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

Commit Message

Mario Kleiner Feb. 12, 2016, 7:30 p.m. UTC

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 <[e-mail]>
Signed-off-by: Mario Kleiner <[e-mail]>
Reviewed-by: Daniel Vetter <[e-mail]>
Tested-by: Vlastimil Babka <[e-mail]>

Cc: <[e-mail]> # 4.4+
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
---
 drivers/gpu/drm/drm_irq.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

Comments

Vlastimil Babka Feb. 24, 2016, 8:59 a.m. UTC | #1

On 02/12/2016 08:30 PM, Mario Kleiner wrote:
> 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 <[e-mail]>
> Signed-off-by: Mario Kleiner <[e-mail]>
> Reviewed-by: Daniel Vetter <[e-mail]>
> Tested-by: Vlastimil Babka <[e-mail]>

FWIW I have applied patches 1-4 of this v2 to 4.4.2 and it works fine so 
far on my radeon. Patch 6/6 was for nouveau so I ignored it, and 5/6 
didn't have stable tag (and from the description I couldn't decide if I 
should include it or not).

Thanks, Vlastimil

Patch

diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
index 92ad62f..055b0fa 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;
+	}
+
+	/*
 	 * 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_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)