, before it's used so that you can import it into
> whatever devices are going to access it. Otherwise if it gets
> allocated before you export it, the allocation won't satisfy the
> constraints of the other devices which will need to access it and
> importing will fail. Assuming of course deferred allocation of the
> backing pages as discussed earlier in the thread.
>
>
>
>> and (b) there isn't
>> really any reason to special case the allocation in the driver because
>> it is going to be exported.
>
> Not sure I follow you here? Surely you absolutely have to special-case
> the allocation if the buffer is to be exported because you have to
> take the other devices' constraints into account when you allocate? Or
> do you mean you don't need to special-case the GEM buffer object
> creation, only the allocation of the backing pages? Though I'm not
> sure how that distinction is useful - at the end of the day, you need
> to special-case allocation of the backing pages.
>

well, you need to consider separately what is (a) in the pages, and
(b) where the pages come from.

By moving the allocation into dmabuf you restrict (b).  For sharing
buffers, (a) may be restricted, but there is at least some examples of
hardware where (b) would not otherwise be restricted by sharing.

>
>> helpers that can be used by simple drivers, yes.  Forcing the way the
>> buffer is allocated, for sure not.  Currently, for example, there is
>> no issue to export a buffer allocated from stolen-mem.
>
> Where stolen-mem is the PC-world's version of a carveout? I.e. A chunk
> of memory reserved at boot for the GPU which the OS can't touch? I
> guess I view such memory as accessible to all media devices on the
> system and as such, needs to be managed by a central allocator which
> dma_buf can use to allocate from.

think carve-out created by bios.  In all the cases I am aware of, the
drm driver handles allocation of buffer(s) from the carveout.

> I guess if that stolen-mem is managed by a single device then in
> essence that device becomes the central allocator you have to use to
> be able to allocate from that stolen mem?
>
>
>> > If a driver needs to allocate memory in a special way for a
>> > particular device, I can't really imagine how it would be able
>> > to share that buffer with another device using dma_buf? I guess
>> > a driver is likely to need some magic voodoo to configure access
>> > to the buffer for its device, but surely that would be done by
>> > the dma_mapping framework when dma_buf_map happens?
>> >
>>
>> if, what it has to configure actually manages to fit in the
>> dma-mapping framework
>
> But if it doesn't, surely that's an issue which needs to be addressed
> in the dma_mapping framework or else you won't be able to import
> buffers for use by that device anyway?
>

I'm not sure if we have to fit everything in dma-mapping framework, at
least in cases where you have something that is specific to one
platform.

Currently dma-buf provides enough flexibility for other drivers to be
able to import these buffers.

>
>> anyways, where the pages come from has nothing to do with whether a
>> buffer can be shared or not
>
> Sure, but where they are located in physical memory really does
> matter.
>

s/does/can/

it doesn't always matter.  And in cases where it does matter, as long
as we can express the restrictions in dma_parms (which we can already
for the case of range-of-memory restrictions) we are covered

>
>> >> At any rate, for both xorg and wayland/gbm, you know when a buffer
>> >> is going to be a scanout buffer.  What I'd recommend is define a
>> >> small userspace API that your customers (the SoC vendors) implement
>> >> to allocate a scanout buffer and hand you back a dmabuf fd.  That
>> >> could be used both for x11 and for gbm.  Inputs should be requested
>> >> width/height and format.  And outputs pitch plus dmabuf fd.
>> >>
>> >> (Actually you might even just want to use gbm as your starting
>> >> point. You could probably just use gbm from xf86-video-armsoc for
>> >> allocation, to have one thing that works for both wayland and x11.
>> >> Scanout and cursor buffers should go to vendor/SoC specific fxn,
>> >> rest can be allocated from mali kernel driver.)
>> >
>> > What does that buy us over just using drm_mode_create_dumb on the
>> > display's DRM driver?
>>
>> well, for example, if there was actually some hw w/ omap's dss + mali,
>> you could actually have mali render transparently to tiled buffers
>> which could be scanned out rotated.  Which would not be possible w/
>> dumb buffers.
>
> Why not? As you said earlier, the format is defined when you setup the
> fb with drm_mode_fb_cmd2. If you wanted to share the buffer between
> devices, you have to be explicit about what format that buffer is in,
> so you'd have to add an entry to drm_fourcc.h for the tiled format.

no, that doesn't really work

in this case, the format to any device (or userspace) accessing the
buffer is not tiled.  (Ie. it would look like normal NV12 or
whatever).  But there are some different requirements on stride.  And
there are cases where you would prefer not to use tiled buffers, but
the kernel doesn't know enough in the dumb-buffer alloc ioctl to make
the correct decision.

> So userspace queries what formats the GPU DRM supports and what
> formats the OMAP DSS DRM supports, selects the tiled format and then
> uses drm_mode_create_dumb to allocate a buffer of the correct size and
> sets the appropriate drm_fourcc.h enum value when creating an fb for
> that buffer. Or have I missed something?
>
>
>
>> >> >> For example, on omapdrm, the SCANOUT flag does nothing on omap4+
>> >> >> (where phys contig is not required for scanout), but causes CMA
>> >> >> (dma_alloc_*()) to be used on omap3.  Userspace doesn't care.
>> >> >> It just knows that it wants to be able to scanout that particular
>> >> >> buffer.
>> >> >
>> >> > I think that's the idea? The omap3's allocator driver would use
>> >> > contiguous memory when it detects the SCANOUT flag whereas the
>> >> > omap4 allocator driver wouldn't have to. No complex negotiation
>> >> > of constraints - it just "knows".
>> >> >
>> >>
>> >> well, it is same allocating driver in both cases (although maybe
>> >> that is unimportant).  The "it" that just knows it wants to scanout
>> >> is userspace.  The "it" that just knows that scanout translates to
>> >> contiguous (or not) is the kernel.  Perhaps we are saying the same
>> >> thing ;-)
>> >
>> > Yeah - I think we are... so what's the issue with having a per-SoC
>> > allocation driver again?
>> >
>>
>> In a way the display driver is a per-SoC allocator.  But not
>> necessarily the *central* allocator for everything.  Ie. no need for
>> display driver to allocate vertex buffers for a separate gpu driver,
>> and that sort of thing.
>
> Again, I'm only talking about allocating buffers which will be shared
> between different devices. At no point have I mentioned the allocation
> of buffers which aren't to be shared between devices. Sorry if that's
> not been clear.

ok, I guess we were talking about slightly different things ;-)

> So for buffers which are to be shared between devices, your suggesting
> that the display driver is the per-SoC allocator? But as I say, and
> how this thread got started, the same display driver can be used on
> different SoCs, so having _it_ be the central allocator isn't ideal.
> Though this is our current solution and why we're "abusing" the dumb
> buffer allocation functions. :-)
>

which is why you want to let userspace figure out the pitch and then
tell the display driver what size it wants, rather than using dumb
buffer ioctl ;-)

Ok, you could have a generic TELL_ME_WHAT_STRIDE_TO_USE ioctl or
property or what have you.. but I think that would be hard to get
right for all cases, and most people don't really care about that
because they already need a gpu/display specific xorg driver and/or
gl/egl talking to their kernel driver.  You are in a slightly special
case, since you are providing GL driver independently of the display
driver.  But I think that is easier to handle by just telling your
customers "here, fill out this function(s) to allocate buffer for
scanout" (and, well, I guess you'd need one to query for
pitch/stride), rather than trying to cram everything into the kernel.

BR,
-R

>
>
> Cheers,
>
> Tom
>
>
>
>
>

Re: [RFC 0/1] drm/pl111: Initial drm/kms driver for pl111

[John Stultz]

On Tue, Aug 6, 2013 at 5:15 AM, Rob Clark <robdclark@gmail.com> wrote:
> On Tue, Aug 6, 2013 at 7:31 AM, Tom Cooksey <tom.cooksey@arm.com> wrote:
>>
>>> > So in some respects, there is a constraint on how buffers which will
>>> > be drawn to using the GPU are allocated. I don't really like the idea
>>> > of teaching the display controller DRM driver about the GPU buffer
>>> > constraints, even if they are fairly trivial like this. If the same
>>> > display HW IP is being used on several SoCs, it seems wrong somehow
>>> > to enforce those GPU constraints if some of those SoCs don't have a
>>> > GPU.
>>>
>>> Well, I suppose you could get min_pitch_alignment from devicetree, or
>>> something like this..
>>>
>>> In the end, the easy solution is just to make the display allocate to
>>> the worst-case pitch alignment.  In the early days of dma-buf
>>> discussions, we kicked around the idea of negotiating or
>>> programatically describing the constraints, but that didn't really
>>> seem like a bounded problem.
>>
>> Yeah - I was around for some of those discussions and agree it's not
>> really an easy problem to solve.
>>
>>
>>
>>> > We may also then have additional constraints when sharing buffers
>>> > between the display HW and video decode or even camera ISP HW.
>>> > Programmatically describing buffer allocation constraints is very
>>> > difficult and I'm not sure you can actually do it - there's some
>>> > pretty complex constraints out there! E.g. I believe there's a
>>> > platform where Y and UV planes of the reference frame need to be in
>>> > separate DRAM banks for real-time 1080p decode, or something like
>>> > that?
>>>
>>> yes, this was discussed.  This is different from pitch/format/size
>>> constraints.. it is really just a placement constraint (ie. where do
>>> the physical pages go).  IIRC the conclusion was to use a dummy
>>> devices with it's own CMA pool for attaching the Y vs UV buffers.
>>>
>>> > Anyway, I guess my point is that even if we solve how to allocate
>>> > buffers which will be shared between the GPU and display HW such that
>>> > both sets of constraints are satisfied, that may not be the end of
>>> > the story.
>>> >
>>>
>>> that was part of the reason to punt this problem to userspace ;-)
>>>
>>> In practice, the kernel drivers doesn't usually know too much about
>>> the dimensions/format/etc.. that is really userspace level knowledge.
>>> There are a few exceptions when the kernel needs to know how to setup
>>> GTT/etc for tiled buffers, but normally this sort of information is up
>>> at the next level up (userspace, and drm_framebuffer in case of
>>> scanout).  Userspace media frameworks like GStreamer already have a
>>> concept of format/caps negotiation.  For non-display<->gpu sharing, I
>>> think this is probably where this sort of constraint negotiation
>>> should be handled.
>>
>> I agree that user-space will know which devices will access the buffer
>> and thus can figure out at least a common pixel format. Though I'm not
>> so sure userspace can figure out more low-level details like alignment
>> and placement in physical memory, etc.
>
> well, let's divide things up into two categories:
>
> 1) the arrangement and format of pixels.. ie. what userspace would
> need to know if it mmap's a buffer.  This includes pixel format,
> stride, etc.  This should be negotiated in userspace, it would be
> crazy to try to do this in the kernel.
>
> 2) the physical placement of the pages.  Ie. whether it is contiguous
> or not.  Which bank the pages in the buffer are placed in, etc.  This
> is not visible to userspace.  This is the purpose of the attach step,
> so you know all the devices involved in sharing up front before
> allocating the backing pages.  (Or in the worst case, if you have a
> "late attacher" you at least know when no device is doing dma access
> to a buffer and can reallocate and move the buffer.)  A long time

One concern I know the Android folks have expressed previously (and
correct me if its no longer an objection), is that this attach time
in-kernel constraint solving / moving or reallocating buffers is
likely to hurt determinism.  If I understood, their perspective was
that userland knows the device path the buffers will travel through,
so why not leverage that knowledge, rather then having the kernel have
to sort it out for itself after the fact.

The concern about determinism even makes them hesitant about CMA, over
things like carveout, as they don't want to be moving pages around at
allocation time (which could hurt reasonable use cases like the time
it takes to launch a camera app - which is quite important). Though
maybe this concern will lessen as more CMA solutions ship.

I worry some of this split between fully general solutions vs
hard-coded known constraints is somewhat intractable. But what might
make it easier to get android folks interested in approaches like the
attach-time allocation / relocating on late-attach you're proposing is
if there is maybe some way, as you suggested, to hint the allocation
when they do know the device paths, so they can provide that insight
and can avoid *all* reallocations.

