Re: [PATCH v2 12/26] drm/exynos: Split manager/display/subdrv

[Thierry Reding <thierry.reding@gmail.com>]

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On Tue, Oct 29, 2013 at 08:46:03PM -0700, Stéphane Marchesin wrote:
> On Tue, Oct 29, 2013 at 1:50 PM, Tomasz Figa <tomasz.figa@gmail.com> wrote:
> 
> > Hi Sean,
> >
> > On Tuesday 29 of October 2013 16:36:47 Sean Paul wrote:
> > > On Mon, Oct 28, 2013 at 7:13 PM, Tomasz Figa <tomasz.figa@gmail.com>
> > wrote:
> > > > Hi,
> > > >
> > > > On Wednesday 23 of October 2013 12:09:06 Sean Paul wrote:
> > > >> On Wed, Oct 23, 2013 at 11:53 AM, Dave Airlie <airlied@gmail.com>
> > wrote:
> > > >> >>>>> I think we need to start considering a framework where
> > > >> >>>>> subdrivers
> > > >> >>>>> just
> > > >> >>>>> add drm objects themselves, then the toplevel node is
> > > >> >>>>> responsible
> > > >> >>>>> for
> > > >> >>>>> knowing that everything for the current configuration is
> > > >> >>>>> loaded.
> > > >> >>>>
> > > >> >>>> It would be nice to specify the various pieces in dt, then have
> > > >> >>>> some
> > > >> >>>> type of drm notifier to the toplevel node when everything has
> > > >> >>>> been
> > > >> >>>> probed. Doing it in the dt would allow standalone
> > > >> >>>> drm_bridge/drm_panel
> > > >> >>>> drivers to be transparent as far as the device's drm driver is
> > > >> >>>> concerned.
> > > >> >>>>
> > > >> >>>> Sean
> > > >> >>>>
> > > >> >>>>> I realise we may need to make changes to the core drm to allow
> > > >> >>>>> this
> > > >> >>>>> but we should probably start to create a strategy for fixing
> > > >> >>>>> the
> > > >> >>>>> API
> > > >> >>>>> issues that this throws up.
> > > >> >>>>>
> > > >> >>>>> Note I'm not yet advocating for dynamic addition of nodes once
> > > >> >>>>> the
> > > >> >>>>> device is in use, or removing them.
> > > >> >>>
> > > >> >>> I do wonder if we had some sort of tag in the device tree for any
> > > >> >>> nodes
> > > >> >>> involved in the display, and the core drm layer would read that
> > > >> >>> list,
> > > >> >>> and when every driver registers tick things off, and when the
> > > >> >>> last
> > > >> >>> one
> > > >> >>> joins we get a callback and init the drm layer, we'd of course
> > > >> >>> have
> > > >> >>> the
> > > >> >>> basic drm layer setup prior to that so we can add the objects as
> > > >> >>> the
> > > >> >>> drivers load. It might make development a bit trickier as you'd
> > > >> >>> need
> > > >> >>> to make sure someone claimed ownership of all the bits for init
> > > >> >>> to
> > > >> >>> proceed.>>
> > > >> >>
> > > >> >> Yeah, that's basically what the strawman looked like in my head.
> > > >> >>
> > > >> >> Instead of a property in each node, I was thinking of having a
> > > >> >> separate gfx pipe nodes that would have dt pointers to the various
> > > >> >> pieces involved in that pipe. This would allow us to associate
> > > >> >> standalone entities like bridges and panels with encoders in dt
> > > >> >> w/o
> > > >> >> doing it in the drm code. I *think* this should be Ok with the dt
> > > >> >> guys
> > > >> >> since it is still describing the hardware, but I think we'd have
> > > >> >> to
> > > >> >> make sure it wasn't drm-specific.
> > > >> >
> > > >> > I suppose the question is how much dynamic pipeline construction
> > > >> > there
> > > >> > is,
> > > >> >
> > > >> > even on things like radeon and i915 we have dynamic clock generator
> > > >> > to
> > > >> > crtc to encoder setups, so I worry about static lists per-pipe, so
> > > >> > I
> > > >> > still think just stating all these devices are needed for display
> > > >> > and
> > > >> > a list of valid interconnections between them, then we can have the
> > > >> > generic code model drm crtc/encoders/connectors on that list, and
