compositor-drm: pageflip timeout implementation

Weston will not repaint until previous update has been acked by a
pageflip event coming from the drm driver. However, some buggy drivers
won’t return those events or will stop sending them at some point and
Weston output repaints will completely freeze. To ease developers’ task
in testing their drivers, this patch makes compositor-drm use a timer
to detect cases where those pageflip events stop coming.

This timeout implementation is software only and includes basic
features usually found in a watchdog. We simply exit Weston gracefully
with a log message and an exit code when the timout is reached.

The timeout value can be set via weston.ini by adding a
pageflip-timeout=<MILLISECONDS> entry under [core]
section. Setting it to 0 disables the timeout feature.

v2:
- Made sure we would get both the pageflip and the vblank events before
  stopping the timer.
- Reordered the error and success cases in
  drm_output_pageflip_timer_create() to be more in line with the rest
  of the code.

v3:
- Reordered (de)arming of the timer with the code around it to avoid it
  being rearmed before the current dearming.
- Return the proper value for the dispatcher in the pageflip_timeout
  callback.
- Also display the output name in case the timer fires.

v4:
- Reordered a forgotten timer rearming after its drmModePageFlip().

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=83884
Signed-off-by: Frederic Plourde <frederic.plourde at collabora.co.uk>
Signed-off-by: Emmanuel Gil Peyrot <emmanuel.peyrot@collabora.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

man/weston.ini.man

@@ -162,6 +162,11 @@ By default, xrgb8888 is used.
 sets Weston's idle timeout in seconds. This idle timeout is the time
 after which Weston will enter an "inactive" mode and screen will fade to
 black. A value of 0 disables the timeout.
+.TP 7
+.BI "pageflip-timeout="milliseconds
+sets Weston's pageflip timeout in milliseconds.  This sets a timer to exit
+gracefully with a log message and an exit code of 1 in case the DRM driver is
+non-responsive.  Setting it to 0 disables this feature.
 
 .IR Important
 : This option may also be set via Weston's '-i' command