Commit 855d30b4 authored Sep 10, 2014 by Daniel Vetter

drm: Only update final vblank count when precise ts is available



Drivers without a hardware vblank counter simply can't account for the
vblanks that happened while the vblank interrupt was off. To check
this grab a vblank timestamp and if the result is dubious follow the
normal save-and-disable logic.

Drivers should prevent this by setting vblank_disable_allowed = false,
but since running vblank interrupts constantly is not good for power
consumption most drivers lie. Testing for precise vblank timestamps is
the next best thing we can check for.

Suggested-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Cc: Mario Kleiner <mario.kleiner.de@gmail.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

parent ab8905f1

drivers/gpu/drm/drm_irq.c

+8 −1

Original line number	Diff line number	Diff line
		@@ -148,8 +148,15 @@ static void vblank_disable_and_save(struct drm_device *dev, int crtc)
		* has been ticking all along until this time. This makes the
		* count account for the entire time between drm_vblank_on() and
		* drm_vblank_off().
		*
		* But only do this if precise vblank timestamps are available.
		* Otherwise we might read a totally bogus timestamp since drivers
		* lacking precise timestamp support rely upon sampling the system clock
		* at vblank interrupt time. Which obviously won't work out well if the
		* vblank interrupt is disabled.
		*/
		if (!vblank->enabled) {
		if (!vblank->enabled &&
		drm_get_last_vbltimestamp(dev, crtc, &tvblank, 0) > 0) {
		drm_update_vblank_count(dev, crtc);
		spin_unlock_irqrestore(&dev->vblank_time_lock, irqflags);
		return;