From: Ville Syrjälä ville.syrjala@linux.intel.com
drm_vblank_restore() exists because certain power saving states can clobber the hardware frame counter. The way it does this is by guesstimating how many frames were missed purely based on the difference between the last stored timestamp vs. a newly sampled timestamp.
If we should call this function before a full frame has elapsed since we sampled the last timestamp we would end up with a possibly slightly different timestamp value for the same frame. Currently we will happily overwrite the already stored timestamp for the frame with the new value. This could cause userspace to observe two different timestamps for the same frame (and the timestamp could even go backwards depending on how much error we introduce when correcting the timestamp based on the scanout position).
To avoid that let's not update the stored timestamp unless we're also incrementing the sequence counter. We do still want to update vblank->last with the freshly sampled hw frame counter value so that subsequent vblank irqs/queries can actually use the hw frame counter to determine how many frames have elapsed.
Cc: Dhinakaran Pandiyan dhinakaran.pandiyan@intel.com Cc: Rodrigo Vivi rodrigo.vivi@intel.com Cc: Daniel Vetter daniel.vetter@ffwll.ch Signed-off-by: Ville Syrjälä ville.syrjala@linux.intel.com --- drivers/gpu/drm/drm_vblank.c | 11 +++++++++++ 1 file changed, 11 insertions(+)
diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c index 893165eeddf3..e127a7db2088 100644 --- a/drivers/gpu/drm/drm_vblank.c +++ b/drivers/gpu/drm/drm_vblank.c @@ -176,6 +176,17 @@ static void store_vblank(struct drm_device *dev, unsigned int pipe,
vblank->last = last;
+ /* + * drm_vblank_restore() wants to always update + * vblank->last since we can't trust the frame counter + * across power saving states. But we don't want to alter + * the stored timestamp for the same frame number since + * that would cause userspace to potentially observe two + * different timestamps for the same frame. + */ + if (vblank_count_inc == 0) + return; + write_seqlock(&vblank->seqlock); vblank->time = t_vblank; atomic64_add(vblank_count_inc, &vblank->count);