[PATCH 3/5] drm/vblank: Do not update vblank count if interrupts are already disabled.

[Dhinakaran Pandiyan]

Updating vblank counts requires register reads and these reads may not
return meaningful values if the device was in a low power state after
vblank interrupts were last disabled. So, update the count only if vblank
interrupts are enabled. Secondly, this means the registers should be read
before disabling vblank interrupts.

v2: Don't check vblank->enabled outside it's lock (Chris)

Cc: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
Cc: Michel Dänzer <michel at daenzer.net>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan at intel.com>
---
 drivers/gpu/drm/drm_vblank.c | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
index f2bf1f5dbaa5..2559d2d7b907 100644
--- a/drivers/gpu/drm/drm_vblank.c
+++ b/drivers/gpu/drm/drm_vblank.c
@@ -347,23 +347,25 @@ void drm_vblank_disable_and_save(struct drm_device *dev, unsigned int pipe)
 	spin_lock_irqsave(&dev->vblank_time_lock, irqflags);
 
 	/*
-	 * Only disable vblank interrupts if they're enabled. This avoids
-	 * calling the ->disable_vblank() operation in atomic context with the
-	 * hardware potentially runtime suspended.
+	 * Update vblank count and disable vblank interrupts only if the
+	 * interrupts were enabled. This avoids calling the ->disable_vblank()
+	 * operation in atomic context with the hardware potentially runtime
+	 * suspended.
 	 */
-	if (vblank->enabled) {
-		__disable_vblank(dev, pipe);
-		vblank->enabled = false;
-	}
+	if (!vblank->enabled)
+		goto out;
 
 	/*
-	 * Always update the count and timestamp to maintain the
+	 * Update the count and timestamp to maintain the
 	 * appearance that the counter has been ticking all along until
 	 * this time. This makes the count account for the entire time
 	 * between drm_crtc_vblank_on() and drm_crtc_vblank_off().
 	 */
 	drm_update_vblank_count(dev, pipe, false);
+	__disable_vblank(dev, pipe);
+	vblank->enabled = false;
 
+out:
 	spin_unlock_irqrestore(&dev->vblank_time_lock, irqflags);
 }
 
-- 
2.11.0