[3.19.y-ckt stable] Patch "drm: Zero out invalid vblank timestamp in drm_update_vblank_count." has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    drm: Zero out invalid vblank timestamp in drm_update_vblank_count.

to the linux-3.19.y-queue branch of the 3.19.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.19.y-queue

This patch is scheduled to be released in version 3.19.8-ckt1.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.19.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

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>From adfa0fa531b0230f24f59f521d96fe8403520f73 Mon Sep 17 00:00:00 2001
From: Mario Kleiner <mario.kleiner.de at gmail.com>
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

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.

Signed-off-by: Mario Kleiner <mario.kleiner.de at gmail.com>

Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
Cc: Daniel Vetter <daniel at ffwll.ch>
Signed-off-by: Dave Airlie <airlied at redhat.com>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 drivers/gpu/drm/drm_irq.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
index 4d79dad..68193a6 100644
--- a/drivers/gpu/drm/drm_irq.c
+++ b/drivers/gpu/drm/drm_irq.c
@@ -131,12 +131,11 @@ static void drm_update_vblank_count(struct drm_device *dev, int crtc)

 	/* 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);
--
1.9.1