Then of course, there's the question of how to consistently hint
things for all the different driver allocation interfaces (and that,
maybe naively, leads to the central allocator approach).

thanks
-john

Re: [RFC 0/1] drm/pl111: Initial drm/kms driver for pl111

[Rob Clark]

On Wed, Aug 7, 2013 at 12:23 AM, John Stultz <john.stultz@linaro.org> wrote:
> On Tue, Aug 6, 2013 at 5:15 AM, Rob Clark <robdclark@gmail.com> wrote:
>> well, let's divide things up into two categories:
>>
>> 1) the arrangement and format of pixels.. ie. what userspace would
>> need to know if it mmap's a buffer.  This includes pixel format,
>> stride, etc.  This should be negotiated in userspace, it would be
>> crazy to try to do this in the kernel.
>>
>> 2) the physical placement of the pages.  Ie. whether it is contiguous
>> or not.  Which bank the pages in the buffer are placed in, etc.  This
>> is not visible to userspace.  This is the purpose of the attach step,
>> so you know all the devices involved in sharing up front before
>> allocating the backing pages.  (Or in the worst case, if you have a
>> "late attacher" you at least know when no device is doing dma access
>> to a buffer and can reallocate and move the buffer.)  A long time
>
> One concern I know the Android folks have expressed previously (and
> correct me if its no longer an objection), is that this attach time
> in-kernel constraint solving / moving or reallocating buffers is
> likely to hurt determinism.  If I understood, their perspective was
> that userland knows the device path the buffers will travel through,
> so why not leverage that knowledge, rather then having the kernel have
> to sort it out for itself after the fact.

If you know the device path, then attach the buffer at all the devices
before you start using it.  Problem solved.. kernel knows all devices
before pages need be allocated ;-)

BR,
-R

Re: [Linaro-mm-sig] [RFC 0/1] drm/pl111: Initial drm/kms driver for pl111

[Lucas Stach]

Am Dienstag, den 06.08.2013, 12:31 +0100 schrieb Tom Cooksey:
> Hi Rob,
> 
> +lkml
> 
> > >> On Fri, Jul 26, 2013 at 11:58 AM, Tom Cooksey <tom.cooksey@arm.com>
> > >> wrote:
> > >> >> >  * It abuses flags parameter of DRM_IOCTL_MODE_CREATE_DUMB to
> > >> >> >    also allocate buffers for the GPU. Still not sure how to 
> > >> >> >    resolve this as we don't use DRM for our GPU driver.
> > >> >>
> > >> >> any thoughts/plans about a DRM GPU driver?  Ideally long term
> > >> >> (esp. once the dma-fence stuff is in place), we'd have 
> > >> >> gpu-specific drm (gpu-only, no kms) driver, and SoC/display
> > >> >> specific drm/kms driver, using prime/dmabuf to share between
> > >> >> the two.
> > >> >
> > >> > The "extra" buffers we were allocating from armsoc DDX were really
> > >> > being allocated through DRM/GEM so we could get an flink name
> > >> > for them and pass a reference to them back to our GPU driver on
> > >> > the client side. If it weren't for our need to access those
> > >> > extra off-screen buffers with the GPU we wouldn't need to
> > >> > allocate them with DRM at all. So, given they are really "GPU"
> > >> > buffers, it does absolutely make sense to allocate them in a
> > >> > different driver to the display driver.
> > >> >
> > >> > However, to avoid unnecessary memcpys & related cache
> > >> > maintenance ops, we'd also like the GPU to render into buffers
> > >> > which are scanned out by the display controller. So let's say
> > >> > we continue using DRM_IOCTL_MODE_CREATE_DUMB to allocate scan
> > >> > out buffers with the display's DRM driver but a custom ioctl
> > >> > on the GPU's DRM driver to allocate non scanout, off-screen
> > >> > buffers. Sounds great, but I don't think that really works
> > >> > with DRI2. If we used two drivers to allocate buffers, which
> > >> > of those drivers do we return in DRI2ConnectReply? Even if we
> > >> > solve that somehow, GEM flink names are name-spaced to a
> > >> > single device node (AFAIK). So when we do a DRI2GetBuffers,
> > >> > how does the EGL in the client know which DRM device owns GEM
> > >> > flink name "1234"? We'd need some pretty dirty hacks.
> > >>
> > >> You would return the name of the display driver allocating the
> > >> buffers.  On the client side you can use generic ioctls to go from
> > >> flink -> handle -> dmabuf.  So the client side would end up opening
> > >> both the display drm device and the gpu, but without needing to know
> > >> too much about the display.
> > >
> > > I think the bit I was missing was that a GEM bo for a buffer imported
> > > using dma_buf/PRIME can still be flink'd. So the display controller's
> > > DRM driver allocates scan-out buffers via the DUMB buffer allocate
> > > ioctl. Those scan-out buffers than then be exported from the
> > > dispaly's DRM driver and imported into the GPU's DRM driver using
> > > PRIME. Once imported into the GPU's driver, we can use flink to get a
> > > name for that buffer within the GPU DRM driver's name-space to return
> > > to the DRI2 client. That same namespace is also what DRI2 back-
> > > buffers are allocated from, so I think that could work... Except...
> > 
> > (and.. the general direction is that things will move more to just use
> > dmabuf directly, ie. wayland or dri3)
> 
> I agree, DRI2 is the only reason why we need a system-wide ID. I also
> prefer buffers to be passed around by dma_buf fd, but we still need to
> support DRI2 and will do for some time I expect.
> 
> 
> 
> > >> > Anyway, that latter case also gets quite difficult. The "GPU"
> > >> > DRM driver would need to know the constraints of the display
> > >> > controller when allocating buffers intended to be scanned out.
> > >> > For example, pl111 typically isn't behind an IOMMU and so
> > >> > requires physically contiguous memory. We'd have to teach the
> > >> > GPU's DRM driver about the constraints of the display HW. Not
> > >> > exactly a clean driver model. :-(
> > >> >
> > >> > I'm still a little stuck on how to proceed, so any ideas
> > >> > would greatly appreciated! My current train of thought is
> > >> > having a kind of SoC-specific DRM driver which allocates
> > >> > buffers for both display and GPU within a single GEM
> > >> > namespace. That SoC-specific DRM driver could then know the
> > >> > constraints of both the GPU and the display HW. We could then
> > >> > use PRIME to export buffers allocated with the SoC DRM driver
> > >> > and import them into the GPU and/or display DRM driver.
> > >>
> > >> Usually if the display drm driver is allocating the buffers that
> > >> might be scanned out, it just needs to have minimal knowledge of 
> > >> the GPU (pitch alignment constraints).  I don't think we need a 
> > >> 3rd device just to allocate buffers.
> > >
> > > While Mali can render to pretty much any buffer, there is a mild
> > > performance improvement to be had if the buffer stride is aligned to
> > > the AXI bus's max burst length when drawing to the buffer.
> > 
> > I suspect the display controllers might frequently benefit if the
> > pitch is aligned to AXI burst length too..
> 
> If the display controller is going to be reading from linear memory
> I don't think it will make much difference - you'll just get an extra
> 1-2 bus transactions per scanline. With a tile-based GPU like Mali,
> you get those extra transactions per _tile_ scan-line and as such,
> the overhead is more pronounced.
> 
> 
> 
> > > So in some respects, there is a constraint on how buffers which will
> > > be drawn to using the GPU are allocated. I don't really like the idea
> > > of teaching the display controller DRM driver about the GPU buffer
> > > constraints, even if they are fairly trivial like this. If the same
> > > display HW IP is being used on several SoCs, it seems wrong somehow
> > > to enforce those GPU constraints if some of those SoCs don't have a
> > > GPU.
> > 
> > Well, I suppose you could get min_pitch_alignment from devicetree, or
> > something like this..
> > 
> > In the end, the easy solution is just to make the display allocate to
> > the worst-case pitch alignment.  In the early days of dma-buf
> > discussions, we kicked around the idea of negotiating or
> > programatically describing the constraints, but that didn't really
> > seem like a bounded problem.
> 
> Yeah - I was around for some of those discussions and agree it's not
> really an easy problem to solve.
> 
> 
> 
> > > We may also then have additional constraints when sharing buffers
> > > between the display HW and video decode or even camera ISP HW.
> > > Programmatically describing buffer allocation constraints is very
> > > difficult and I'm not sure you can actually do it - there's some
> > > pretty complex constraints out there! E.g. I believe there's a
> > > platform where Y and UV planes of the reference frame need to be in
> > > separate DRAM banks for real-time 1080p decode, or something like
> > > that?
> > 
> > yes, this was discussed.  This is different from pitch/format/size
> > constraints.. it is really just a placement constraint (ie. where do
> > the physical pages go).  IIRC the conclusion was to use a dummy
> > devices with it's own CMA pool for attaching the Y vs UV buffers.
> > 
> > > Anyway, I guess my point is that even if we solve how to allocate
> > > buffers which will be shared between the GPU and display HW such that
> > > both sets of constraints are satisfied, that may not be the end of
> > > the story.
> > >
> > 
> > that was part of the reason to punt this problem to userspace ;-)
> >
> > In practice, the kernel drivers doesn't usually know too much about
> > the dimensions/format/etc.. that is really userspace level knowledge.
> > There are a few exceptions when the kernel needs to know how to setup
> > GTT/etc for tiled buffers, but normally this sort of information is up
> > at the next level up (userspace, and drm_framebuffer in case of
> > scanout).  Userspace media frameworks like GStreamer already have a
> > concept of format/caps negotiation.  For non-display<->gpu sharing, I
> > think this is probably where this sort of constraint negotiation
> > should be handled.
> 
> I agree that user-space will know which devices will access the buffer
> and thus can figure out at least a common pixel format. Though I'm not
> so sure userspace can figure out more low-level details like alignment
> and placement in physical memory, etc.
> 
> Anyway, assuming user-space can figure out how a buffer should be 
> stored in memory, how does it indicate this to a kernel driver and 
> actually allocate it? Which ioctl on which device does user-space
> call, with what parameters? Are you suggesting using something like
> ION which exposes the low-level details of how buffers are laid out in
> physical memory to userspace? If not, what?
> 

I strongly disagree with exposing low-level hardware details like tiling
to userspace. If we have to do the negotiation of those things in
userspace we will end up with having to pipe those information through
things like the wayland protocol. I don't see how this could ever be
considered a good idea.

I would rather see kernel drivers negotiating those things at dmabuf
attach time in way invisible to userspace. I agree that this negotiation
thing isn't easy to get right for the plethora of different hardware
constraints we see today, but I would rather see this in-kernel, where
we have the chance to fix things up if needed, than in a fixed userspace
interface.

Regards,
Lucas
-- 
Pengutronix e.K.                           | Lucas Stach                 |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
Peiner Str. 6-8, 31137 Hildesheim, Germany | Phone: +49-5121-206917-5076 |
Amtsgericht Hildesheim, HRA 2686           | Fax:   +49-5121-206917-5555 |

Re: [Linaro-mm-sig] [RFC 0/1] drm/pl111: Initial drm/kms driver for pl111

[Rob Clark]

