[Intel-gfx] [PATCH] drm/i915: Sleep after restarting the GPU after reset

[Chris Wilson]

A piece of magic that prevents Baytrail from doing a timewarp. (Perhaps
executing an incorrect portion of the ring?) If left to itself after a
few hundred resets, the GPU will write a stale seqno causing all
requests to be completed early and the GPU then appears idle.

A msleep(1000) before the intel_gpu_reset, or after it is not
sufficient, but a short delay of a few hundred microseconds after
writing the RING_CTL is. Less than a few hundred us, or inserting a few
more posting reads into the init_ring_common is not enough. To limit the
impact, it appears we only need one after all RING_CTL writes and also we
only want to apply it when writing nonzero values into the RING_HEAD/TAIL
i.e. after reset.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98836
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/i915_gem.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index bbc6f1c9f175..54e6d8b43c2b 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -4581,6 +4581,9 @@ static int __i915_gem_restart_engines(void *data)
 			return err;
 	}
 
+	/* Small piece of magic to stablize resets of an active GPU */
+	msleep(1);
+
 	return 0;
 }
 
-- 
2.11.0