Patch "drm: Zero out invalid vblank timestamp in drm_update_vblank_count." has been added to the 4.0-stable tree

This is a note to let you know that I've just added the patch titled

drm: Zero out invalid vblank timestamp in drm_update_vblank_count.

to the 4.0-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
drm-zero-out-invalid-vblank-timestamp-in-drm_update_vblank_count.patch
and it can be found in the queue-4.0 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Mario Kleiner <[email protected]>
Date: Tue, 7 Apr 2015 06:31:09 +0200
Subject: drm: Zero out invalid vblank timestamp in drm_update_vblank_count.
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

From: Mario Kleiner <[email protected]>

commit fdb68e09bbb1c981f24608d7022c7d93cc47b326 upstream.

Since commit 844b03f27739135fe1fed2fef06da0ffc4c7a081 we make sure that after vblank irq off, we return the last valid (vblank count, vblank timestamp) pair to clients, e.g., during modesets, which is good.

An overlooked side effect of that commit for kms drivers without support for precise vblank timestamping is that at vblank irq enable, when we update the vblank counter from the hw counter, we can't update the corresponding vblank timestamp, so now we have a totally mismatched timestamp for the new count to confuse clients.

Restore old client visible behaviour from before Linux 3.17, but zero out the timestamp at vblank counter update (instead of disable as in original implementation) if we can't generate a meaningful timestamp immediately for the new vblank counter. This will fix this regression, so callers know they need to retry again later if they need a valid timestamp, but at the same time preserves the improvements made in the commit mentioned above.

---
 drivers/gpu/drm/drm_irq.c |    9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

@@ -131,12 +131,11 @@ static void drm_update_vblank_count(stru
 
 	/* Reinitialize corresponding vblank timestamp if high-precision query
 	 * available. Skip this step if query unsupported or failed. Will
-	 * reinitialize delayed at next vblank interrupt in that case.
+	 * reinitialize delayed at next vblank interrupt in that case and
+	 * assign 0 for now, to mark the vblanktimestamp as invalid.
 	 */
-	if (rc) {
-		tslot = atomic_read(&vblank->count) + diff;
-		vblanktimestamp(dev, crtc, tslot) = t_vblank;
-	}
+	tslot = atomic_read(&vblank->count) + diff;
+	vblanktimestamp(dev, crtc, tslot) = rc ? t_vblank : (struct timeval) {0, 0};
 
 	smp_mb__before_atomic();
 	atomic_add(diff, &vblank->count);

Patches currently in stable-queue which might be from [email protected] are

queue-4.0/drm-zero-out-invalid-vblank-timestamp-in-drm_update_vblank_count.patch