On Tue, Aug 6, 2013 at 8:18 AM, Lucas Stach <l.stach@pengutronix.de> wrote:
> Am Dienstag, den 06.08.2013, 12:31 +0100 schrieb Tom Cooksey:
>> Hi Rob,
>>
>> +lkml
>>
>> > >> On Fri, Jul 26, 2013 at 11:58 AM, Tom Cooksey <tom.cooksey@arm.com>
>> > >> wrote:
>> > >> >> >  * It abuses flags parameter of DRM_IOCTL_MODE_CREATE_DUMB to
>> > >> >> >    also allocate buffers for the GPU. Still not sure how to
>> > >> >> >    resolve this as we don't use DRM for our GPU driver.
>> > >> >>
>> > >> >> any thoughts/plans about a DRM GPU driver?  Ideally long term
>> > >> >> (esp. once the dma-fence stuff is in place), we'd have
>> > >> >> gpu-specific drm (gpu-only, no kms) driver, and SoC/display
>> > >> >> specific drm/kms driver, using prime/dmabuf to share between
>> > >> >> the two.
>> > >> >
>> > >> > The "extra" buffers we were allocating from armsoc DDX were really
>> > >> > being allocated through DRM/GEM so we could get an flink name
>> > >> > for them and pass a reference to them back to our GPU driver on
>> > >> > the client side. If it weren't for our need to access those
>> > >> > extra off-screen buffers with the GPU we wouldn't need to
>> > >> > allocate them with DRM at all. So, given they are really "GPU"
>> > >> > buffers, it does absolutely make sense to allocate them in a
>> > >> > different driver to the display driver.
>> > >> >
>> > >> > However, to avoid unnecessary memcpys & related cache
>> > >> > maintenance ops, we'd also like the GPU to render into buffers
>> > >> > which are scanned out by the display controller. So let's say
>> > >> > we continue using DRM_IOCTL_MODE_CREATE_DUMB to allocate scan
>> > >> > out buffers with the display's DRM driver but a custom ioctl
>> > >> > on the GPU's DRM driver to allocate non scanout, off-screen
>> > >> > buffers. Sounds great, but I don't think that really works
>> > >> > with DRI2. If we used two drivers to allocate buffers, which
>> > >> > of those drivers do we return in DRI2ConnectReply? Even if we
>> > >> > solve that somehow, GEM flink names are name-spaced to a
>> > >> > single device node (AFAIK). So when we do a DRI2GetBuffers,
>> > >> > how does the EGL in the client know which DRM device owns GEM
>> > >> > flink name "1234"? We'd need some pretty dirty hacks.
>> > >>
>> > >> You would return the name of the display driver allocating the
>> > >> buffers.  On the client side you can use generic ioctls to go from
>> > >> flink -> handle -> dmabuf.  So the client side would end up opening
>> > >> both the display drm device and the gpu, but without needing to know
>> > >> too much about the display.
>> > >
>> > > I think the bit I was missing was that a GEM bo for a buffer imported
>> > > using dma_buf/PRIME can still be flink'd. So the display controller's
>> > > DRM driver allocates scan-out buffers via the DUMB buffer allocate
>> > > ioctl. Those scan-out buffers than then be exported from the
>> > > dispaly's DRM driver and imported into the GPU's DRM driver using
>> > > PRIME. Once imported into the GPU's driver, we can use flink to get a
>> > > name for that buffer within the GPU DRM driver's name-space to return
>> > > to the DRI2 client. That same namespace is also what DRI2 back-
>> > > buffers are allocated from, so I think that could work... Except...
>> >
>> > (and.. the general direction is that things will move more to just use
>> > dmabuf directly, ie. wayland or dri3)
>>
>> I agree, DRI2 is the only reason why we need a system-wide ID. I also
>> prefer buffers to be passed around by dma_buf fd, but we still need to
>> support DRI2 and will do for some time I expect.
>>
>>
>>
>> > >> > Anyway, that latter case also gets quite difficult. The "GPU"
>> > >> > DRM driver would need to know the constraints of the display
>> > >> > controller when allocating buffers intended to be scanned out.
>> > >> > For example, pl111 typically isn't behind an IOMMU and so
>> > >> > requires physically contiguous memory. We'd have to teach the
>> > >> > GPU's DRM driver about the constraints of the display HW. Not
>> > >> > exactly a clean driver model. :-(
>> > >> >
>> > >> > I'm still a little stuck on how to proceed, so any ideas
>> > >> > would greatly appreciated! My current train of thought is
>> > >> > having a kind of SoC-specific DRM driver which allocates
>> > >> > buffers for both display and GPU within a single GEM
>> > >> > namespace. That SoC-specific DRM driver could then know the
>> > >> > constraints of both the GPU and the display HW. We could then
>> > >> > use PRIME to export buffers allocated with the SoC DRM driver
>> > >> > and import them into the GPU and/or display DRM driver.
>> > >>
>> > >> Usually if the display drm driver is allocating the buffers that
>> > >> might be scanned out, it just needs to have minimal knowledge of
>> > >> the GPU (pitch alignment constraints).  I don't think we need a
>> > >> 3rd device just to allocate buffers.
>> > >
>> > > While Mali can render to pretty much any buffer, there is a mild
>> > > performance improvement to be had if the buffer stride is aligned to
>> > > the AXI bus's max burst length when drawing to the buffer.
>> >
>> > I suspect the display controllers might frequently benefit if the
>> > pitch is aligned to AXI burst length too..
>>
>> If the display controller is going to be reading from linear memory
>> I don't think it will make much difference - you'll just get an extra
>> 1-2 bus transactions per scanline. With a tile-based GPU like Mali,
>> you get those extra transactions per _tile_ scan-line and as such,
>> the overhead is more pronounced.
>>
>>
>>
>> > > So in some respects, there is a constraint on how buffers which will
>> > > be drawn to using the GPU are allocated. I don't really like the idea
>> > > of teaching the display controller DRM driver about the GPU buffer
>> > > constraints, even if they are fairly trivial like this. If the same
>> > > display HW IP is being used on several SoCs, it seems wrong somehow
>> > > to enforce those GPU constraints if some of those SoCs don't have a
>> > > GPU.
>> >
>> > Well, I suppose you could get min_pitch_alignment from devicetree, or
>> > something like this..
>> >
>> > In the end, the easy solution is just to make the display allocate to
>> > the worst-case pitch alignment.  In the early days of dma-buf
>> > discussions, we kicked around the idea of negotiating or
>> > programatically describing the constraints, but that didn't really
>> > seem like a bounded problem.
>>
>> Yeah - I was around for some of those discussions and agree it's not
>> really an easy problem to solve.
>>
>>
>>
>> > > We may also then have additional constraints when sharing buffers
>> > > between the display HW and video decode or even camera ISP HW.
>> > > Programmatically describing buffer allocation constraints is very
>> > > difficult and I'm not sure you can actually do it - there's some
>> > > pretty complex constraints out there! E.g. I believe there's a
>> > > platform where Y and UV planes of the reference frame need to be in
>> > > separate DRAM banks for real-time 1080p decode, or something like
>> > > that?
>> >
>> > yes, this was discussed.  This is different from pitch/format/size
>> > constraints.. it is really just a placement constraint (ie. where do
>> > the physical pages go).  IIRC the conclusion was to use a dummy
>> > devices with it's own CMA pool for attaching the Y vs UV buffers.
>> >
>> > > Anyway, I guess my point is that even if we solve how to allocate
>> > > buffers which will be shared between the GPU and display HW such that
>> > > both sets of constraints are satisfied, that may not be the end of
>> > > the story.
>> > >
>> >
>> > that was part of the reason to punt this problem to userspace ;-)
>> >
>> > In practice, the kernel drivers doesn't usually know too much about
>> > the dimensions/format/etc.. that is really userspace level knowledge.
>> > There are a few exceptions when the kernel needs to know how to setup
>> > GTT/etc for tiled buffers, but normally this sort of information is up
>> > at the next level up (userspace, and drm_framebuffer in case of
>> > scanout).  Userspace media frameworks like GStreamer already have a
>> > concept of format/caps negotiation.  For non-display<->gpu sharing, I
>> > think this is probably where this sort of constraint negotiation
>> > should be handled.
>>
>> I agree that user-space will know which devices will access the buffer
>> and thus can figure out at least a common pixel format. Though I'm not
>> so sure userspace can figure out more low-level details like alignment
>> and placement in physical memory, etc.
>>
>> Anyway, assuming user-space can figure out how a buffer should be
>> stored in memory, how does it indicate this to a kernel driver and
>> actually allocate it? Which ioctl on which device does user-space
>> call, with what parameters? Are you suggesting using something like
>> ION which exposes the low-level details of how buffers are laid out in
>> physical memory to userspace? If not, what?
>>
>
> I strongly disagree with exposing low-level hardware details like tiling
> to userspace. If we have to do the negotiation of those things in
> userspace we will end up with having to pipe those information through
> things like the wayland protocol. I don't see how this could ever be
> considered a good idea.

well, unless userspace mmap's via a de-tiling gart type thing, I don't
think tiling can be invisible to userspace.

But if two GPU's have some overlap in supportable tiled formats, and
you have one gpu doing app and one doing compositor, and you want to
use tiling for the shared buffers, you need something in the wayland
protocol to figure out what the common supported formats are between
the two sides.  I suppose it shouldn't be too hard to add a
standardized (cross-driver) format-negotiation protocol, and in the
absence of that fallback to non tiled format for shared buffers.

> I would rather see kernel drivers negotiating those things at dmabuf
> attach time in way invisible to userspace. I agree that this negotiation
> thing isn't easy to get right for the plethora of different hardware
> constraints we see today, but I would rather see this in-kernel, where
> we have the chance to fix things up if needed, than in a fixed userspace
> interface.

Well, if you can think of a sane way to add that to dev->dma_params,
and if it isn't visible if userspace mmap's a buffer, then we could
handle that in the kernel.  But I don't think that will be the case.

BR,
-R

>
> Regards,
> Lucas
> --
> Pengutronix e.K.                           | Lucas Stach                 |
> Industrial Linux Solutions                 | http://www.pengutronix.de/  |
> Peiner Str. 6-8, 31137 Hildesheim, Germany | Phone: +49-5121-206917-5076 |
> Amtsgericht Hildesheim, HRA 2686           | Fax:   +49-5121-206917-5555 |
>

Re: [Linaro-mm-sig] [RFC 0/1] drm/pl111: Initial drm/kms driver for pl111

[Lucas Stach]

