Patch "drm: No-Op redundant calls to drm_vblank_off() (v2)" has been added to the 4.4-stable tree

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

drm: No-Op redundant calls to drm_vblank_off() (v2)

to the 4.4-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-no-op-redundant-calls-to-drm_vblank_off-v2.patch
and it can be found in the queue-4.4 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: Fri, 12 Feb 2016 20:30:27 +0100
Subject: drm: No-Op redundant calls to drm_vblank_off() (v2)

From: Mario Kleiner <[email protected]>

commit e8235891b33799d597ff4ab5e45afe173a65da30 upstream.

Otherwise if a kms driver calls into drm_vblank_off() more than once before calling drm_vblank_on() again, the redundant calls to vblank_disable_and_save() will call drm_update_vblank_count() while hw vblank counters and vblank timestamping are in a undefined state during modesets, dpms off etc.

At least with the legacy drm helpers it is not unusual to get multiple calls to drm_vblank_off and drm_vblank_on, e.g., half a dozen calls to drm_vblank_off and two calls to drm_vblank_on were observed on radeon-kms during dpms-off -> dpms-on transition.

We don't no-op calls from atomic modesetting drivers, as they should do a proper job of tracking hw state.

Fixes large jumps of the software maintained vblank counter due to the hardware vblank counter resetting to zero during dpms off or modeset, e.g., if radeon-kms is modified to use drm_vblank_off/on instead of drm_vblank_pre/post_modeset().

This fixes a regression caused by the changes made to drm_update_vblank_count() in Linux 4.4.

v2: Don't no-op on atomic modesetting drivers, per suggestion
of Daniel Vetter.

---
 drivers/gpu/drm/drm_irq.c |   11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

@@ -1313,7 +1313,13 @@ void drm_vblank_off(struct drm_device *d
 	spin_lock_irqsave(&dev->event_lock, irqflags);
 
 	spin_lock(&dev->vbl_lock);
-	vblank_disable_and_save(dev, pipe);
+	DRM_DEBUG_VBL("crtc %d, vblank enabled %d, inmodeset %d\n",
+		      pipe, vblank->enabled, vblank->inmodeset);
+
+	/* Avoid redundant vblank disables without previous drm_vblank_on(). */
+	if (drm_core_check_feature(dev, DRIVER_ATOMIC) || !vblank->inmodeset)
+		vblank_disable_and_save(dev, pipe);
+
 	wake_up(&vblank->queue);
 
 	/*

@@ -1415,6 +1421,9 @@ void drm_vblank_on(struct drm_device *de
 		return;
 
 	spin_lock_irqsave(&dev->vbl_lock, irqflags);
+	DRM_DEBUG_VBL("crtc %d, vblank enabled %d, inmodeset %d\n",
+		      pipe, vblank->enabled, vblank->inmodeset);
+
 	/* Drop our private "prevent drm_vblank_get" refcount */
 	if (vblank->inmodeset) {
 		atomic_dec(&vblank->refcount);

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

queue-4.4/drm-prevent-vblank-counter-bumps-1-with-active-vblank-clients.-v2.patch queue-4.4/drm-fix-treatment-of-drm_vblank_offdelay-in-drm_vblank_on-v2.patch queue-4.4/drm-nouveau-display-enable-vblank-irqs-after-display-engine-is-on-again.patch queue-4.4/drm-fix-drm_vblank_pre-post_modeset-regression-from-linux-4.4.patch queue-4.4/drm-radeon-don-t-hang-in-radeon_flip_work_func-on-disabled-crtc.-v2.patch queue-4.4/drm-no-op-redundant-calls-to-drm_vblank_off-v2.patch queue-4.4/drm-amdgpu-don-t-hang-in-amdgpu_flip_work_func-on-disabled-crtc.patch