[PATCH 5/6] drm: Prevent vblank counter jumps with timestamp based update method.

Mario Kleiner mario.kleiner.de at gmail.com
Mon Feb 8 01:13:28 UTC 2016

The changes to drm_update_vblank_count() in Linux 4.4 added a
method to emulate a hardware vblank counter by use of high
precision vblank timestamps if a kms driver supports those,
but doesn't suppport hw vblank counters.

That method assumes that the old timestamp from a previous
invocation is valid, but that is not always the case. E.g.,
if drm_reset_vblank_timestamp() gets called during drm_vblank_on()
or drm_update_vblank_count() gets called outside vblank irq and
the high precision timestamping can't deliver a precise timestamp,
ie. drm_get_last_vbltimestamp() delivers a return value of false,
then those functions will initialize the old timestamp to zero
to mark it as invalid.

A following call to drm_update_vblank_count() would then calculate
elapsed time with vblank irqs off as current vblank timestamp minus
the zero old timestamp and compute a software vblank counter increment
that corresponds to system uptime, causing a large forward jump of the
software vblank counter. That jump in turn can cause too long waits
in drmWaitVblank and very long delays in delivery of vblank events,
resulting in hangs of userspace clients.

This problem can be observed on nouveau-kms during machine
suspend->resume cycles, where drm_vblank_off is called during
suspend, drm_vblank_on is called during resume and the first
queries to drm_get_last_vbltimestamp() don't deliver high
precision timestamps, resulting in a large harmful counter jump.

Fix this by checking if the old timestamp used for this calculations
is zero == invalid. If so, perform a counter increment of +1 to
prevent large counter jumps and reinitialize the timestamps to
sane values.

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 | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
index fb17c45..88bdf19 100644
--- a/drivers/gpu/drm/drm_irq.c
+++ b/drivers/gpu/drm/drm_irq.c
@@ -216,6 +216,13 @@ static void drm_update_vblank_count(struct drm_device *dev, unsigned int pipe,
 			DRM_DEBUG_VBL("crtc %u: Redundant vblirq ignored."
 				      " diff_ns = %lld, framedur_ns = %d)\n",
 				      pipe, (long long) diff_ns, framedur_ns);
+		/* No valid t_old to calculate diff? Bump +1 to force reinit. */
+		if (t_old->tv_sec == 0 && t_old->tv_usec == 0) {
+			DRM_DEBUG_VBL("crtc %u: No baseline ts. Bump +1.\n",
+				      pipe);
+			diff = 1;
+		}
 	} else {
 		/* some kind of default for drivers w/o accurate vbl timestamping */
 		diff = (flags & DRM_CALLED_FROM_VBLIRQ) != 0;

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