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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. |
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| .. | ||
| common.h | ||
| context_android.c | ||
| context_display.c | ||
| context_wayland.c | ||
| context_win.c | ||
| context_xlib.c | ||
| context.c | ||
| context.h | ||
| utils.c | ||
| utils.h | ||