[PATCH 2/4] drm: Only update final vblank count when precise ts is available

[Daniel Vetter]

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 at gmail.com>
Cc: Mario Kleiner <mario.kleiner.de at gmail.com>
Cc: Matt Roper <matthew.d.roper at intel.com>
Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
---
 drivers/gpu/drm/drm_irq.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
index 6eb015020af2..922721ead29a 100644
--- a/drivers/gpu/drm/drm_irq.c
+++ b/drivers/gpu/drm/drm_irq.c
@@ -163,8 +163,15 @@ 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;
-- 
1.9.3