On Tue, 16 Aug 2016 10:46:13 +0900
Michel Dänzer <mic...@daenzer.net> wrote:

> On 16/08/16 10:32 AM, Mario Kleiner wrote:
> > Cc'ing Daniel Stone and Pekka Paalanen, because this relates to wayland.
> > 
> > Wrt. having a new pageflip parameter struct, i wonder if it wouldn't
> > make sense to then already prepare some space in it for specifying an
> > absolute target time, e.g., in u64 microseconds? Or make this part of
> > the atomic pageflip framework?  
> I feel like this is too much hassle for the DRM_IOCTL_MODE_PAGE_FLIP
> ioctl, probably makes sense to only deal with this in the atomic API.
> > In Wayland we now have the presentation_feedback extension (maybe it got
> > a new name?), which is great to get feedback when and how presentation
> > was completed, essentially the equivalent of X11's GLX_INTEL_swap_events
> > + some useful extra info / the feedback half of OML_sync_control.
> > 
> > That was supposed to be complemented by a presentation_queue extension
> > which would provide the other half of OML_sync_control, the part where
> > an app can tell the compositor when and how to present surface updates.
> > I remember that a year ago inclusion of that extension was blocked on
> > some other more urgent important blocker bugs? Did this make progress?


sorry, I'm pretty poor at following dri-devel@.

Yeah, the Wayland extension for queueing frames to be presented at
specific times has not been going anywhere for a long time. IIRC the
blockers have since been resolved, now it would need dusting off and
seeing how it interacts with those protocol additions. I wasn't too
happy with the design at the time, either, because of the question of
which state to queue and which not.

Nowadays presentation_feedback lives in
and has been declared stable.

> > For timing sensitive applications such an extension is even more
> > important in a wayland world than under X11. On X11 most desktops allow
> > unredirecting fullscreen windows, so an app can drive the flip timing
> > rather direct. At least on Weston as i remember it from my last tests a
> > year ago, that wasn't possible, and an app that doesn't want to present
> > at each video refresh, but at specific target times had to guess what
> > the specific compositors scheduling strategy might be and then "play
> > games" wrt. to timing surface commit's to trick the compositor into sort
> > of doing the right thing - very fragile.
> > 
> > Anyway, the idea of presentation_queue was to specify the requested time
> > of presentation as actual time, not as a target vblank count. This
> > because applications that care about precise presentation timing, like
> > my kind of neuroscience/medical visual stimulation software, or also
> > video players, or e.g., at least the VDPAU video presentation api
> > "think" in absolute time, not in refresh cycles. Also a target vblank
> > count for presentation is less meaningful than a target time for things
> > like variable framerate displays/adaptive sync, or displays which don't
> > have a classic refresh cycle at all. It might also be useful if one
> > thinks about something like VR compositors, where precise timing control
> > could help for some of the tricks ("time warping" iirc?) they use to
> > hide/cope with latency from head tracking -> display.
> > 
> > It would be nice if the kernel could help compositors which would
> > implement presentation_queue or similar to be robust/precise in face of
> > future stuff like Freesync, or of added delays due to Optimus/Prime
> > hybrid-graphics setups. If we wanted to synchronize presentation of
> > multiple displays in a Freesync type display setup, absolute target
> > times could also be helpful.  
> I agree with all of this though.

I'm very happy to hear the idea has support!


> I'd like to add that the X11 Present extension specification already
> includes support for specifying a target time instead of a target
> refresh cycle. This isn't implemented yet though, but it should be
> relatively straightforward to implement using the kind of kernel API you
> describe.

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