Pageflip completion event has impossible msc related lockups

Submitted by Steve Wills @swills

Assigned to xf86-video-ati maintainers

Link to original bug (#112255)

Description

I've been seeing lockups with messages of this form:

(WW) AMDGPU(0): amdgpu_dri2_flip_event_handler: Pageflip completion event has impossible msc XXXXXXX < target_msc YYYYYYY

After commenting this line, it seems to have gone away:

https://gitlab.freedesktop.org/xorg/driver/xf86-video-amdgpu/blob/master/src/amdgpu_dri2.c#L494

That is, the messages are still logged, of course, but the lockups are gone because the client is no longer signaling failure to the client. I haven't seen any other adverse effects.

Should this be a warning only? One thing I noticed is the time is always 1 less than expected:

[ 20308.275] (WW) AMDGPU(0): amdgpu_dri2_flip_event_handler: Pageflip completion event has impossible msc 1868426 < target_msc 1868427
[716312.737] (WW) AMDGPU(0): amdgpu_dri2_flip_event_handler: Pageflip completion event has impossible msc 43628303 < target_msc 43628304
[752121.770] (WW) AMDGPU(0): amdgpu_dri2_flip_event_handler: Pageflip completion event has impossible msc 45776848 < target_msc 45776849
[833776.925] (WW) AMDGPU(0): amdgpu_dri2_flip_event_handler: Pageflip completion event has impossible msc 50676148 < target_msc 50676149

Perhaps there is an off by one error somewhere? Also I believe, but can't recall for sure sure, this problem only started once I enabled the TearFree option in the driver.