Re: [PATCH 1/2] drm: Add DRM_CAP_PRIME_SCANOUT.

[Lucas Stach <l.stach@pengutronix.de>]

Am Mittwoch, den 05.04.2017, 11:59 +0200 schrieb Daniel Vetter:
> On Wed, Apr 05, 2017 at 10:15:44AM +0200, Lucas Stach wrote:
> > Am Mittwoch, den 05.04.2017, 00:20 +0000 schrieb Christopher James Halse
> > Rogers:
> > > 
> > > 
> > > On Tue, Apr 4, 2017 at 9:53 PM Daniel Vetter <daniel@ffwll.ch> wrote:
> > > 
> > >         On Tue, Apr 4, 2017 at 12:43 PM, Lucas Stach
> > >         <l.stach@pengutronix.de> wrote:
> > >         >> If I could guarantee that I'd only ever run on
> > >         4.13-or-later kernels
> > >         >> (I think that's when the previous patches will land?), then
> > >         this would
> > >         >> indeed be mostly unnecessary. It would save me a bunch of
> > >         addfb calls
> > >         >> that would predictably fail, but they're cheap.
> > >         >
> > >         > I don't think we ever had caps for "things are working now,
> > >         as they are
> > >         > supposed to". i915 wasn't properly synchronizing on foreign
> > >         fences for a
> > >         > long time, yet we didn't gain a cap for "cross device sync
> > >         works now".
> > >         >
> > >         > If your distro use-case relies on those things working it's
> > >         probably
> > >         > best to just backport the relevant fixes.
> > >         
> > >         The even better solution for this is to push the backports
> > >         through
> > >         upstream -stable kernels. This stuff here is simple enough
> > >         that we can
> > >         do it. Same could have been done for the fairly minimal
> > >         fencing fixes
> > >         for i915 (at least some of them, the ones in the page-flip).
> > >         
> > >         Otherwise we'll end up with tons IM_NOT_BUGGY and
> > >         IM_SLIGHTLY_LESS_BUGGY and
> > >         IM_NOT_BUGGY_EXCEPT_THIS_BOTCHED_BACKPORT
> > >         flags where no one at all knows what they mean, usage between
> > >         different drivers and different userspace is entirely
> > >         inconsistent and
> > >         they just all add to the confusion. They're just bugs, lets
> > >         treat them
> > >         like that.
> > > 
> > > 
> > > It's not *quite* DRM_CAP_PRIME_SCANOUT_NOT_BUGGY - while the relevant
> > > hardware allegedly supports it, nouveau/radeon/amdgpu don't do scanout
> > > of GTT, so the lack of this cap indicates that there's no point in
> > > trying to call addfb2.
> > > 
> > > 
> > > But calling addfb2 and it failing is not expensive, so this is rather
> > > niche.
> > > 
> > > 
> > > In practice I can just restrict attempting to scanout of imported
> > > buffers to i915, as that's the only driver that it'll work on. By the
> > > time nouveau/radeon/amdgpu get patches to scanout of GTT the fixes
> > > should be old enough that I don't need to care about unfixed kernels.
> > > 
> > So given that most discreet hardware won't ever be able to scanout from
> > GTT (latency and iso requirements will be hard to meet), can't we just
> > fix the case of the broken prime sharing when migrating to VRAM?
> > 
> > I'm thinking about attaching an exclusive fence to the dma-buf when the
> > migration to VRAM happens, then when the GPU is done with the buffer we
> > can either write back any changes to GTT, or just drop the fence in case
> > the GPU didn't modify the buffer.
> 
> We could, but someone needs to type the code for it. There's also the
> problem that you need to migrate back, and then doing all that behind
> userspaces back might not be the best idea.

Drivers with separate VRAM and GTT are already doing a lot of migration
behind the userspaces back. The only issue with dma-buf migration to
VRAM is that you probably don't want to migrate the pages, but duplicate
them in VRAM, doubling memory consumption with possible OOM. But then
you could alloc the memory on addfb where you are able to return proper
errors.

I guess what I'm saying is that userspace really should check if addfb
for imported buffers works and base its decisions on that, not some
arbitrary "is this driver XY" or "does it have magic DRM_CAP".  This way
we can enable transparent VRAM migration (making addfb on them work) if
someone is interested in this, without further changes to the UAPI.

Regards,
Lucas

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