Presentation feedback events are delayed by 1 frame if VRR is enabled

[mahkoh]

Consider the following sequence of events with VRR enabled:

[3882782.418] {Default Queue}  -> wl_surface#9.frame(new id wl_callback#41)
[3882782.423] {Default Queue}  -> wp_presentation#18.feedback(wl_surface#9, new id wp_presentation_feedback#54)
[3882782.442] {Default Queue}  -> wl_surface#9.attach(wl_buffer#64, 0, 0)
[3882782.448] {Default Queue}  -> wl_surface#9.damage_buffer(0, 0, 1920, 1080)

[3882815.531] {Default Queue}  -> wl_surface#10.commit()
[3882815.546] {Default Queue}  -> wl_surface#9.commit()
[3882815.551] {Default Queue}  -> wl_surface#8.commit()
[3882815.566] {Display Queue} wl_display#1.delete_id(67)
[3882815.570] {Default Queue} wl_buffer#66.release()
[3882815.575] {Default Queue} wp_presentation_feedback#67.sync_output(wl_output#30)
[3882815.580] {Default Queue} wp_presentation_feedback#67.presented(0, 344085, 138497000, 8333333, 0, 1830556, 15)
[3882815.707] {Display Queue} wl_display#1.delete_id(41)
[3882815.717] {Default Queue} wl_callback#41.done(344085171)

You can see that the timestamp in the presented event is much older than the timestamp in the done event and that the id of presentation feedback event is different from the id used in the commit sequence.

VRR otherwise works as expected. I have not checked if presentation itself is delayed by 1 frame, which would be a different bug.

[emersion]

The timestamp given by wl_surface.frame is the wall-clock time when the output commit is performed, not when a buffer is presented. The wl_surface.frame timestamp will be earlier than the presentation timestamp.

The commit creates wp_presentation_feedback#54, but we're looking at wp_presentation_feedback#67 here, so feedback for another commit (likely the previous one?).

[mahkoh]

The timestamp given by wl_surface.frame is the wall-clock time when the output commit is performed, not when a buffer is presented.

The difference is roughly 1/30s while playing a 30hz video.

The commit creates wp_presentation_feedback#54, but we're looking at wp_presentation_feedback#67 here, so feedback for another commit (likely the previous one?).

Yes, after committing it sends the presented event for the commit that was made 1/30s ago. But since the video was played on a 180hz monitor, the distance between a commit and the presented event should never be more than 1/180s.