Commits on Mar 2, 2020

1. output: remove wlr_output_impl.schedule_frame …

   This function allowed backends to provide a custom function for frame
   scheduling. Before resuming the rendering loop, the DRM and Wayland
   backends would wait for vsync.
   
   There isn't a clear benefit of doing this. The only upside is that we
   get more stable timings: the delay between two repaints doesn't change too
   much and is close to a mutliple of the refresh rate.
   
   However this introduces latency, especially when a client misses a
   frame. For instance a fullscreen game missing vblank will need to wait
   more than a whole frame before being able to display new content. This
   worst case scenario happens as follows:
   
   - Client is still rendering its frame and cannot submit it in time
   - Deadline is reached
   - Compositor decides to stop the rendering loop since nothing changed on
     screen
   - Client finally manages to render its frame, submits it
   - Compositor calls wlr_output_schedule_frame
   - DRM backend waits for next vblank
   - The wlr_output frame event is fired, compositor draws new content on screen
   - On the second next vblank, the new content reaches the screen
   
   With this patch, the wlr_output frame event is fired immediately when
   the client submits its late frame.
   
   This change also makes it easier to support variable refresh rate, since
   VRR is all about being able to present too-late frames earlier.
   
   References: swaywm#1925

   

   emersion committed Mar 2, 2020
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