Am Dienstag, den 06.08.2013, 10:14 -0400 schrieb Rob Clark:
> On Tue, Aug 6, 2013 at 8:18 AM, Lucas Stach <l.stach@pengutronix.de> wrote:
> > Am Dienstag, den 06.08.2013, 12:31 +0100 schrieb Tom Cooksey:
> >> Hi Rob,
> >>
> >> +lkml
> >>
> >> > >> On Fri, Jul 26, 2013 at 11:58 AM, Tom Cooksey <tom.cooksey@arm.com>
> >> > >> wrote:
> >> > >> >> >  * It abuses flags parameter of DRM_IOCTL_MODE_CREATE_DUMB to
> >> > >> >> >    also allocate buffers for the GPU. Still not sure how to
> >> > >> >> >    resolve this as we don't use DRM for our GPU driver.
> >> > >> >>
> >> > >> >> any thoughts/plans about a DRM GPU driver?  Ideally long term
> >> > >> >> (esp. once the dma-fence stuff is in place), we'd have
> >> > >> >> gpu-specific drm (gpu-only, no kms) driver, and SoC/display
> >> > >> >> specific drm/kms driver, using prime/dmabuf to share between
> >> > >> >> the two.
> >> > >> >
> >> > >> > The "extra" buffers we were allocating from armsoc DDX were really
> >> > >> > being allocated through DRM/GEM so we could get an flink name
> >> > >> > for them and pass a reference to them back to our GPU driver on
> >> > >> > the client side. If it weren't for our need to access those
> >> > >> > extra off-screen buffers with the GPU we wouldn't need to
> >> > >> > allocate them with DRM at all. So, given they are really "GPU"
> >> > >> > buffers, it does absolutely make sense to allocate them in a
> >> > >> > different driver to the display driver.
> >> > >> >
> >> > >> > However, to avoid unnecessary memcpys & related cache
> >> > >> > maintenance ops, we'd also like the GPU to render into buffers
> >> > >> > which are scanned out by the display controller. So let's say
> >> > >> > we continue using DRM_IOCTL_MODE_CREATE_DUMB to allocate scan
> >> > >> > out buffers with the display's DRM driver but a custom ioctl
> >> > >> > on the GPU's DRM driver to allocate non scanout, off-screen
> >> > >> > buffers. Sounds great, but I don't think that really works
> >> > >> > with DRI2. If we used two drivers to allocate buffers, which
> >> > >> > of those drivers do we return in DRI2ConnectReply? Even if we
> >> > >> > solve that somehow, GEM flink names are name-spaced to a
> >> > >> > single device node (AFAIK). So when we do a DRI2GetBuffers,
> >> > >> > how does the EGL in the client know which DRM device owns GEM
> >> > >> > flink name "1234"? We'd need some pretty dirty hacks.
> >> > >>
> >> > >> You would return the name of the display driver allocating the
> >> > >> buffers.  On the client side you can use generic ioctls to go from
> >> > >> flink -> handle -> dmabuf.  So the client side would end up opening
> >> > >> both the display drm device and the gpu, but without needing to know
> >> > >> too much about the display.
> >> > >
> >> > > I think the bit I was missing was that a GEM bo for a buffer imported
> >> > > using dma_buf/PRIME can still be flink'd. So the display controller's
> >> > > DRM driver allocates scan-out buffers via the DUMB buffer allocate
> >> > > ioctl. Those scan-out buffers than then be exported from the
> >> > > dispaly's DRM driver and imported into the GPU's DRM driver using
> >> > > PRIME. Once imported into the GPU's driver, we can use flink to get a
> >> > > name for that buffer within the GPU DRM driver's name-space to return
> >> > > to the DRI2 client. That same namespace is also what DRI2 back-
> >> > > buffers are allocated from, so I think that could work... Except...
> >> >
> >> > (and.. the general direction is that things will move more to just use
> >> > dmabuf directly, ie. wayland or dri3)
> >>
> >> I agree, DRI2 is the only reason why we need a system-wide ID. I also
> >> prefer buffers to be passed around by dma_buf fd, but we still need to
> >> support DRI2 and will do for some time I expect.
> >>
> >>
> >>
> >> > >> > Anyway, that latter case also gets quite difficult. The "GPU"
> >> > >> > DRM driver would need to know the constraints of the display
> >> > >> > controller when allocating buffers intended to be scanned out.
> >> > >> > For example, pl111 typically isn't behind an IOMMU and so
> >> > >> > requires physically contiguous memory. We'd have to teach the
> >> > >> > GPU's DRM driver about the constraints of the display HW. Not
> >> > >> > exactly a clean driver model. :-(
> >> > >> >
> >> > >> > I'm still a little stuck on how to proceed, so any ideas
> >> > >> > would greatly appreciated! My current train of thought is
> >> > >> > having a kind of SoC-specific DRM driver which allocates
> >> > >> > buffers for both display and GPU within a single GEM
> >> > >> > namespace. That SoC-specific DRM driver could then know the
> >> > >> > constraints of both the GPU and the display HW. We could then
> >> > >> > use PRIME to export buffers allocated with the SoC DRM driver
> >> > >> > and import them into the GPU and/or display DRM driver.
> >> > >>
> >> > >> Usually if the display drm driver is allocating the buffers that
> >> > >> might be scanned out, it just needs to have minimal knowledge of
> >> > >> the GPU (pitch alignment constraints).  I don't think we need a
> >> > >> 3rd device just to allocate buffers.
> >> > >
> >> > > While Mali can render to pretty much any buffer, there is a mild
> >> > > performance improvement to be had if the buffer stride is aligned to
> >> > > the AXI bus's max burst length when drawing to the buffer.
> >> >
> >> > I suspect the display controllers might frequently benefit if the
> >> > pitch is aligned to AXI burst length too..
> >>
> >> If the display controller is going to be reading from linear memory
> >> I don't think it will make much difference - you'll just get an extra
> >> 1-2 bus transactions per scanline. With a tile-based GPU like Mali,
> >> you get those extra transactions per _tile_ scan-line and as such,
> >> the overhead is more pronounced.
> >>
> >>
> >>
> >> > > So in some respects, there is a constraint on how buffers which will
> >> > > be drawn to using the GPU are allocated. I don't really like the idea
> >> > > of teaching the display controller DRM driver about the GPU buffer
> >> > > constraints, even if they are fairly trivial like this. If the same
> >> > > display HW IP is being used on several SoCs, it seems wrong somehow
> >> > > to enforce those GPU constraints if some of those SoCs don't have a
> >> > > GPU.
> >> >
> >> > Well, I suppose you could get min_pitch_alignment from devicetree, or
> >> > something like this..
> >> >
> >> > In the end, the easy solution is just to make the display allocate to
> >> > the worst-case pitch alignment.  In the early days of dma-buf
> >> > discussions, we kicked around the idea of negotiating or
> >> > programatically describing the constraints, but that didn't really
> >> > seem like a bounded problem.
> >>
> >> Yeah - I was around for some of those discussions and agree it's not
> >> really an easy problem to solve.
> >>
> >>
> >>
> >> > > We may also then have additional constraints when sharing buffers
> >> > > between the display HW and video decode or even camera ISP HW.
> >> > > Programmatically describing buffer allocation constraints is very
> >> > > difficult and I'm not sure you can actually do it - there's some
> >> > > pretty complex constraints out there! E.g. I believe there's a
> >> > > platform where Y and UV planes of the reference frame need to be in
> >> > > separate DRAM banks for real-time 1080p decode, or something like
> >> > > that?
> >> >
> >> > yes, this was discussed.  This is different from pitch/format/size
> >> > constraints.. it is really just a placement constraint (ie. where do
> >> > the physical pages go).  IIRC the conclusion was to use a dummy
> >> > devices with it's own CMA pool for attaching the Y vs UV buffers.
> >> >
> >> > > Anyway, I guess my point is that even if we solve how to allocate
> >> > > buffers which will be shared between the GPU and display HW such that
> >> > > both sets of constraints are satisfied, that may not be the end of
> >> > > the story.
> >> > >
> >> >
> >> > that was part of the reason to punt this problem to userspace ;-)
> >> >
> >> > In practice, the kernel drivers doesn't usually know too much about
> >> > the dimensions/format/etc.. that is really userspace level knowledge.
> >> > There are a few exceptions when the kernel needs to know how to setup
> >> > GTT/etc for tiled buffers, but normally this sort of information is up
> >> > at the next level up (userspace, and drm_framebuffer in case of
> >> > scanout).  Userspace media frameworks like GStreamer already have a
> >> > concept of format/caps negotiation.  For non-display<->gpu sharing, I
> >> > think this is probably where this sort of constraint negotiation
> >> > should be handled.
> >>
> >> I agree that user-space will know which devices will access the buffer
> >> and thus can figure out at least a common pixel format. Though I'm not
> >> so sure userspace can figure out more low-level details like alignment
> >> and placement in physical memory, etc.
> >>
> >> Anyway, assuming user-space can figure out how a buffer should be
> >> stored in memory, how does it indicate this to a kernel driver and
> >> actually allocate it? Which ioctl on which device does user-space
> >> call, with what parameters? Are you suggesting using something like
> >> ION which exposes the low-level details of how buffers are laid out in
> >> physical memory to userspace? If not, what?
> >>
> >
> > I strongly disagree with exposing low-level hardware details like tiling
> > to userspace. If we have to do the negotiation of those things in
> > userspace we will end up with having to pipe those information through
> > things like the wayland protocol. I don't see how this could ever be
> > considered a good idea.
> 
> well, unless userspace mmap's via a de-tiling gart type thing, I don't
> think tiling can be invisible to userspace.
> 
Why is mmap considered to be such a strong use-case for DMABUFs? After
all we are trying to _avoid_ mmapping shared buffers where ever
possible.

> But if two GPU's have some overlap in supportable tiled formats, and
> you have one gpu doing app and one doing compositor, and you want to
> use tiling for the shared buffers, you need something in the wayland
> protocol to figure out what the common supported formats are between
> the two sides.  I suppose it shouldn't be too hard to add a
> standardized (cross-driver) format-negotiation protocol, and in the
> absence of that fallback to non tiled format for shared buffers.
> 
I don't see how tiling format negotiation would be easier in userspace
than in the kernel. If we can come up with a scheme for that, we can as
well do it in the kernel.

> > I would rather see kernel drivers negotiating those things at dmabuf
> > attach time in way invisible to userspace. I agree that this negotiation
> > thing isn't easy to get right for the plethora of different hardware
> > constraints we see today, but I would rather see this in-kernel, where
> > we have the chance to fix things up if needed, than in a fixed userspace
> > interface.
> 
> Well, if you can think of a sane way to add that to dev->dma_params,
> and if it isn't visible if userspace mmap's a buffer, then we could
> handle that in the kernel.  But I don't think that will be the case.
> 
I'm sure we can come up with something sane to put it in there. The
userspace mmap thing is a bit complicated to deal with, but I have the
feeling that going through a slow path if you really need mmap is a
reasonable thing to do.
For example we could just make mmap some form of attach that forces
linear layout if the exporter isn't able to give userspace a linear
mapping from a tiled buffer by using GART or VM. 

Regards,
Lucas
-- 
Pengutronix e.K.                           | Lucas Stach                 |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
Peiner Str. 6-8, 31137 Hildesheim, Germany | Phone: +49-5121-206917-5076 |
Amtsgericht Hildesheim, HRA 2686           | Fax:   +49-5121-206917-5555 |

Re: [Linaro-mm-sig] [RFC 0/1] drm/pl111: Initial drm/kms driver for pl111

[Rob Clark]

On Tue, Aug 6, 2013 at 10:36 AM, Lucas Stach <l.stach@pengutronix.de> wrote:
> Am Dienstag, den 06.08.2013, 10:14 -0400 schrieb Rob Clark:
>> On Tue, Aug 6, 2013 at 8:18 AM, Lucas Stach <l.stach@pengutronix.de> wrote:
>> > Am Dienstag, den 06.08.2013, 12:31 +0100 schrieb Tom Cooksey:
>> >> Hi Rob,
>> >>
>> >> +lkml
>> >>
>> >> > >> On Fri, Jul 26, 2013 at 11:58 AM, Tom Cooksey <tom.cooksey@arm.com>
>> >> > >> wrote:
>> >> > >> >> >  * It abuses flags parameter of DRM_IOCTL_MODE_CREATE_DUMB to
>> >> > >> >> >    also allocate buffers for the GPU. Still not sure how to
>> >> > >> >> >    resolve this as we don't use DRM for our GPU driver.
>> >> > >> >>
>> >> > >> >> any thoughts/plans about a DRM GPU driver?  Ideally long term
>> >> > >> >> (esp. once the dma-fence stuff is in place), we'd have
>> >> > >> >> gpu-specific drm (gpu-only, no kms) driver, and SoC/display
>> >> > >> >> specific drm/kms driver, using prime/dmabuf to share between
>> >> > >> >> the two.
>> >> > >> >
>> >> > >> > The "extra" buffers we were allocating from armsoc DDX were really
>> >> > >> > being allocated through DRM/GEM so we could get an flink name
>> >> > >> > for them and pass a reference to them back to our GPU driver on
>> >> > >> > the client side. If it weren't for our need to access those
>> >> > >> > extra off-screen buffers with the GPU we wouldn't need to
>> >> > >> > allocate them with DRM at all. So, given they are really "GPU"
>> >> > >> > buffers, it does absolutely make sense to allocate them in a
>> >> > >> > different driver to the display driver.
>> >> > >> >
>> >> > >> > However, to avoid unnecessary memcpys & related cache
>> >> > >> > maintenance ops, we'd also like the GPU to render into buffers
>> >> > >> > which are scanned out by the display controller. So let's say
>> >> > >> > we continue using DRM_IOCTL_MODE_CREATE_DUMB to allocate scan
>> >> > >> > out buffers with the display's DRM driver but a custom ioctl
>> >> > >> > on the GPU's DRM driver to allocate non scanout, off-screen
>> >> > >> > buffers. Sounds great, but I don't think that really works
>> >> > >> > with DRI2. If we used two drivers to allocate buffers, which
>> >> > >> > of those drivers do we return in DRI2ConnectReply? Even if we
>> >> > >> > solve that somehow, GEM flink names are name-spaced to a
>> >> > >> > single device node (AFAIK). So when we do a DRI2GetBuffers,
>> >> > >> > how does the EGL in the client know which DRM device owns GEM
>> >> > >> > flink name "1234"? We'd need some pretty dirty hacks.
>> >> > >>
>> >> > >> You would return the name of the display driver allocating the
>> >> > >> buffers.  On the client side you can use generic ioctls to go from
>> >> > >> flink -> handle -> dmabuf.  So the client side would end up opening
>> >> > >> both the display drm device and the gpu, but without needing to know
>> >> > >> too much about the display.
>> >> > >
>> >> > > I think the bit I was missing was that a GEM bo for a buffer imported
>> >> > > using dma_buf/PRIME can still be flink'd. So the display controller's
>> >> > > DRM driver allocates scan-out buffers via the DUMB buffer allocate
>> >> > > ioctl. Those scan-out buffers than then be exported from the
>> >> > > dispaly's DRM driver and imported into the GPU's DRM driver using
>> >> > > PRIME. Once imported into the GPU's driver, we can use flink to get a
>> >> > > name for that buffer within the GPU DRM driver's name-space to return
>> >> > > to the DRI2 client. That same namespace is also what DRI2 back-
>> >> > > buffers are allocated from, so I think that could work... Except...
>> >> >
>> >> > (and.. the general direction is that things will move more to just use
>> >> > dmabuf directly, ie. wayland or dri3)
>> >>
>> >> I agree, DRI2 is the only reason why we need a system-wide ID. I also
>> >> prefer buffers to be passed around by dma_buf fd, but we still need to
>> >> support DRI2 and will do for some time I expect.
>> >>
>> >>
>> >>
>> >> > >> > Anyway, that latter case also gets quite difficult. The "GPU"
>> >> > >> > DRM driver would need to know the constraints of the display
>> >> > >> > controller when allocating buffers intended to be scanned out.
>> >> > >> > For example, pl111 typically isn't behind an IOMMU and so
>> >> > >> > requires physically contiguous memory. We'd have to teach the
>> >> > >> > GPU's DRM driver about the constraints of the display HW. Not
>> >> > >> > exactly a clean driver model. :-(
>> >> > >> >
>> >> > >> > I'm still a little stuck on how to proceed, so any ideas
>> >> > >> > would greatly appreciated! My current train of thought is
>> >> > >> > having a kind of SoC-specific DRM driver which allocates
>> >> > >> > buffers for both display and GPU within a single GEM
>> >> > >> > namespace. That SoC-specific DRM driver could then know the
>> >> > >> > constraints of both the GPU and the display HW. We could then
>> >> > >> > use PRIME to export buffers allocated with the SoC DRM driver
>> >> > >> > and import them into the GPU and/or display DRM driver.
>> >> > >>
>> >> > >> Usually if the display drm driver is allocating the buffers that
>> >> > >> might be scanned out, it just needs to have minimal knowledge of
>> >> > >> the GPU (pitch alignment constraints).  I don't think we need a
>> >> > >> 3rd device just to allocate buffers.
>> >> > >
>> >> > > While Mali can render to pretty much any buffer, there is a mild
>> >> > > performance improvement to be had if the buffer stride is aligned to
>> >> > > the AXI bus's max burst length when drawing to the buffer.
>> >> >
>> >> > I suspect the display controllers might frequently benefit if the
>> >> > pitch is aligned to AXI burst length too..
>> >>
>> >> If the display controller is going to be reading from linear memory
>> >> I don't think it will make much difference - you'll just get an extra
>> >> 1-2 bus transactions per scanline. With a tile-based GPU like Mali,
>> >> you get those extra transactions per _tile_ scan-line and as such,
>> >> the overhead is more pronounced.
>> >>
>> >>
>> >>
>> >> > > So in some respects, there is a constraint on how buffers which will
>> >> > > be drawn to using the GPU are allocated. I don't really like the idea
>> >> > > of teaching the display controller DRM driver about the GPU buffer
>> >> > > constraints, even if they are fairly trivial like this. If the same
>> >> > > display HW IP is being used on several SoCs, it seems wrong somehow
>> >> > > to enforce those GPU constraints if some of those SoCs don't have a
>> >> > > GPU.
>> >> >
>> >> > Well, I suppose you could get min_pitch_alignment from devicetree, or
>> >> > something like this..
>> >> >
>> >> > In the end, the easy solution is just to make the display allocate to
>> >> > the worst-case pitch alignment.  In the early days of dma-buf
>> >> > discussions, we kicked around the idea of negotiating or
>> >> > programatically describing the constraints, but that didn't really
>> >> > seem like a bounded problem.
>> >>
>> >> Yeah - I was around for some of those discussions and agree it's not
>> >> really an easy problem to solve.
>> >>
>> >>
>> >>
>> >> > > We may also then have additional constraints when sharing buffers
>> >> > > between the display HW and video decode or even camera ISP HW.
>> >> > > Programmatically describing buffer allocation constraints is very
>> >> > > difficult and I'm not sure you can actually do it - there's some
>> >> > > pretty complex constraints out there! E.g. I believe there's a
>> >> > > platform where Y and UV planes of the reference frame need to be in
>> >> > > separate DRAM banks for real-time 1080p decode, or something like
>> >> > > that?
>> >> >
>> >> > yes, this was discussed.  This is different from pitch/format/size
>> >> > constraints.. it is really just a placement constraint (ie. where do
>> >> > the physical pages go).  IIRC the conclusion was to use a dummy
>> >> > devices with it's own CMA pool for attaching the Y vs UV buffers.
>> >> >
>> >> > > Anyway, I guess my point is that even if we solve how to allocate
>> >> > > buffers which will be shared between the GPU and display HW such that
>> >> > > both sets of constraints are satisfied, that may not be the end of
>> >> > > the story.
>> >> > >
>> >> >
>> >> > that was part of the reason to punt this problem to userspace ;-)
>> >> >
>> >> > In practice, the kernel drivers doesn't usually know too much about
>> >> > the dimensions/format/etc.. that is really userspace level knowledge.
>> >> > There are a few exceptions when the kernel needs to know how to setup
>> >> > GTT/etc for tiled buffers, but normally this sort of information is up
>> >> > at the next level up (userspace, and drm_framebuffer in case of
>> >> > scanout).  Userspace media frameworks like GStreamer already have a
>> >> > concept of format/caps negotiation.  For non-display<->gpu sharing, I
>> >> > think this is probably where this sort of constraint negotiation
>> >> > should be handled.
>> >>
>> >> I agree that user-space will know which devices will access the buffer
>> >> and thus can figure out at least a common pixel format. Though I'm not
>> >> so sure userspace can figure out more low-level details like alignment
>> >> and placement in physical memory, etc.
>> >>
>> >> Anyway, assuming user-space can figure out how a buffer should be
>> >> stored in memory, how does it indicate this to a kernel driver and
>> >> actually allocate it? Which ioctl on which device does user-space
>> >> call, with what parameters? Are you suggesting using something like
>> >> ION which exposes the low-level details of how buffers are laid out in
>> >> physical memory to userspace? If not, what?
>> >>
>> >
>> > I strongly disagree with exposing low-level hardware details like tiling
>> > to userspace. If we have to do the negotiation of those things in
>> > userspace we will end up with having to pipe those information through
>> > things like the wayland protocol. I don't see how this could ever be
>> > considered a good idea.
>>
>> well, unless userspace mmap's via a de-tiling gart type thing, I don't
>> think tiling can be invisible to userspace.
>>
> Why is mmap considered to be such a strong use-case for DMABUFs? After
> all we are trying to _avoid_ mmapping shared buffers where ever
> possible.

