[PATCH 3/6] drm: Fix drm_vblank_pre/post_modeset regression from Linux 4.4

[Mario Kleiner]

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.

Reported-by: Vlastimil Babka <vbabka at suse.cz>
Signed-off-by: Mario Kleiner <mario.kleiner.de at gmail.com>
Cc: <stable at vger.kernel.org> # 4.4+
Cc: michel at daenzer.net
Cc: vbabka at suse.cz
Cc: ville.syrjala at linux.intel.com
Cc: daniel.vetter at ffwll.ch
Cc: dri-devel at lists.freedesktop.org
Cc: alexander.deucher at amd.com
Cc: christian.koenig at amd.com
---
 drivers/gpu/drm/drm_irq.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
index aa2c74b..5c27ad3 100644
--- a/drivers/gpu/drm/drm_irq.c
+++ b/drivers/gpu/drm/drm_irq.c
@@ -222,6 +222,21 @@ static void drm_update_vblank_count(struct drm_device *dev, unsigned int pipe,
 	}
 
 	/*
+	 * 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;
+	}
+
+	/*
 	 * Restrict the bump of the software vblank counter to a safe maximum
 	 * value of +1 whenever there is the possibility that concurrent readers
 	 * of vblank timestamps could be active at the moment, as the current
@@ -1573,6 +1588,7 @@ void drm_vblank_post_modeset(struct drm_device *dev, unsigned int pipe)
 	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)
-- 
1.9.1