Re: [PATCH v3] drm: Use USB controller's DMA mask when importing dmabufs

[Sudip Mukherjee <sudipm.mukherjee@gmail.com>]

On Tue, Feb 23, 2021 at 02:09:58PM +0100, Greg KH wrote:
> On Tue, Feb 23, 2021 at 01:51:09PM +0100, Thomas Zimmermann wrote:
> > Hi
> > 
> > Am 23.02.21 um 13:24 schrieb Greg KH:
> > > On Tue, Feb 23, 2021 at 01:14:30PM +0100, Daniel Vetter wrote:
> > > > On Tue, Feb 23, 2021 at 1:02 PM Greg KH <gregkh@linuxfoundation.org> wrote:
> > > > > 
> > > > > On Tue, Feb 23, 2021 at 12:46:20PM +0100, Daniel Vetter wrote:
> > > > > > On Tue, Feb 23, 2021 at 12:19:56PM +0100, Greg KH wrote:
> > > > > > > On Tue, Feb 23, 2021 at 11:58:42AM +0100, Thomas Zimmermann wrote:
> > > > > > > > USB devices cannot perform DMA and hence have no dma_mask set in their
> > > > > > > > device structure. Importing dmabuf into a USB-based driver fails, which
> > > > > > > > break joining and mirroring of display in X11.
> > > > > > > > 
> > > > > > > > For USB devices, pick the associated USB controller as attachment device,
> > > > > > > > so that it can perform DMA. If the DMa controller does not support DMA
> > > > > > > > transfers, we're aout of luck and cannot import.
> > > > > > > > 
> > > > > > > > Drivers should use DRM_GEM_SHMEM_DROVER_OPS_USB to initialize their
> > > > > > > > instance of struct drm_driver.
> > > > > > > > 
> > > > > > > > Tested by joining/mirroring displays of udl and radeon un der Gnome/X11.
> > > > > > > > 
> > > > > > > > v3:
> > > > > > > >    * drop gem_create_object
> > > > > > > >    * use DMA mask of USB controller, if any (Daniel, Christian, Noralf)
> > > > > > > > v2:
> > > > > > > >    * move fix to importer side (Christian, Daniel)
> > > > > > > >    * update SHMEM and CMA helpers for new PRIME callbacks
> > > > > > > > 
> > > > > > > > Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
> > > > > > > > Fixes: 6eb0233ec2d0 ("usb: don't inherity DMA properties for USB devices")
> > > > > > > > Cc: Christoph Hellwig <hch@lst.de>
> > > > > > > > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > > > > > > > Cc: Johan Hovold <johan@kernel.org>
> > > > > > > > Cc: Alan Stern <stern@rowland.harvard.edu>
> > > > > > > > Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> > > > > > > > Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> > > > > > > > Cc: Mathias Nyman <mathias.nyman@linux.intel.com>
> > > > > > > > Cc: Oliver Neukum <oneukum@suse.com>
> > > > > > > > Cc: Thomas Gleixner <tglx@linutronix.de>
> > > > > > > > Cc: <stable@vger.kernel.org> # v5.10+
> > > > > > > > ---
<snip>
> > > > > > > 
> > > > > > > There shouldn't be anything "special" about a DRM driver that needs this
> > > > > > > vs. any other driver that might want to know about DMA things related to
> > > > > > > a specific USB device.  Why isn't this an issue with the existing
> > > > > > > storage or v4l USB devices?
> > > > > > 
> > > > > > The trouble is that this is a regression fix for 5.9, because the dma-api
> > > > > > got more opinionated about what it allows. The proper fix is a lot more
> > > > > > invasive (we essentially need to rework the drm_prime.c to allow dma-buf
> > > > > > importing for just cpu access), and that's a ton more invasive than just a
> > > > > > small patch with can stuff into stable kernels.
> > > > > > 
> > > > > > This here is ugly, but it should at least get rid of black screens again.
> > > > > > 
> > > > > > I think solid FIXME comment explaining the situation would be good.
> > > > > 
> > > > > Why can't I take a USB patch for a regression fix?  Is drm somehow
> > > > > stand-alone that you make changes here that should belong in other
> > > > > subsystems?
> > > > > 
> > > > > {hint, it shouldn't be}
> > > > > 
> > > > > When you start poking in the internals of usb controller structures,
> > > > > that logic belongs in the USB core for all drivers to use, not in a
> > > > > random tiny subsystem where no USB developer will ever notice it?  :)
> > > > 
> > > > Because the usb fix isn't the right fix here, it's just the duct-tape.
> > > > We don't want to dig around in these internals, it's just a convenient
> > > > way to shut up the dma-api until drm has sorted out its confusion.
> > > > 
> > > > We can polish the turd if you want, but the thing is, it's still a turd ...
> > > > 
> > > > The right fix is to change drm_prime.c code to not call dma_map_sg
> > > > when we don't need it. The problem is that roughly 3 layers of code
> > > > (drm_prime, dma-buf, gem shmem helpers) are involved. Plus, since
> > > > drm_prime is shared by all drm drivers, all other drm drivers are
> > > > impacted too. We're not going to be able to cc: stable that kind of
> > > > stuff. Thomas actually started with that series, until I pointed out
> > > > how bad things really are.
> > > > 
> > > > And before you ask: The dma-api change makes sense, and dma-api vs drm
> > > > relations are strained since years, so we're not going ask for some
> > > > hack there for our abuse to paper over the regression. I've been in
> > > > way too many of those threads, only result is shouting and failed
> > > > anger management.
> > > 
> > > Let's do it right.  If this is a regression from 5.9, it isn't a huge
> > > one as that kernel was released last October.  I don't like to see this
> > > messing around with USB internals in non-USB-core code please.
> > 
> > I get
> > 
> >  > git tag --contains 6eb0233ec2d0
> >  ...
> >  v5.10-rc1
> >  ...
> 
> Ah, I thought you said 5.9 was when the problem happened, ok, yes, 5.10
> is slow to get out to a lot of distros that do not update frequently :(

iiuc, Debian Bullseye release will be having v5.10.y.

Ben ?


--
Regards
Sudip