Buffers don't become readable in time with GuC submission
KWin git master checks for dmabuf fd readability before doing atomic commits with the relevant buffer(s). Two users reported that this causes severe performance degradation on their laptops when GuC submission is enabled - the output drops to 30Hz on 60Hz screens.
Rendering starts about a full frame before vblank (15ms or so), and according to OpenGL it takes between 2 and 10ms to complete (CPU + GPU time). The fds are checked for readability less than 2ms before vblank, so they should always be readable. If the checks for fd readability are removed and commits are done even with buffers that aren't ready, the pageflips do not get delayed by the kernel, so rendering really does complete in time. Likewise, if GuC submission is disabled, the buffers are readable when they should be.