linux-kernel - Re: [RFC 0/4] dma-fence: Deadline awareness

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Date:   Tue, 27 Jul 2021 08:12:45 -0700
From:   Rob Clark <robdclark@...il.com>
To:     Michel Dänzer <michel@...nzer.net>
Cc:     Matthew Brost <matthew.brost@...el.com>,
        Rob Clark <robdclark@...omium.org>,
        Jack Zhang <Jack.Zhang1@....com>, Roy Sun <Roy.Sun@....com>,
        Christian König <ckoenig.leichtzumerken@...il.com>,
        open list <linux-kernel@...r.kernel.org>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>,
        Luben Tuikov <luben.tuikov@....com>,
        Gustavo Padovan <gustavo@...ovan.org>,
        Alex Deucher <alexander.deucher@....com>,
        Tian Tao <tiantao6@...ilicon.com>,
        Lee Jones <lee.jones@...aro.org>,
        Christian König <christian.koenig@....com>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>
Subject: Re: [RFC 0/4] dma-fence: Deadline awareness

On Tue, Jul 27, 2021 at 7:50 AM Michel Dänzer <michel@...nzer.net> wrote:
>
> On 2021-07-27 1:38 a.m., Rob Clark wrote:
> > From: Rob Clark <robdclark@...omium.org>
> >
> > Based on discussion from a previous series[1] to add a "boost" mechanism
> > when, for example, vblank deadlines are missed.  Instead of a boost
> > callback, this approach adds a way to set a deadline on the fence, by
> > which the waiter would like to see the fence signalled.
> >
> > I've not yet had a chance to re-work the drm/msm part of this, but
> > wanted to send this out as an RFC in case I don't have a chance to
> > finish the drm/msm part this week.
> >
> > Original description:
> >
> > In some cases, like double-buffered rendering, missing vblanks can
> > trick the GPU into running at a lower frequence, when really we
> > want to be running at a higher frequency to not miss the vblanks
> > in the first place.
> >
> > This is partially inspired by a trick i915 does, but implemented
> > via dma-fence for a couple of reasons:
> >
> > 1) To continue to be able to use the atomic helpers
> > 2) To support cases where display and gpu are different drivers
> >
> > [1] https://patchwork.freedesktop.org/series/90331/
>
> Unfortunately, none of these approaches will have the full intended effect once Wayland compositors start waiting for client buffers to become idle before using them for an output frame (to prevent output frames from getting delayed by client work). See https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1880 (shameless plug :) for a proof of concept of this for mutter. The boost will only affect the compositor's own GPU work, not the client work (which means no effect at all for fullscreen apps where the compositor can scan out the client buffers directly).
>

I guess you mean "no effect at all *except* for fullscreen..."?  Games
are usually running fullscreen, it is a case I care about a lot ;-)

I'd perhaps recommend that wayland compositors, in cases where only a
single layer is changing, not try to be clever and just push the
update down to the kernel.

BR,
-R

>
> --
> Earthling Michel Dänzer               |               https://redhat.com
> Libre software enthusiast             |             Mesa and X developer