I don't know if I'd call it a strong use-case, as much as a good
worst-case to consider

>> But if two GPU's have some overlap in supportable tiled formats, and
>> you have one gpu doing app and one doing compositor, and you want to
>> use tiling for the shared buffers, you need something in the wayland
>> protocol to figure out what the common supported formats are between
>> the two sides.  I suppose it shouldn't be too hard to add a
>> standardized (cross-driver) format-negotiation protocol, and in the
>> absence of that fallback to non tiled format for shared buffers.
>>
> I don't see how tiling format negotiation would be easier in userspace
> than in the kernel. If we can come up with a scheme for that, we can as
> well do it in the kernel.

well.. I guess I don't see how it would be easier in the kernel than
in userspace ;-)

But if you can come up with a good way to add it to dev->dma_params
that everyone can agree on, I don't mind.  I suppose I'm starting with
the assumption that it will be difficult/impossible to get everyone to
agree on this, but I don't mind being proved wrong.

>> > I would rather see kernel drivers negotiating those things at dmabuf
>> > attach time in way invisible to userspace. I agree that this negotiation
>> > thing isn't easy to get right for the plethora of different hardware
>> > constraints we see today, but I would rather see this in-kernel, where
>> > we have the chance to fix things up if needed, than in a fixed userspace
>> > interface.
>>
>> Well, if you can think of a sane way to add that to dev->dma_params,
>> and if it isn't visible if userspace mmap's a buffer, then we could
>> handle that in the kernel.  But I don't think that will be the case.
>>
> I'm sure we can come up with something sane to put it in there. The
> userspace mmap thing is a bit complicated to deal with, but I have the
> feeling that going through a slow path if you really need mmap is a
> reasonable thing to do.
> For example we could just make mmap some form of attach that forces
> linear layout if the exporter isn't able to give userspace a linear
> mapping from a tiled buffer by using GART or VM.

I guess I don't object to mmap being a slow path (as long is it
doesn't end up requiring a slow path on hw where this otherwise
wouldn't be needed).

BR,
-R

> Regards,
> Lucas
> --
> Pengutronix e.K.                           | Lucas Stach                 |
> Industrial Linux Solutions                 | http://www.pengutronix.de/  |
> Peiner Str. 6-8, 31137 Hildesheim, Germany | Phone: +49-5121-206917-5076 |
> Amtsgericht Hildesheim, HRA 2686           | Fax:   +49-5121-206917-5555 |
>

Re: [Linaro-mm-sig] [RFC 0/1] drm/pl111: Initial drm/kms driver for pl111

[Daniel Vetter]

On Tue, Aug 6, 2013 at 2:18 PM, Lucas Stach <l.stach@pengutronix.de> wrote:
> I strongly disagree with exposing low-level hardware details like tiling
> to userspace. If we have to do the negotiation of those things in
> userspace we will end up with having to pipe those information through
> things like the wayland protocol. I don't see how this could ever be
> considered a good idea.
>
> I would rather see kernel drivers negotiating those things at dmabuf
> attach time in way invisible to userspace. I agree that this negotiation
> thing isn't easy to get right for the plethora of different hardware
> constraints we see today, but I would rather see this in-kernel, where
> we have the chance to fix things up if needed, than in a fixed userspace
> interface.

The problem with negotiating tiling in the kernel for at least
drm/i915 is that userspace needs to know the tiling and for allocating
actually be in charge of it. The approach used by prime thus far is to
simply use linear buffers between different gpus. If you want
something better I guess you need to have a fairly tightly integrate
userspace driver, so I don't see how passing around the tiling
information should be an issue for those cases.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch

Re: [Linaro-mm-sig] [RFC 0/1] drm/pl111: Initial drm/kms driver for pl111

[Daniel Stone]

Hi,

On Tue, 2013-08-06 at 12:31 +0100, Tom Cooksey wrote:
> > >> On Fri, Jul 26, 2013 at 11:58 AM, Tom Cooksey <tom.cooksey@arm.com>
> > >> wrote:
> > that was part of the reason to punt this problem to userspace ;-)
> >
> > In practice, the kernel drivers doesn't usually know too much about
> > the dimensions/format/etc.. that is really userspace level knowledge.
> > There are a few exceptions when the kernel needs to know how to setup
> > GTT/etc for tiled buffers, but normally this sort of information is up
> > at the next level up (userspace, and drm_framebuffer in case of
> > scanout).  Userspace media frameworks like GStreamer already have a
> > concept of format/caps negotiation.  For non-display<->gpu sharing, I
> > think this is probably where this sort of constraint negotiation
> > should be handled.

Egads.  GStreamer's caps negotiation is already close to unbounded time;
seems like most of the optimisation work that goes into it these days is
all about _reducing_ the complexity of caps negotiation!

> I agree that user-space will know which devices will access the buffer
> and thus can figure out at least a common pixel format.

Hm, are you sure about that? The answer is yes for 'userspace' as a
broad handwave, but not necessarily for individual processes.  Take, for
instance, media decode through GStreamer, being displayed by Wayland
using a KMS plane/overlay/sprite/cursor/etc.  The media player knows
that the buffers are coming from the decode engine, and Wayland knows
that the buffers are going to a KMS plane, but neither of them knows the
full combination of the two.

Though this kinda feeds into an idea I've been kicking around for a
while, which is an 'optimal hint' mechanism in the Wayland protocol.  So
for our hypothetical dmabuf-using protocol, we'd start off with buffers
which satisfied all the constraints of our media decode engine, but
perhaps just the GPU rather than display controller.  At this point,
we'd note that we could place the video in a plane if only the buffers
were better-allocated, and send an event to the client letting it know
how to tweak its buffer allocation for more optimal display.

But ...

> Though I'm not
> so sure userspace can figure out more low-level details like alignment
> and placement in physical memory, etc.
> 
> Anyway, assuming user-space can figure out how a buffer should be 
> stored in memory, how does it indicate this to a kernel driver and 
> actually allocate it? Which ioctl on which device does user-space
> call, with what parameters? Are you suggesting using something like
> ION which exposes the low-level details of how buffers are laid out in
> physical memory to userspace? If not, what?

