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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.
This commit is contained in:
Sultan Alsawaf 2023-01-24 18:43:05 -08:00 committed by sfan5
parent 8e2dc8b02b
commit db524988b1
3 changed files with 3 additions and 9 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

4
video/out/opengl/context_glx.c
View File

@ -206,11 +206,9 @@ static bool glx_check_visible(struct ra_ctx *ctx)
static void glx_swap_buffers(struct ra_ctx *ctx)
{
glXSwapBuffers(ctx->vo->x11->display, ctx->vo->x11->window);
if (ctx->vo->x11->use_present) {
vo_x11_present(ctx->vo);
if (ctx->vo->x11->use_present)
present_sync_swap(ctx->vo->x11->present);
}
}
static void glx_get_vsync(struct ra_ctx *ctx, struct vo_vsync_info *info)
{

4
video/out/opengl/context_x11egl.c
View File

@ -82,11 +82,9 @@ static void mpegl_swap_buffers(struct ra_ctx *ctx)
struct priv *p = ctx->priv;
eglSwapBuffers(p->egl_display, p->egl_surface);
if (ctx->vo->x11->use_present) {
vo_x11_present(ctx->vo);
if (ctx->vo->x11->use_present)
present_sync_swap(ctx->vo->x11->present);
}
}
static void mpegl_get_vsync(struct ra_ctx *ctx, struct vo_vsync_info *info)
{

4
video/out/vulkan/context_xlib.c
View File

@ -34,11 +34,9 @@ static bool xlib_check_visible(struct ra_ctx *ctx)
static void xlib_vk_swap_buffers(struct ra_ctx *ctx)
{
if (ctx->vo->x11->use_present) {
vo_x11_present(ctx->vo);
if (ctx->vo->x11->use_present)
present_sync_swap(ctx->vo->x11->present);
}
}
static void xlib_vk_get_vsync(struct ra_ctx *ctx, struct vo_vsync_info *info)
{