Re: [PATCH 1/2] drm: Fix dirtyfb stalls

[Rob Clark]

On Fri, May 14, 2021 at 12:54 AM Pekka Paalanen <ppaalanen@xxxxxxxxx> wrote:
>
> On Wed, 12 May 2021 07:57:26 -0700
> Rob Clark <robdclark@xxxxxxxxx> wrote:
>
> > On Wed, May 12, 2021 at 1:23 AM Pekka Paalanen <ppaalanen@xxxxxxxxx> wrote:
> > >
> > > On Tue, 11 May 2021 18:44:17 +0200
> > > Daniel Vetter <daniel@xxxxxxxx> wrote:
> > >
> > > > On Mon, May 10, 2021 at 12:06:05PM -0700, Rob Clark wrote:
> > > > > On Mon, May 10, 2021 at 10:44 AM Daniel Vetter <daniel@xxxxxxxx> wrote:
> > > > > >
> > > > > > On Mon, May 10, 2021 at 6:51 PM Rob Clark <robdclark@xxxxxxxxx> wrote:
> > > > > > >
> > > > > > > On Mon, May 10, 2021 at 9:14 AM Daniel Vetter <daniel@xxxxxxxx> wrote:
> > > > > > > >
> > > > > > > > On Sat, May 08, 2021 at 12:56:38PM -0700, Rob Clark wrote:
> > > > > > > > > From: Rob Clark <robdclark@xxxxxxxxxxxx>
> > > > > > > > >
> > > > > > > > > drm_atomic_helper_dirtyfb() will end up stalling for vblank on "video
> > > > > > > > > mode" type displays, which is pointless and unnecessary.  Add an
> > > > > > > > > optional helper vfunc to determine if a plane is attached to a CRTC
> > > > > > > > > that actually needs dirtyfb, and skip over them.
> > > > > > > > >
> > > > > > > > > Signed-off-by: Rob Clark <robdclark@xxxxxxxxxxxx>
>
> ...
>
> > > > > > > But we could re-work drm_framebuffer_funcs::dirty to operate on a
> > > > > > > per-crtc basis and hoist the loop and check if dirtyfb is needed out
> > > > > > > of drm_atomic_helper_dirtyfb()
> > > > > >
> > > > > > That's still using information that userspace doesn't have, which is a
> > > > > > bit irky. We might as well go with your thing here then.
> > > > >
> > > > > arguably, this is something we should expose to userspace.. for DSI
> > > > > command-mode panels, you probably want to make a different decision
> > > > > with regard to how many buffers in your flip-chain..
> > > > >
> > > > > Possibly we should add/remove the fb_damage_clips property depending
> > > > > on the display type (ie. video/pull vs cmd/push mode)?
> > > >
> > > > I'm not sure whether atomic actually needs this exposed:
> > > > - clients will do full flips for every frame anyway, I've not heard of
> > > >   anyone seriously doing frontbuffer rendering.
> > >
> > > That may or may not be changing, depending on whether the DRM drivers
> > > will actually support tearing flips. There has been a huge amount of
> > > debate for needing tearing for Wayland [1], and while I haven't really
> > > joined that discussion, using front-buffer rendering (blits) to work
> > > around the driver inability to flip-tear might be something some people
> > > will want.
> >
> > jfwiw, there is a lot of hw that just can't do tearing pageflips.. I
> > think this probably includes most arm hw.  What is done instead is to
> > skip the pageflip and render directly to the front-buffer.
> >
> > EGL_KHR_mutable_render_buffer is a thing you might be interested in..
> > it is wired up for android on i965 and there is a WIP MR[1] for
> > mesa/st (gallium):
> >
> > Possibly it could be useful to add support for platform_wayland?
> >
> > [1] https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10685
>
> Thanks Rob, that's interesting.
>
> I would like to say that EGL Wayland platform cannot and has no reason
> to support frontbuffer rendering in Wayland clients, because the
> compositor may be reading the current client frontbuffer at any time,
> including *not reading it again* until another update is posted via
> Wayland. So if a Wayland client is doing frontbuffer rendering, then I
> would expect it to be very likely that the window might almost never
> show a "good" picture, meaning that it is literally just the
> half-rendered frame after the current one with continuously updating
> clients.
>
> That is because a Wayland client doing frontbuffer rendering is
> completely unrelated to the Wayland compositor putting the client
> buffer on scanout.
>
> Frontbuffer rendering used by a Wayland compositor would be used for
> fallback tearing updates, where the rendering is roughly just a blit
> from a client buffer. By definition, it means blit instead of scanout
> from client buffers, so the performance/power hit is going to be...
> let's say observable.
>
> If a Wayland client did frontbuffer rendering, and then it used a
> shadow buffer of its own to minimise the level of garbage on screen by
> doing only blits into the frontbuffer, that would again be a blit. And
> then the compositor might be doing another blit because it doesn't know
> the client is doing frontbuffer rendering which would theoretically
> allow the compositor to scan out the client buffer.
>
> There emerges the need for a Wayland extension for clients to be
> telling the compositor explicitly that they are going to be doing
> frontbuffer rendering now, it is ok to put the client buffer on scanout
> even if you want to do tearing updates on hardware that cannot
> tear-flip.
>
> However, knowing that a client buffer is good for scanout is not
> sufficient for scanout in a compositor, so it might still not be
> scanned out. If the compositor is instead render-compositing, you have
> the first problem of "likely never a good picture".

I think if a client is doing front-buffer rendering, then "tearing" is
the clients problem.

The super-big-deal use-case for this is stylus, because you want to
minimize the latency of pen-to-pixel.. tearing isn't really a problem
because things aren't changing much from-by-frame

I'm going to predict there will be at least one wayland compositor
supporting this, maybe via custom protocol, idk. ;-)

BR,
-R

> I'm sure there can be specialised use cases (e.g. a game console
> Wayland compositor) where scanout can be guaranteed, but generally
> for desktops it's not so.
>
> I believe there will be people wanting EGL Wayland platform frontbuffer
> rendering for very special cases, and I also believe it will just break
> horribly everywhere else. Would it be worth it to implement? No idea.
>
> Maybe there would need to be a Wayland extension so that compositors
> can control the availability of frontbuffer rendering in EGL Wayland
> platform?
>
> There is the dmabuf-hints Wayland addition that is aimed at dynamically
> providing information to help optimise for scanout and
> render-compositing. If a compositor could control frontbuffer rendering
> in a client, it could tell the client to use frontbuffer rendering when
> it does hit scanout, and tell the client to stop frontbuffer rendering
> when scanout is no longer possible. The problem with the latter is a
> glitch, but since frontbuffer rendering is by definition glitchy (when
> done in clients), maybe that is acceptable to some?
>
>
> Thanks,
> pq