... this is still rather unresolved. ;)

Cheers,
Daniel


> 
> Cheers,
> 
> Tom
> 
> 
> 
> 
> 
> 
> _______________________________________________
> Linaro-mm-sig mailing list
> Linaro-mm-sig@lists.linaro.org
> http://lists.linaro.org/mailman/listinfo/linaro-mm-sig

Re: [RFC 0/1] drm/pl111: Initial drm/kms driver for pl111

[John Stultz]

On Mon, Jul 29, 2013 at 7:58 AM, Rob Clark <robdclark@gmail.com> wrote:
> On Fri, Jul 26, 2013 at 11:58 AM, Tom Cooksey <tom.cooksey@arm.com> wrote:
>> Hi Rob,
>>
>>> >  * It abuses flags parameter of DRM_IOCTL_MODE_CREATE_DUMB to also
>>> >    allocate buffers for the GPU. Still not sure how to resolve this
>>> >    as we don't use DRM for our GPU driver.
>>>
>>> any thoughts/plans about a DRM GPU driver?  Ideally long term (esp.
>>> once the dma-fence stuff is in place), we'd have gpu-specific drm
>>> (gpu-only, no kms) driver, and SoC/display specific drm/kms driver,
>>> using prime/dmabuf to share between the two.
>>
>> The "extra" buffers we were allocating from armsoc DDX were really
>> being allocated through DRM/GEM so we could get an flink name
>> for them and pass a reference to them back to our GPU driver on
>> the client side. If it weren't for our need to access those
>> extra off-screen buffers with the GPU we wouldn't need to
>> allocate them with DRM at all. So, given they are really "GPU"
>> buffers, it does absolutely make sense to allocate them in a
>> different driver to the display driver.
>>
>> However, to avoid unnecessary memcpys & related cache
>> maintenance ops, we'd also like the GPU to render into buffers
>> which are scanned out by the display controller. So let's say
>> we continue using DRM_IOCTL_MODE_CREATE_DUMB to allocate scan
>> out buffers with the display's DRM driver but a custom ioctl
>> on the GPU's DRM driver to allocate non scanout, off-screen
>> buffers. Sounds great, but I don't think that really works
>> with DRI2. If we used two drivers to allocate buffers, which
>> of those drivers do we return in DRI2ConnectReply? Even if we
>> solve that somehow, GEM flink names are name-spaced to a
>> single device node (AFAIK). So when we do a DRI2GetBuffers,
>> how does the EGL in the client know which DRM device owns GEM
>> flink name "1234"? We'd need some pretty dirty hacks.
>
> You would return the name of the display driver allocating the
> buffers.  On the client side you can use generic ioctls to go from
> flink -> handle -> dmabuf.  So the client side would end up opening
> both the display drm device and the gpu, but without needing to know
> too much about the display.
>
> (Probably in (for example) mesa there needs to be a bit of work to
> handle this better, but I think that would be needed as well for
> sharing between, say, nouveau and udl displaylink driver.. which is
> really the same scenario.)
>
>> So then we looked at allocating _all_ buffers with the GPU's
>> DRM driver. That solves the DRI2 single-device-name and single
>> name-space issue. It also means the GPU would _never_ render
>> into buffers allocated through DRM_IOCTL_MODE_CREATE_DUMB.
>
> Well, I think we can differentiate between shared buffers and internal
> buffers (textures, vertex upload, etc, etc).
>
> For example, in mesa/gallium driver there are two paths to getting a
> buffer...  ->resource_create() and ->resource_from_handle(), the
> latter is the path you go for dri2 buffers shared w/ x11.  The former
> is buffers that are just internal to gpu (if !(bind &
> PIPE_BIND_SHARED)).  I guess you must have something similar in mali
> driver.
>
>> One thing I wasn't sure about is if there was an objection
>> to using PRIME to export scanout buffers allocated with
>> DRM_IOCTL_MODE_CREATE_DUMB and then importing them into a GPU
>> driver to be rendered into? Is that a concern?
>
> well.. it wasn't quite the original intention of the 'dumb' ioctls.
> But I guess in the end if you hand the gpu a buffer with a
> layout/format that it can't grok, then gpu does a staging texture plus
> copy.  If you had, for example, some setup w/ gpu that could only
> render to tiled format, plus a display that could scanout that format,
> then your DDX driver would need to allocate the dri2 buffers with
> something other than dumb ioctl.  (But at that point, you probably
> need to implement your own EXA so you could hook in your own custom
> upload-to/download-from screen fxns for sw fallbacks, so you're not
> talking about using generic DDX anyways.)
>
>> Anyway, that latter case also gets quite difficult. The "GPU"
>> DRM driver would need to know the constraints of the display
>> controller when allocating buffers intended to be scanned out.
>> For example, pl111 typically isn't behind an IOMMU and so
>> requires physically contiguous memory. We'd have to teach the
>> GPU's DRM driver about the constraints of the display HW. Not
>> exactly a clean driver model. :-(
>>
>> I'm still a little stuck on how to proceed, so any ideas
>> would greatly appreciated! My current train of thought is
>> having a kind of SoC-specific DRM driver which allocates
>> buffers for both display and GPU within a single GEM
>> namespace. That SoC-specific DRM driver could then know the
>> constraints of both the GPU and the display HW. We could then
>> use PRIME to export buffers allocated with the SoC DRM driver
>> and import them into the GPU and/or display DRM driver.
>
> Usually if the display drm driver is allocating the buffers that might
> be scanned out, it just needs to have minimal knowledge of the GPU
> (pitch alignment constraints).  I don't think we need a 3rd device
> just to allocate buffers.
>
> Really I don't think the separate display drm and gpu drm device is
> much different from desktop udl/displaylink case.  Or desktop
> integrated-gpu + external gpu.

Although on many SoCs, aren't the pipelines a bit more complicated?
(ie: camera data -> doing face detection, -> applying filters w/ the
gpu -> and displaying it).

While ION does have interface issues (in that it requires userland to
hard-code particular constraints for that specific SoC) just the idea
of having the centralized allocator that manages the constraints does
seem to make sense for these larger pipelines (and also the fact that
there's different allocation APIs for various devices).

One idea that was discussed in some of the hallway discussions at
connect was if there was some way for the hardware to expose a
"constraint cookie" via sysfs (basically a bit field where the meaning
of each bit is opaque to userland - so each bit can mean a different
constraint on different devices), userland could determine which
combination of devices it wanted to pipe a buffer through, then AND
the device constraint cookies together,  and pass that to a
centralized allocator. This allows the constraint solving to be pushed
out to userland, but also avoids what I see as the biggest negatives
in ION (ie: the heap mask is defined to userland, making it
inflexible, as well as the lack of a method for userland to discover
device constraints, so they're instead hard-coded in userland for each
device).

>> Note: While it doesn't use the DRM framework, the Mali T6xx
>> kernel driver has supported importing buffers through dma_buf
>> for some time. I've even written an EGL extension :-):
>>
>> <http://www.khronos.org/registry/egl/extensions/EXT/EGL_EXT_image_dma_buf_im
>> port.txt>
>>
>>
>>> I'm not entirely sure that the directions that the current CDF
>>> proposals are headed is necessarily the right way forward.  I'd prefer
>>> to see small/incremental evolution of KMS (ie. add drm_bridge and
>>> drm_panel, and refactor the existing encoder-slave).  Keeping it
>>> inside drm means that we can evolve it more easily, and avoid layers
>>> of glue code for no good reason.
>>
>> I think CDF could allow vendors to re-use code they've written
>> for their Android driver stack in DRM drivers more easily. Though
>> I guess ideally KMS would evolve to a point where it could be used
>> by an Android driver stack. I.e. Support explicit fences.
>>
>
> yeah, the best would be evolving DRM/KMS to the point that it meets
> android requirements.  Because I really don't think android has any
> special requirements compared to, say, a wayland compositor.  (Other
> than the way they do synchronization.  But that doesn't really *need*
> to be different, as far as I can tell.)  It would certainly be easier
> if android dev's actually participated in upstream linux graphics, and
> talked about what they needed, and maybe even contributed a patch or
> two.  But that is a whole different topic.

It also couldn't hurt to cc them on these discussions (Rebecca and Erik cc'ed :)

I think Ross covered much of the KMS issues at Linaro Connect (Video
here if you want to spend the time: http://youtu.be/AuoFABKS_Dk), and
indeed, the explicit syncing requirement is probably the biggest
barrier. My understanding is the additional parallelism/performance
that the explicit method provides is the key requirement (though maybe
Erik or Ross can chime in with further details? - I've been told Erik
has a long list as to why explicit is preferred over implicit and it
would be good to get that documented somewhere :)

I understand the implicit model is easier to program for, and Daniel
mentioned dynamic buffer eviction as having a requirement on the
implicit model, so I'm currently in the camp of assuming the
synchronization needs are different enough that one method alone (of
the two currently presented) likely won't work. My current hope is
there's some hybrid approach that can be used (implicitly maybe for
the majority of cases, but with the option of doing explicit syncing
when necessary).

thanks
-john (who's unfortunately heading off on vacation soon, so might not
be fast to reply)

Re: [RFC 1/1] drm/pl111: Initial drm/kms driver for pl111

[Daniel Vetter]

Just comment a bit on Rob's review with my own opinion.

On Wed, Aug 07, 2013 at 12:17:21PM -0400, Rob Clark wrote:
> On Thu, Jul 25, 2013 at 1:17 PM,  <tom.cooksey@arm.com> wrote:
> > From: Tom Cooksey <tom.cooksey@arm.com>
> >
> > This is a mode-setting driver for the pl111 CLCD display controller
> > found on various ARM reference platforms such as the Versatile
> > Express. The driver supports setting of a single mode (640x480) and
> > has only been tested on Versatile Express with a Cortex-A9 core tile.
> >
> > Known issues:
> >  * It still includes code to use KDS, which is not going upstream.
> 
> review's on http://lists.freedesktop.org/archives/dri-devel/2013-July/042462.html
> can't hurt
> 
> although you might consider submitting a reduced functionality driver
> w/ KDS bits removed in the mean time.. then when the fence stuff is
> merged it is just an incremental patch rather than a whole driver ;-)

Yeah, I think the KDS bits and comments need to go first before merginge.


> > +/*
> > + * Number of flips allowed in flight at any one time. Any more flips requested
> > + * beyond this value will cause the caller to block until earlier flips have
> > + * completed.
> > + *
> > + * For performance reasons, this must be greater than the number of buffers
> > + * used in the rendering pipeline. Note that the rendering pipeline can contain
> > + * different types of buffer, e.g.:
> > + * - 2 final framebuffers
> > + * - >2 geometry buffers for GPU use-cases
> > + * - >2 vertex buffers for GPU use-cases
> > + *
> > + * For example, a system using 5 geometry buffers could have 5 flips in flight,
> > + * and so NR_FLIPS_IN_FLIGHT_THRESHOLD must be 5 or greater.
> > + *
> > + * Whilst there may be more intermediate buffers (such as vertex/geometry) than
> > + * final framebuffers, KDS is used to ensure that GPU rendering waits for the
> > + * next off-screen buffer, so it doesn't overwrite an on-screen buffer and
> > + * produce tearing.
> > + */
> > +
> 
> fwiw, this is at least different from how other drivers do triple (or
> > double) buffering.  In other drivers (intel, omap, and
> msm/freedreno, that I know of, maybe others too) the xorg driver dri2
> bits implement the double buffering (ie. send flip event back to
> client immediately and queue up the flip and call page-flip after the
> pageflip event back from kernel.
> 
> I'm not saying not to do it this way, I guess I'd like to hear what
> other folks think.  I kinda prefer doing this in userspace as it keeps
> the kernel bits simpler (plus it would then work properly on exynosdrm
> or other kms drivers).

Yeah, if this is just a sw queue then I don't think it makes sense to have
it in the kernel. Afaik the current pageflip interface drm exposes allows
one oustanding flip only, and you _must_ wait for the flip complete event
before you can submit the second one.

Ofc if your hardware as a hw-based flip queue (maybe even with frame
targets) that's a different matter, but currently we don't have a drm
interface to expose this. I'd say for merging the basic driver first we
should go with the existing simple pageflip semantics.

And tbh I don't understand why the amount of buffers you keep in the
render pipeline side of things matters here at all. But I also haven't
read the details of your driver code.

> 
> > +/*
> > + * Here, we choose a conservative value. A lower value is most likely
> > + * suitable for GPU use-cases.
> > + */
> > +#define NR_FLIPS_IN_FLIGHT_THRESHOLD 16
> > +
> > +#define CLCD_IRQ_NEXTBASE_UPDATE (1u<<2)
> > +
> > +struct pl111_drm_flip_resource;
> > +struct pl111_drm_cursor_plane;
> > +
> > +enum pl111_bo_type {
> > +       PL111_BOT_DMA,
> > +       PL111_BOT_SHM
> > +};
> > +
> > +struct pl111_gem_bo_dma {
> > +       dma_addr_t fb_dev_addr;
> > +       void *fb_cpu_addr;
> > +};
> > +
> > +struct pl111_gem_bo_shm {
> > +       struct page **pages;
> > +       dma_addr_t *dma_addrs;
> > +};
> > +
> > +struct pl111_gem_bo {
> > +       struct drm_gem_object gem_object;
> > +       enum pl111_bo_type type;
> > +       union {
> > +               struct pl111_gem_bo_dma dma;
> > +               struct pl111_gem_bo_shm shm;
> > +       } backing_data;
> > +       struct drm_framebuffer *fb;
> 
> this is at least a bit odd.. normally the fb has ref to the bo(s) and
> not the other way around.  And the same bo could be referenced by
> multiple fb's which would kinda fall down with this approach.

I'd say that's just backwards, framebuffers are created from backing
storage objects (which for a gem based driver is a gem object), not the
other way round. What's this exactly used for?

[snip]

> > +
> > +       /*
> > +        * Used to prevent race between pl111_dma_buf_release and
> > +        * drm_gem_prime_handle_to_fd
> > +        */
> > +       struct mutex export_dma_buf_lock;
> 
> hmm, seems a bit suspicious.. the handle reference should keep the
> object live.  Ie. either drm_gem_object_lookup() will fail because the
> object is gone (userspace has closed it's handle ref and
> dmabuf->release() already dropped it's ref) or it will succeed and
> you'll have a reference to the bo keeping it from going away if the
> release() comes after.

