Patch "drm: Fix drm_vblank_pre/post_modeset regression from Linux 4.4" 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: Fix drm_vblank_pre/post_modeset regression from Linux 4.4

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-fix-drm_vblank_pre-post_modeset-regression-from-linux-4.4.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 20:30:29 +0100
Subject: drm: Fix drm_vblank_pre/post_modeset regression from Linux 4.4

From: Mario Kleiner <[email protected]>

commit c61934ed9a0e3911a9935df26858726a7ec35ec0 upstream.

Changes to drm_update_vblank_count() in Linux 4.4 broke the behaviour of the pre/post modeset functions as the new update code doesn't deal with hw vblank counter resets inbetween calls to drm_vblank_pre_modeset an drm_vblank_post_modeset, as it should.

This causes mistreatment of such hw counter resets as counter wraparound, and thereby large forward jumps of the software vblank counter which in turn cause vblank event dispatching and vblank waits to fail/hang --> userspace clients hang.

This symptom was reported on radeon-kms to cause a infinite hang of KDE Plasma 5 shell's login procedure, preventing users from logging in.

Fix this by detecting when drm_update_vblank_count() is called inside a pre->post modeset interval. If so, clamp valid vblank increments to the safe values 0 and 1, pretty much restoring the update behavior of the old update code of Linux 4.3 and earlier. Also reset the last recorded hw vblank count at call to drm_vblank_post_modeset() to be safe against hw that after modesetting, dpms on etc. only fires its first vblank irq after drm_vblank_post_modeset() was already called.

---
 drivers/gpu/drm/drm_irq.c |   16 ++++++++++++++++
 1 file changed, 16 insertions(+)

@@ -222,6 +222,21 @@ static void drm_update_vblank_count(stru
 	}
 
 	/*
+	 * Within a drm_vblank_pre_modeset - drm_vblank_post_modeset
+	 * interval? If so then vblank irqs keep running and it will likely
+	 * happen that the hardware vblank counter is not trustworthy as it
+	 * might reset at some point in that interval and vblank timestamps
+	 * are not trustworthy either in that interval. Iow. this can result
+	 * in a bogus diff >> 1 which must be avoided as it would cause
+	 * random large forward jumps of the software vblank counter.
+	 */
+	if (diff > 1 && (vblank->inmodeset & 0x2)) {
+		DRM_DEBUG_VBL("clamping vblank bump to 1 on crtc %u: diffr=%u"
+			      " due to pre-modeset.\n", pipe, diff);
+		diff = 1;
+	}
+
+	/*
 	 * FIMXE: Need to replace this hack with proper seqlocks.
 	 *
 	 * Restrict the bump of the software vblank counter to a safe maximum

@@ -1575,6 +1590,7 @@ void drm_vblank_post_modeset(struct drm_
 	if (vblank->inmodeset) {
 		spin_lock_irqsave(&dev->vbl_lock, irqflags);
 		dev->vblank_disable_allowed = true;
+		drm_reset_vblank_timestamp(dev, pipe);
 		spin_unlock_irqrestore(&dev->vbl_lock, irqflags);
 
 		if (vblank->inmodeset & 0x2)

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