> > > >> > construct the possible_crtcs /possible_clones etc at that stage.
> > > >>
> > > >> I'm, without excuse, hopeless at devicetree, so there are probably
> > > >> some violations, but something like:
> > > >>
> > > >> display-pipelines {
> > > >>
> > > >>   required-elements = <&bridge-a &panel-a &encoder-x &encoder-y
> > > >>
> > > >> &crtc-x &crtc-y>;
> > > >>
> > > >>   pipe1 {
> > > >>
> > > >>     bridge = <&bridge-a>;
> > > >>     encoder = <&encoder-x>;
> > > >>     crtc = <&crtc-y>;
> > > >>
> > > >>   };
> > > >>   pipe2 {
> > > >>
> > > >>     encoder = <&encoder-x>;
> > > >>     crtc = <&crtc-x>;
> > > >>
> > > >>   };
> > > >>   pipe3 {
> > > >>
> > > >>     panel = <&panel-a>;
> > > >>     encoder = <&encoder-y>;
> > > >>     crtc = <&crtc-y>;
> > > >>
> > > >>   };
> > > >>
> > > >> };
> > > >>
> > > >> I'm tempted to add connector to the pipe nodes as well, so it's
> > > >> obvious which connector should be used in cases where multiple
> > > >> entities in the pipe implement drm_connector. However, I'm not sure
> > > >> if
> > > >> that would be NACKed by dt people.
> > > >>
> > > >> I'm also not sure if there are too many combinations for i915 and
> > > >> radeon to make this unreasonable. I suppose those devices could just
> > > >> use required-elements and leave the pipe nodes out.
> > > >
> > > > Just to put my two cents in, as one of the people involved into "the
> > > > device tree movement", I'd say that instead of creating artifical
> > > > entities, such as display-pipelines and all of the pipeX'es, device
> > > > tree should represent relations between nodes.
> > > >
> > > > According to the generic DT bindings we already have for
> > > > video-interfaces
> > > > [1] your example connection layout would look as follows:
> > > Hi Tomasz
> > > Thanks for sending this along.
> > >
> > > I think the general consensus is that each drm driver should be
> > > implemented as a singular driver. That is, N:1 binding to driver
> > > mapping, where there are N IP blocks. Optional devices (such as
> > > bridges, panels) probably make sense to spin off as standalone
> > > drivers.
> >
> > I believe this is a huge step backwards from current kernel design
> > standards, which prefer modularity.
> >
> > Having multiple IPs being part of the DRM subsystem in a SoC, it would be
> > nice to have the possibility to compile just a subset of support for them
> > into the kernel and load rest of them as modules. (e.g. basic LCD
> > controller on a mobile phone compiled in and external connectors, like
> > HDMI as modules)
> >
> > Not even saying that from development perspective, a huge single driver
> > would be much more difficult to test and debug, than several smaller
> > drivers, which could be developed separately.
> >
> 
> This is the opposite of our experience, though. A series of small drivers
> like what's in drm/exynos can become really tedious/difficult to
> coordinate. If you have separate drivers, everything needs to be
> synchronized, but also has to account for potentially different loading
> order.
> 
> It seems you're only thinking about the basic case, where you only support
> a single resolution, no dpms, no suspend to ram... But when you want full
> fledged functionality, then the issues I described become much more
> prevalent.

I fail to see how this is relevant here. It's fairly clear that even if
a DRM driver is composed of more than a single platform driver, there's
still a single point of coordination (the DRM driver). How does that
have any impact on what features the driver can support? All of the
features will be exposed via DRM, whether you use multiple drivers or a
single monolithic one underneath is completely irrelevant.

Thierry

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