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Message-ID: <20230224112630.313d7b76@eldfell>
Date: Fri, 24 Feb 2023 11:26:30 +0200
From: Pekka Paalanen <ppaalanen@...il.com>
To: Rob Clark <robdclark@...il.com>
Cc: Luben Tuikov <luben.tuikov@....com>,
Rob Clark <robdclark@...omium.org>,
Gustavo Padovan <gustavo@...ovan.org>,
Tvrtko Ursulin <tvrtko.ursulin@...el.com>,
Christian König <ckoenig.leichtzumerken@...il.com>,
Michel Dänzer <michel@...nzer.net>,
open list <linux-kernel@...r.kernel.org>,
dri-devel@...ts.freedesktop.org,
Christian König <christian.koenig@....com>,
"moderated list:DMA BUFFER SHARING FRAMEWORK"
<linaro-mm-sig@...ts.linaro.org>,
Rodrigo Vivi <rodrigo.vivi@...el.com>,
Alex Deucher <alexander.deucher@....com>,
freedreno@...ts.freedesktop.org,
Sumit Semwal <sumit.semwal@...aro.org>,
"open list:SYNC FILE FRAMEWORK" <linux-media@...r.kernel.org>
Subject: Re: [PATCH v4 06/14] dma-buf/sync_file: Support (E)POLLPRI
On Thu, 23 Feb 2023 10:51:48 -0800
Rob Clark <robdclark@...il.com> wrote:
> On Thu, Feb 23, 2023 at 1:38 AM Pekka Paalanen <ppaalanen@...il.com> wrote:
> >
> > On Wed, 22 Feb 2023 07:37:26 -0800
> > Rob Clark <robdclark@...il.com> wrote:
> >
> > > On Wed, Feb 22, 2023 at 1:49 AM Pekka Paalanen <ppaalanen@...il.com> wrote:
...
> > > > On another matter, if the application uses SET_DEADLINE with one
> > > > timestamp, and the compositor uses SET_DEADLINE on the same thing with
> > > > another timestamp, what should happen?
> > >
> > > The expectation is that many deadline hints can be set on a fence.
> > > The fence signaller should track the soonest deadline.
> >
> > You need to document that as UAPI, since it is observable to userspace.
> > It would be bad if drivers or subsystems would differ in behaviour.
> >
>
> It is in the end a hint. It is about giving the driver more
> information so that it can make better choices. But the driver is
> even free to ignore it. So maybe "expectation" is too strong of a
> word. Rather, any other behavior doesn't really make sense. But it
> could end up being dictated by how the hw and/or fw works.
It will stop being a hint once it has been implemented and used in the
wild long enough. The kernel userspace regression rules make sure of
that.
See the topic of implementing triple-buffering in Mutter in order to
put more work to the GPU in order to have the GPU ramp up clocks in
order to not miss rendering deadlines. I don't think that patch set has
landed in Mutter upstream, but I hear distributions in downstream are
already carrying it.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1383
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1441
Granted, GPU clocks are just one side of that story it seems, and
triple-buffering may have other benefits.
If SET_DEADLINE would fix that problem without triple-buffering, it is
definitely userspace observable, expected and eventually required
behaviour.
Thanks,
pq
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