db52498...
by
Sultan Alsawaf <email address hidden>
x11: remove PresentNotifyMSC from egl/glx/vulkan to fix xpresent timing
PresentNotifyMSC turns out to be not only redundant, but also harmful with
mesa-backed egl/glx/vulkan VOs because for all of them, mesa uses
PresentPixmap behind the scenes when DRI3 is available, which already
spawns a PresentCompleteNotify event when the buffer swap actually
finishes. This is important because without using the timing information
from these PresentCompleteKindPixmap events, there's no way for mpv to know
exactly when a frame becomes visible on the display.
By using PresentNotifyMSC in conjunction with DRI3-enabled mesa, two
problems are created:
1. mpv assumes that a vblank won't elapse (i.e., it assumes the current MSC
won't change) between the time when mesa enqueues the buffer swap and
the time when mpv calls PresentNotifyMSC to ask xorg for a notification
at the next MSC, relative to the current MSC at the time that xorg reads
it for the PresentNotifyMSC call. This means that mpv could get a
notification one or more vblanks later than it expects, since the
intention here is for mpv to get a notification at the MSC that the
buffer swap completes.
2. mpv assumes that a buffer swap always takes one vblank to complete,
which isn't always true. A buffer swap (i.e., a page flip) could take
longer than that depending on hardware conditions (if the GPU is running
slowly or needs to exit a low-power state), scheduling delays (under
heavy system or GPU load), or unfortunate timing (if the raster scan
line happens to be at one of the last few rows of pixels and a vblank
elapses just before the buffer swap is enqueued).
This causes mpv to have a faulty assumption of when frames become visible.
Since mpv already receives the PresentCompleteNotify events generated by
mesa's buffer swaps under the hood, the PresentNotifyMSC usage is unneeded
and just throws a wrench in mpv's vsync timing when xpresent is enabled.
Simply removing the PresentNotifyMSC usage from the egl, glx, and vulkan
VOs fixes the xpresent vsync timing.
Officially, the most cursed part of the wayland code in mpv (third or
fourth try now?) This time, let's allocate a pool during init
(arbitrarily set to the maximum swapchain length mpv allows, 8; don't
even know if this actually works in wayland). Then we add/remove
feedbacks from the pool during the lifecycle of the VO, and clean it up
all at the end. Hopefully, this does the trick for good this time.