The race is real, I have an evil testcase here which Oopses my kernel. I'm
working on a fix (v1 of my patches is submitted a few weeks back, awaiting
review), but I need to rework a few things since now I've also spotted a
leak or two ;-)

[snip]

> > +static void vsync_worker(struct work_struct *work)
> > +{
> > +       struct pl111_drm_flip_resource *flip_res;
> > +       struct pl111_gem_bo *bo;
> > +       struct pl111_drm_crtc *pl111_crtc;
> > +       struct drm_device *dev;
> > +       int flips_in_flight;
> > +       flip_res > > +               container_of(work, struct pl111_drm_flip_resource, vsync_work);
> > +
> > +       pl111_crtc = to_pl111_crtc(flip_res->crtc);
> > +       dev = pl111_crtc->crtc.dev;
> > +
> > +       DRM_DEBUG_KMS("DRM Finalizing flip_res=%p\n", flip_res);
> > +
> > +       bo = PL111_BO_FROM_FRAMEBUFFER(flip_res->fb);
> > +#ifdef CONFIG_DMA_SHARED_BUFFER_USES_KDS
> > +       if (flip_res->worker_release_kds = true) {
> > +               spin_lock(&pl111_crtc->current_displaying_lock);
> > +               release_kds_resource_and_display(flip_res);
> > +               spin_unlock(&pl111_crtc->current_displaying_lock);
> > +       }
> > +#endif
> > +       /* Release DMA buffer on this flip */
> > +       if (bo->gem_object.export_dma_buf != NULL)
> > +               dma_buf_put(bo->gem_object.export_dma_buf);
> 
> I think you just want to unref the outgoing bo, and let it drop the
> dmabuf ref when the file ref of the imported bo goes.  Or actually, it
> would be better to hold/drop ref's to the fb, rather than the bo.  At
> least this will make things simpler if you ever have multi-planar
> support.

Drivers have no business frobbing around the dma-buf refcount of imported
objects imo, at least if they use all the standard drm prime
infrastructure. And if they're bugs they need to be fixed there, not in
drivers.

[snip]

> > +struct drm_framebuffer *pl111_fb_create(struct drm_device *dev,
> > +                                       struct drm_file *file_priv,
> > +                                       struct drm_mode_fb_cmd2 *mode_cmd)
> > +{
> > +       struct pl111_drm_framebuffer *pl111_fb = NULL;
> > +       struct drm_framebuffer *fb = NULL;
> > +       struct drm_gem_object *gem_obj;
> > +       struct pl111_gem_bo *bo;
> > +
> > +       pr_info("DRM %s\n", __func__);
> > +       gem_obj = drm_gem_object_lookup(dev, file_priv, mode_cmd->handles[0]);
> > +       if (gem_obj = NULL) {
> > +               DRM_ERROR("Could not get gem obj from handle to create fb\n");
> > +               goto out;
> > +       }
> > +
> > +       bo = PL111_BO_FROM_GEM(gem_obj);
> > +       /* Don't even attempt PL111_BOT_SHM, it's not contiguous */
> > +       BUG_ON(bo->type != PL111_BOT_DMA);
> 
> umm, no BUG_ON() is not really a good way to validate userspace input..
> 
>   if (bo->type != ...)
>     return ERR_PTR(-EINVAL);

Yep.

> > +
> > +       switch ((char)(mode_cmd->pixel_format & 0xFF)) {
> > +       case 'Y':
> > +       case 'U':
> > +       case 'V':
> > +       case 'N':
> > +       case 'T':
> 
> perhaps we should instead add a drm_format_is_yuv().. or you could
> (ab)use drm_fb_get_bpp_depth()..

Yeah, I think a new drm_format_is_yuv is asked-for here. Now the bigger
question is why you need this, since the drm core should filter out
formats not in your list of supported ones. Or at least it should ...

Cheers, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch

Re: [RFC 1/1] drm/pl111: Initial drm/kms driver for pl111

[Rob Clark]

On Fri, Aug 9, 2013 at 12:15 PM, Tom Cooksey <tom.cooksey@arm.com> wrote:
> Hi Daniel, Rob.
>
> Thank you both for your reviews - greatly appreciated!
>
>> > > Known issues:
>> > >  * It still includes code to use KDS, which is not going upstream.
>> >
>> > review's on <http://lists.freedesktop.org/archives/dri-devel/2013-
>> > July/042462.html> can't hurt
>> >
>> > although you might consider submitting a reduced functionality driver
>> > w/ KDS bits removed in the mean time.. then when the fence stuff is
>> > merged it is just an incremental patch rather than a whole driver ;-)
>>
>> Yeah, I think the KDS bits and comments need to go first before
>> merginge.
>
> Right, as I expected really. Though as I said we'll probably wait for
> fences to land and switch over to that before asking for it to be
> merged. A pl111 KMS driver with neither KDS nor implicit fences is
> useless to us. Having said that, if someone else would have a use for
> a fence/KDS-less pl111 KMS driver, please let me know!
>

well, it would make it easier to review the patches adding fence
support if it was on top of basic KMS support.  So there still is some
benefit to a fence-less pl111, even if it is just for purposes of git
history and patch review ;-)

>
>> > > +/*
>> > > + * Number of flips allowed in flight at any one time. Any more
>> > > + * flips requested beyond this value will cause the caller to
>> > > + * block until earlier flips have completed.
>> > > + *
>> > > + * For performance reasons, this must be greater than the number
>> > > + * of buffers used in the rendering pipeline. Note that the
>> > > + * rendering pipeline can contain different types of buffer, e.g.:
>> > > + * - 2 final framebuffers
>> > > + * - >2 geometry buffers for GPU use-cases
>> > > + * - >2 vertex buffers for GPU use-cases
>> > > + *
>> > > + * For example, a system using 5 geometry buffers could have 5
>> > > + * flips in flight, and so NR_FLIPS_IN_FLIGHT_THRESHOLD must be
>> > > + * 5 or greater.
>> > > + *
>> > > + * Whilst there may be more intermediate buffers (such as
>> > > + * vertex/geometry) than final framebuffers, KDS is used to
>> > > + * ensure that GPU rendering waits for the next off-screen
>> > > + * buffer, so it doesn't overwrite an on-screen buffer and
>> > > + * produce tearing.
>> > > + */
>> > > +
>> >
>> > fwiw, this is at least different from how other drivers do triple
>> > (or > double) buffering.  In other drivers (intel, omap, and
>> > msm/freedreno, that I know of, maybe others too) the xorg driver
>> > dri2 bits implement the double buffering (ie. send flip event back
>> > to client immediately and queue up the flip and call page-flip
>> > after the pageflip event back from kernel.
>> >
>> > I'm not saying not to do it this way, I guess I'd like to hear
>> > what other folks think.  I kinda prefer doing this in userspace
>> > as it keeps the kernel bits simpler (plus it would then work
>> > properly on exynosdrm or other kms drivers).
>>
>> Yeah, if this is just a sw queue then I don't think it makes sense
>> to have it in the kernel. Afaik the current pageflip interface drm
>> exposes allows one oustanding flip only, and you _must_ wait for
>> the flip complete event before you can submit the second one.
>
> Right, I'll have a think about this. I think our idea was to issue
> enough page-flips into the kernel to make sure that any process
> scheduling latencies on a heavily loaded system don't cause us to
> miss a v_sync deadline. At the moment we issue the page flip from DRI2
> schedule_swap. If we were to move that to the page flip event handler
> of the previous page-flip, we're potentially adding in extra latency.
>
> I.e. Currently we have:
>
> DRI2SwapBuffers
>  - drm_mode_page_flip to buffer B
> DRI2SwapBuffers
>  - drm_mode_page_flip to buffer A (gets queued in kernel)
> ...
> v_sync! (at this point buffer B is scanned out)
>  - release buffer A's KDS resource/signal buffer A's fence
>     - queued GPU job to render next frame to buffer A scheduled on HW
> ...
> GPU interrupt! (at this point buffer A is ready to be scanned out)
>  - release buffer A's KDS resource/signal buffer A's fence
>     - second page flip executed, buffer A's address written to scanout
>       register, takes effect on next v_sync.
>
>
> So in the above, after X receives the second DRI2SwapBuffers, it
> doesn't need to get scheduled again for the next frame to be both
> rendered by the GPU and issued to the display for scanout.

well, this is really only an issue if you are so loaded that you don't
get a chance to schedule for ~16ms.. which is pretty long time.  If
you are triple buffering, it should not end up in the critical path
(since the gpu already has the 3rd buffer to start on the next frame).
 And, well, if you do it all in the kernel you probably need to toss
things over to a workqueue anyways.

>
>
> If we were to move to a user-space queue, I think we have something
> like this:
>
> DRI2SwapBuffers
>  - drm_mode_page_flip to buffer B
> DRI2SwapBuffers
>  - queue page flip to buffer A in DDX
> ...
> v_sync! (at this point buffer B is scanned out)
>  - release buffer A's KDS resource/signal buffer A's fence
>     - queued GPU job to render next frame to buffer A scheduled on HW
>  - Send page flip event to X
> ...
> GPU interrupt! (at this point buffer A is ready to be scanned out)
>  - Release buffer A's KDS resource/signal buffer A's fence - but nothing
>    is waiting on it....
> ...
> X gets scheduled, runs page flip handler
>  - drm_mode_page_flip to buffer A
>    - buffer A's address written to scanout register, takes effect on
>      next v_sync.
>
>
> So here, X must get scheduled again after processing the second
> DRI2SwapBuffers in order to have the next frame displayed. This
> increases the likely-hood that we're not able to write the address of
> buffer A to the display HW's scan-out buffer in time to catch the next
> v_sync, especially on a loaded system.
>
> Anyway, I think that's our rational for keeping the queue in kernel
> space, but I don't see there's much value in queuing more than 2 page
> flips in kernel space.
>
>> Ofc if your hardware as a hw-based flip queue (maybe even with frame
>> targets) that's a different matter, but currently we don't have a drm
>> interface to expose this. I'd say for merging the basic driver first we
>> should go with the existing simple pageflip semantics.
>
> Sure - I think it would mean slightly increased jank, but probably
> something we can address later.
>
>
>> > > +enum pl111_bo_type {
>> > > +       PL111_BOT_DMA,
>> > > +       PL111_BOT_SHM
>> > > +};
>> > > +
>> > > +struct pl111_gem_bo_dma {
>> > > +       dma_addr_t fb_dev_addr;
>> > > +       void *fb_cpu_addr;
>> > > +};
>> > > +
>> > > +struct pl111_gem_bo_shm {
>> > > +       struct page **pages;
>> > > +       dma_addr_t *dma_addrs;
>> > > +};
>> > > +
>> > > +struct pl111_gem_bo {
>> > > +       struct drm_gem_object gem_object;
>> > > +       enum pl111_bo_type type;
>> > > +       union {
>> > > +               struct pl111_gem_bo_dma dma;
>> > > +               struct pl111_gem_bo_shm shm;
>> > > +       } backing_data;
>> > > +       struct drm_framebuffer *fb;
>> >
>> > this is at least a bit odd.. normally the fb has ref to the bo(s) and
>> > not the other way around.  And the same bo could be referenced by
>> > multiple fb's which would kinda fall down with this approach.
>>
>> I'd say that's just backwards, framebuffers are created from backing
>> storage objects (which for a gem based driver is a gem object), not the
>> other way round. What's this exactly used for?
>
> Yup.
>
>
>> > > +static void vsync_worker(struct work_struct *work)
>> > > +{
>> > > +       struct pl111_drm_flip_resource *flip_res;
>> > > +       struct pl111_gem_bo *bo;
>> > > +       struct pl111_drm_crtc *pl111_crtc;
>> > > +       struct drm_device *dev;
>> > > +       int flips_in_flight;
>> > > +       flip_res >> > > +               container_of(work, struct pl111_drm_flip_resource,
>> > > +                            vsync_work);
>> > > +
>> > > +       pl111_crtc = to_pl111_crtc(flip_res->crtc);
>> > > +       dev = pl111_crtc->crtc.dev;
>> > > +
>> > > +       DRM_DEBUG_KMS("DRM Finalizing flip_res=%p\n", flip_res);
>> > > +
>> > > +       bo = PL111_BO_FROM_FRAMEBUFFER(flip_res->fb);
>> > > +#ifdef CONFIG_DMA_SHARED_BUFFER_USES_KDS
>> > > +       if (flip_res->worker_release_kds = true) {
>> > > +               spin_lock(&pl111_crtc->current_displaying_lock);
>> > > +               release_kds_resource_and_display(flip_res);
>> > > +               spin_unlock(&pl111_crtc->current_displaying_lock);
>> > > +       }
>> > > +#endif
>> > > +       /* Release DMA buffer on this flip */
>> > > +       if (bo->gem_object.export_dma_buf != NULL)
>> > > +               dma_buf_put(bo->gem_object.export_dma_buf);
>> >
>> > I think you just want to unref the outgoing bo, and let it drop the
>> > dmabuf ref when the file ref of the imported bo goes.  Or actually,
>> > it would be better to hold/drop ref's to the fb, rather than the bo.
>> > At least this will make things simpler if you ever have multi-planar
>> > support.
>>
>> Drivers have no business frobbing around the dma-buf refcount of
>> imported objects imo, at least if they use all the standard drm
>> prime infrastructure. And if they're bugs they need to be fixed
>> there, not in drivers.
>
> Good point. I guess the fb holds a ref on the bo and the bo holds a
> ref on the imported dma_buf. Don't know what this was for...
>
>
>> > > +       BUG_ON(bo->type != PL111_BOT_DMA);
>> >
>> > umm, no BUG_ON() is not really a good way to validate userspace
>> > input..
>>
>> Yep.
>
> :-D
>
>
>> > > +
>> > > +       switch ((char)(mode_cmd->pixel_format & 0xFF)) {
>> > > +       case 'Y':
>> > > +       case 'U':
>> > > +       case 'V':
>> > > +       case 'N':
>> > > +       case 'T':
>> >
>> > perhaps we should instead add a drm_format_is_yuv().. or you could
>> > (ab)use drm_fb_get_bpp_depth()..
>>
>> Yeah, I think a new drm_format_is_yuv is asked-for here. Now the bigger
>> question is why you need this, since the drm core should filter out
>> formats not in your list of supported ones. Or at least it should ...
>
> Probably unnecessary belts & braces. I'll see if I can find some DRM
> test which tries to create an fb using a yuv format and see where, if
> anywhere, it gets rejected.
>

