drm_output_start_repaint_loop() incurred a delay of one refresh cycle by using a no-op page-flip to get an accurate vblank timestamp as reference. This causes unwanted lag whenever Weston exited its repaint loop, e.g., whenever an application wants to repaint with less than full video refresh rate but still minimum lag.
Try to use the drmWaitVblank ioctl to get a proper timestamp instantaneously without lag. If that does not work, fall back to the old method of idle page-flip.
This optimization will work on any drm/kms driver which supports high precision vblank timestamping. As of Linux 4.0 these would be intel, radeon and nouveau on all their supported gpu's.
On kms drivers without instant high precision timestamping support, the kernel is supposed to return a timestamp of zero when calling drmWaitVblank() to query the current
vblank count and time iff vblank irqs are currently disabled, because the only way to get a valid timestamp on such kms drivers is to enable vblank interrupts and then wait a bit for the next vblank irq to take a new valid timestamp. The caller is supposed to poll until at next
vblank irq it gets a valid non-zero timestamp if it needs a timestamp.
This zero-timestamp signalling works up to Linux 3.17, but got broken due to a regression in Linux 3.18 and later. On Linux 3.18+ with kms drivers that don't have high precision timestamping, the kernel erroneously returns a stale timestamp from an earlier vblank, ie. the vblank count and timestamp are mismatched. A patch is under way to fix this, but to deal with broken kernels, we also check non-zero timestamps if they are more than one refresh duration in the past, as this indicates a stale/invalid timestamp, so we need to take the page-flip fallback for restarting the repaint loop.
v2: Implement review suggestions by Pekka Paalanen, especially extend the commit message to describe when and why the instant restart won't work due to missing Linux kernel functionality or a Linux kernel regression.
f507ec3 compositor-drm: Allow instant start of repaint loop. (v4)
Makefile.am | 1 +
src/compositor-drm.c | 43 ++++++++++++++++++++++++++++++++++++++++++-
2 files changed, 43 insertions(+), 1 deletion(-)