[PATCH 4.4 231/342] drm: Fix drm_vblank_pre/post_modeset regression from Linux 4.4

[Greg Kroah-Hartman]

4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Mario Kleiner <mario.kleiner.de@xxxxxxxxx>

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.

Reported-by: Vlastimil Babka <vbabka@xxxxxxx>
Signed-off-by: Mario Kleiner <mario.kleiner.de@xxxxxxxxx>
Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
Tested-by: Vlastimil Babka <vbabka@xxxxxxx>
Cc: michel@xxxxxxxxxxx
Cc: vbabka@xxxxxxx
Cc: ville.syrjala@xxxxxxxxxxxxxxx
Cc: daniel.vetter@xxxxxxxx
Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
Cc: alexander.deucher@xxxxxxx
Cc: christian.koenig@xxxxxxx
Signed-off-by: Dave Airlie <airlied@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

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

--- a/drivers/gpu/drm/drm_irq.c
+++ b/drivers/gpu/drm/drm_irq.c
@@ -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)