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

[Christopher James Halse Rogers <christopher.halse.rogers@canonical.com>]

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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.

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