fwiw, it should fail, not when you create the (at least for most
drivers) but when you try to attach it to a plane or crtc.

If we had primary plane's in drm core, we could do a bit better error
checking in the core and bail out on fb creation for a format that no
crtc/plane could scanout.

BR,
-R

>
> Thanks again!!
>
>
> Cheers,
>
> Tom
>
>
>
>
>

Re: [RFC 1/1] drm/pl111: Initial drm/kms driver for pl111

[Daniel Vetter]

On Fri, Aug 09, 2013 at 12:34:55PM -0400, Rob Clark wrote:
> On Fri, Aug 9, 2013 at 12:15 PM, Tom Cooksey <tom.cooksey@arm.com> wrote:
> >> > fwiw, this is at least different from how other drivers do triple
> >> > (or > double) buffering.  In other drivers (intel, omap, and
> >> > msm/freedreno, that I know of, maybe others too) the xorg driver
> >> > dri2 bits implement the double buffering (ie. send flip event back
> >> > to client immediately and queue up the flip and call page-flip
> >> > after the pageflip event back from kernel.
> >> >
> >> > I'm not saying not to do it this way, I guess I'd like to hear
> >> > what other folks think.  I kinda prefer doing this in userspace
> >> > as it keeps the kernel bits simpler (plus it would then work
> >> > properly on exynosdrm or other kms drivers).
> >>
> >> Yeah, if this is just a sw queue then I don't think it makes sense
> >> to have it in the kernel. Afaik the current pageflip interface drm
> >> exposes allows one oustanding flip only, and you _must_ wait for
> >> the flip complete event before you can submit the second one.
> >
> > Right, I'll have a think about this. I think our idea was to issue
> > enough page-flips into the kernel to make sure that any process
> > scheduling latencies on a heavily loaded system don't cause us to
> > miss a v_sync deadline. At the moment we issue the page flip from DRI2
> > schedule_swap. If we were to move that to the page flip event handler
> > of the previous page-flip, we're potentially adding in extra latency.
> >
> > I.e. Currently we have:
> >
> > DRI2SwapBuffers
> >  - drm_mode_page_flip to buffer B
> > DRI2SwapBuffers
> >  - drm_mode_page_flip to buffer A (gets queued in kernel)
> > ...
> > v_sync! (at this point buffer B is scanned out)
> >  - release buffer A's KDS resource/signal buffer A's fence
> >     - queued GPU job to render next frame to buffer A scheduled on HW
> > ...
> > GPU interrupt! (at this point buffer A is ready to be scanned out)
> >  - release buffer A's KDS resource/signal buffer A's fence
> >     - second page flip executed, buffer A's address written to scanout
> >       register, takes effect on next v_sync.
> >
> >
> > So in the above, after X receives the second DRI2SwapBuffers, it
> > doesn't need to get scheduled again for the next frame to be both
> > rendered by the GPU and issued to the display for scanout.
> 
> well, this is really only an issue if you are so loaded that you don't
> get a chance to schedule for ~16ms.. which is pretty long time.  If
> you are triple buffering, it should not end up in the critical path
> (since the gpu already has the 3rd buffer to start on the next frame).
>  And, well, if you do it all in the kernel you probably need to toss
> things over to a workqueue anyways.

Just a quick comment on the kernel flip queue issue.

16 ms scheduling latency sounds awful but totally doable with a less than
stellar ddx driver going into limbo land and so preventing your single
threaded X from doing more useful stuff. Is this really the linux
scheduler being stupid?

At least my impression was that the hw/kernel flip queue is to save power
so that you can queue up a few frames and everything goes to sleep for
half a second or so (at 24fps or whatever movie your showing). Needing to
schedule 5 frames ahead with pageflips under load is just guaranteed to
result in really horrible interactivity and so awful user experience ...
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch

Re: [RFC 1/1] drm/pl111: Initial drm/kms driver for pl111

[Rob Clark]

On Fri, Aug 9, 2013 at 1:31 PM, Tom Cooksey <tom.cooksey@arm.com> wrote:
>> > > So in the above, after X receives the second DRI2SwapBuffers, it
>> > > doesn't need to get scheduled again for the next frame to be both
>> > > rendered by the GPU and issued to the display for scanout.
>> >
>> > well, this is really only an issue if you are so loaded that you
>> > don't get a chance to schedule for ~16ms.. which is pretty long time.
>
> Yes - it really is 16ms (minus interrupt/workqueue latency) isn't it?
> Hmmm, that does sound very long. Will try out some experiments and see.
>

yeah

>
>> > If you are triple buffering, it should not end up in the critical
>> > path (since the gpu already has the 3rd buffer to start on the next
>> > frame). And, well, if you do it all in the kernel you probably need
>> > to toss things over to a workqueue anyways.
>>
>> Just a quick comment on the kernel flip queue issue.
>>
>> 16 ms scheduling latency sounds awful but totally doable with a less
>> than stellar ddx driver going into limbo land and so preventing your
>> single threaded X from doing more useful stuff. Is this really the
>> linux scheduler being stupid?
>
> Ahahhaaa!! Yes!!! Really good point. We generally don't have 2D HW and
> so rely on pixman to perform all 2D operations which does indeed tie
> up that thread for fairly long periods of time.
>
> We've had internal discussions about introducing a thread (gulp) in
> the DDX to off-load drawing operations to. I think we were all a bit
> scared by that idea though.
>

thread does sound a bit scary.. it probably could be done if you treat
it like a virtual cpu and have WaitMarker or PrepareAccess for sw
fallbacks synchronize properly..

I bet you'd be much better off just making non-scanout pixmaps cached
and doing cache sync ops when needed for dri2 buffers.  Sw fallbacks
on uncached buffers probably aren't exactly the hot ticket.

>
> BTW: I wasn't suggesting it was the linux scheduler being stupid, just
> that there is sometimes lots of contention over the CPU cores and X
> is just one thread among many wanting to run.
>
>
>> At least my impression was that the hw/kernel flip queue is to save
>> power so that you can queue up a few frames and everything goes to
>> sleep for half a second or so (at 24fps or whatever movie your
>> showing). Needing to schedule 5 frames ahead with pageflips under
>> load is just guaranteed to result in really horrible interactivity
>> and so awful user experience
>
> Agreed. There's always a tradeoff between tolerance to variable frame
> rendering time/system latency (lot of buffers) and UI latency (few
> buffers).
>
> As a side note, video playback is one use-case for explicit sync
> objects which implicit/buffer-based sync doesn't handle: Queue up lots
> of video frames for display, but mark those "display buffer"
> operations as depending on explicit sync objects which get signalled
> by the audio clock. Not sure Android actually does that yet though.
> Anyway, off topic.
>

w/ dmafence, rather than explicit fences, I suppose you could add some
way to queue the buffer to the audio device and have the audio device
signal the fence.  I suppose it does sound a bit funny for ALSA to
have a DMA_BUF_AV_SYNC ioctl for this sort of case?

I don't think there is anything like it in EGL, but there is
oml_sync_control extension for more precise control of presentation
time.  But this is all implemented in userspace and doesn't really
work out w/ >double buffering.  This is part of the reason for the
timing information in vblank events.  Of course it doesn't have any
tie in to audio subsystem, but in practice this really shouldn't be
needed.  Audio samples are either rendered at a very predictable rate,
or sound like sh** with lots of pops and cut outs.

BR,
-R

>
> Cheers,
>
> Tom
>
>
>
>
>

Re: [RFC 1/1] drm/pl111: Initial drm/kms driver for pl111

[Rob Clark]

On Tue, Aug 13, 2013 at 10:35 AM, Tom Cooksey <tom.cooksey@arm.com> wrote:
>> > > > So in the above, after X receives the second DRI2SwapBuffers, it
>> > > > doesn't need to get scheduled again for the next frame to be both
>> > > > rendered by the GPU and issued to the display for scanout.
>> > >
>> > > well, this is really only an issue if you are so loaded that you
>> > > don't get a chance to schedule for ~16ms.. which is pretty long
>> > > time.
>>
>> Yes - it really is 16ms (minus interrupt/workqueue latency) isn't it?
>> Hmmm, that does sound very long. Will try out some experiments and see.
>
> We're looking at moving the flip queue into the DDX driver, however
> it's not as straight-forward as I thought. With the current design,
> all rate-limiting happens on the client side. So even if you only have
> double buffering, using KDS you can queue up as many asynchronous
> GPU-render/scan-out pairs as you want. It's up to EGL in the client
> application to figure out there's a lot of frames in-flight and so
> should probably block the application's render thread in
> eglSwapBuffers to let the GPU and/or display catch up a bit.
>
> If we only allow a single outstanding page-flip job in DRM, there'd be
> a race if we returned a buffer to the client which had an outstanding
> page-flip queued up in the DDX: The client could issue a render job to
> the buffer just as the DDX processed the page-flip from the queue,
> making the scan-out block until the GPU rendered the next frame. It
> would also mean the previous frame would have been lost as it never
> got scanned out before the GPU rendered the next-next frame to it.

You wouldn't unconditionally send the swap-done event to the client
when the queue is "full".  (Well, for omap and msm, the queue depth is
1, for triple buffer.. I think usually you don't want to do more than
triple buffer.)  The client would never get a buffer that wasn't
already done being scanned out, so there shouldn't be a race.

Basically, in DDX, when you get a ScheduleSwap, there are two cases:
1) you are still waiting for previous page-flip event from kernel, in
which case you queue the swap and don't immediately send the event
back to the client.  When the previous page flip completes, you
schedule the new one and then send back the event to the client.
2) you are not waiting for a previous page-flip, in which case you
schedule the new page-flip and send the event to the client.

(I hope that is clear.. I suppose maybe a picture here would help, but
sadly I don't have anything handy)

The potential drawback is that the client doesn't necessarily have any
control over double vs triple buffering.  In omap ddx I solved this by
adding a special attachment point that the client could request to
tell the DDX that it wanted to triple buffer.  But the upside is that
you never need to worry about a modeset when there is more than one
flip pending.

BR,
-R

> So instead, I think we'll have to block (suspend?) a client in
> ScheduleSwap if the next buffer it would obtain with DRI2GetBuffers
> has an outstanding page-flip in the user-space queue. We then wake
> the client up again _after_ we get the page-flip event for the
> previous page flip and have issued the page-flip to the next buffer
> to the DRM. That way the DRM display driver has already registered its
> intention to use the buffer with KDS before the client ever gets hold
> of it.
>
> Note: I say KDS here, but I assume the same issues will apply on any
> implicit buffer-based synchronization. I.e. dma-fence.
>
> It's not really a problem I don't think, but mention it to see if you
> can see a reason why the above wouldn't work before we go and
> implement it - it's a fairly big change to the DDX. Can you see any
> issues with it? PrepareAccess gets interesting...
>
>
>
> Cheers,
>
> Tom
>
>
>
>
>