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

["Christian König" <christian.koenig@amd.com>]

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Am 05.04.2017 um 13:13 schrieb Christopher James Halse Rogers:
> On Wed, Apr 5, 2017 at 8:14 PM Lucas Stach <l.stach@pengutronix.de 
> <mailto:l.stach@pengutronix.de>> wrote:
>
>     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 <mailto:daniel@ffwll.ch>> wrote:
>     > > >
>     > > >         On Tue, Apr 4, 2017 at 12:43 PM, Lucas Stach
>     > > >         <l.stach@pengutronix.de
>     <mailto: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?
>
>
> At least some nouveau and AMD devs seem to think that their hardware 
> is capable of doing it. Shrug.

Wow, wait a second. Recent AMD GPU can scanout from system memory, 
that's true.

But you need to met quite a bunch of special allocation requirements to 
do this.

When we are talking about sharing between AMD GPUs, (e.g. both exporter 
and importer are AMD hardware) than that might work.

But I think it's unrealistic that an imported BO (created by an external 
driver stack) will ever meet those requirements.

Christian.

>
>     > >
>     > > 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 would *love* for the driver to copy the pages for me into VRAM for 
> scanout, rather than me having to spin up an EGL context and run the 
> trivial blitting shader across an EGLImage.
>
> Are you offering to do it? :)
>
> I'll still need to, for the short term, assume that only i915 can do 
> this without breaking, though.
>
>
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