Composite redraw speedup?

[Alexander E. Patrakov]

On Thu, Feb 13, 2020 at 1:12 AM Carsten Haitzler <raster at rasterman.com> wrote:
>
> On Wed, 12 Feb 2020 16:45:50 +0100 Michel Dänzer <michel at daenzer.net> said:
>
> > On 2020-02-12 4:34 p.m., Carsten Haitzler wrote:
> > > On Wed, 12 Feb 2020 15:36:06 +0100 Michel Dänzer <michel at daenzer.net> said:
> > >
> > >> On 2020-02-12 2:06 p.m., Carsten Haitzler wrote:
> > >>> On Wed, 12 Feb 2020 13:23:28 +0200 Pekka Paalanen <ppaalanen at gmail.com>
> > >>> said:
> > >>>
> > >>>> On Wed, 12 Feb 2020 11:07:56 +0000
> > >>>> Carsten Haitzler <raster at rasterman.com> wrote:
> > >>>>
> > >>>>> On Wed, 12 Feb 2020 12:40:15 +0200 Pekka Paalanen <ppaalanen at gmail.com>
> > >>>>> said:
> > >>>>>
> > >>>>>> On Wed, 12 Feb 2020 10:21:02 +0000
> > >>>>>> Carsten Haitzler (The Rasterman) <raster at rasterman.com> wrote:
> > >>>>>>
> > >>>>>>> even better - if the /dev/dri/card0
> > >>>>>>> device exists, dlopen libdrm and get some symbols from it and ... use
> > >>>>>>> it to request the drm device sent you vsync events so you can use the
> > >>>>>>> vsync interrupt as your frame event. this will be another fd to listen
> > >>>>>>> on in select() and of course you can turn this vblank event stream on
> > >>>>>>> and off.
> > >>>>>>
> > >>>>>> Please don't. Talk to the X server instead.
> > >>>>>
> > >>>>> and what vsync events does the xserver provide?
> > >>>>
> > >>>> You don't want vsync events. You have no idea what they
> > >>>> correspond to, or even if you opened the right device.
> > >>>>
> > >>>> https://gitlab.freedesktop.org/xorg/proto/xorgproto/blob/master/presentproto.txt
> > >>>
> > >>> I wrote the drm support before the present extension existed. The drm
> > >>> path is  easy to support - only open if a single card exists (if multiple
> > >>> - don't do it and fall back to timer based animation) and you can filter
> > >>> for multiple screens as you get events for all screens. Yes - you end up
> > >>> syncing with a single chosen screen if you filter for just one of the
> > >>> vblank events, but it's better than using the system clock.
> > >>
> > >> You only get an event for the CRTC you ask for in the
> > >> DRM_IOCTL_WAIT_VBLANK ioctl. How do yo know which CRTC to pick?
> > >
> > > drmVBlank vbl;
> > > vbl.request.type = DRM_VBLANK_RELATIVE | DRM_VBLANK_EVENT;
> > > vbl.request.sequence = 1;
> > > vbl.request.signal = 0;
> > > drmWaitVBlank(drm_fd, &vbl);
> > >
> > > has worked a charm for maybe give or take close to a decade (as i said -
> > > before present existed).
> >
> > That selects the first CRTC. It'll work most of the time, except when
> > the first CRTC isn't enabled for some reason.
>
> I've seen it select multiple crtc's as I get multiple vblank events with the
> same frame number... (thus filter out the dups to only get one).
>
> > >>> I have found x present XPresentNotifyMSC() to be unreliable where the
> > >>> drm back-door above is far more reliable. For example - on my amdgpu
> > >>> driver here it just refuses to produce any events (yes - extension is
> > >>> there), [...]
> > >>
> > >> I haven't seen any such reports, so I'm not aware of such issues with
> > >> xf86-video-amdgpu.
> > >
> > > i'm staring at it not working :) admittedly it's a slightly older rx550 with
> > > amdgpu on an aarch64 host... :) i did play with xpresent in the early days
> > > it came out and found it to be iffy and stuck to the solution we already had
> > > (above) which is more reliable.
> >
> > A proper issue report with all the needed information will be needed to
> > do anything about this.
>
> It hasn't been something I've needed to do as I have the other method above and
> no xpresent one to have to support and make work :)
>
> > To be clear, even the Present extension is really too low level to be
> > used directly for this by a compositor (unless it also uses it directly
> > for the actual presentation). The DRM vblank ioctl even more so, it's
> > for the display server.
> >
> > A compositor which uses GLX/EGL for presentation should use
> > corresponding GLX/EGL (extension) APIs for this.
>
> Aaaah therein lies the rub. I need this to work with our without EGL/GLX. I
> need it work for compositors and clients - the whole shebang as the same
> library, infra etc. is used on both sides. There already is an "animator" infra
> where you request a callback to be called whenever a frame needs to be
> produced. The idea is that you just modify objects in the scene graph and this
> then percolates down to there being changes that then results in a render that
> may or may not use GL. at once point I had an xrender engine I gave up on so
> i'm down to just software and GL (GLX or EGL and mostly a gles2 susbset of GL).
> The animation is far removed from the rendering. In the end the pipeline
> depends on their being some event to say "wake-up! time to render!". The vblank
> events are just that. That means there is an entire frame worth of budget to go
> set up the changes, render them and get a swap or xshmputimage queued up and
> ready to go. The GL infra for this is certainly not ideal and in most cases
> just hasn't been able to do this in my experience in the past. I need this
> event even though I have not actually rendered anything previously.

Please also consider the corner case of Xvfb, xf86-video-dummy, or
maybe (untested) rendering on a headless GPU on a EC2 instance and
displaying the result via VNC. In these cases, there will be no vblank
events at all.

-- 
Alexander E. Patrakov