Re: [PATCH v4 06/14] dma-buf/sync_file: Support (E)POLLPRI

[Tvrtko Ursulin]

On 24/02/2023 09:26, Pekka Paalanen wrote:
> On Thu, 23 Feb 2023 10:51:48 -0800
> Rob Clark <robdclark@xxxxxxxxx> wrote:
>
> > On Thu, Feb 23, 2023 at 1:38 AM Pekka Paalanen <ppaalanen@xxxxxxxxx> wrote:
> > > On Wed, 22 Feb 2023 07:37:26 -0800
> > > Rob Clark <robdclark@xxxxxxxxx> wrote:
> > >   
> > > > On Wed, Feb 22, 2023 at 1:49 AM Pekka Paalanen <ppaalanen@xxxxxxxxx> 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.

Yeah, tricky and maybe a gray area in this case. I think we eluded 
elsewhere in the thread that renaming the thing might be an option.

So maybe instead of deadline, which is a very strong word, use something 
along the lines of "present time hint", or "signalled time hint"? Maybe 
reads clumsy. Just throwing some ideas for a start.

Regards,

Tvrtko

> 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