The issue we have is that the crc worker might fall behind. We've
tried to handle this by tracking both the earliest frame for which it
still needs to compute a crc, and the last one. Plus when the
crtc_state changes, we have a new work item, which are all run in
order due to the ordered workqueue we allocate for each vkms crtc.

Trouble is there's been a few small issues in the current code:
- we need to capture frame_end in the vblank hrtimer, not in the
  worker. The worker might run much later, and then we generate a lot
  of crc for which there's already a different worker queued up.
- frame number might be 0, so create a new crc_pending boolean to
  track this without confusion.
- we need to atomically grab frame_start/end and clear it, so do that
  all in one go. This is not going to create a new race, because if we
  race with the hrtimer then our work will be re-run.
- only race that can happen is the following:
  1. worker starts
  2. hrtimer runs and updates frame_end
  3. worker grabs frame_start/end, already reading the new frame_end,
  and clears crc_pending
  4. hrtimer calls queue_work()
  5. worker completes
  6. worker gets  re-run, crc_pending is false
  Explain this case a bit better by rewording the comment.

v2: Demote warning level output to debug when we fail to requeue, this
is expected under high load when the crc worker can't quite keep up.

Cc: Shayenne Moura <shayenneluzmoura@gmail.com>
Cc: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Cc: Haneen Mohammed <hamohammed.sa@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Tested-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190606222751.32567-2-daniel.vetter@ffwll.ch