[Intel-gfx] [PATCH] drm/i915: Double check ring is idle before declaring the GPU wedged

[Chris Wilson]

During ring initialisation, sometimes we observe, though not in
production hardware, that the idle flag is not set even though the ring
is empty. Double check before giving up.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Damien Lespiau <damien.lespiau at intel.com>
---
 drivers/gpu/drm/i915/intel_ringbuffer.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
index a0831c309eab..d72d5e0e693d 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -467,7 +467,12 @@ static bool stop_ring(struct intel_engine_cs *ring)
 		I915_WRITE_MODE(ring, _MASKED_BIT_ENABLE(STOP_RING));
 		if (wait_for((I915_READ_MODE(ring) & MODE_IDLE) != 0, 1000)) {
 			DRM_ERROR("%s : timed out trying to stop ring\n", ring->name);
-			return false;
+			/* Sometimes we observe that the idle flag is not
+			 * set even though the ring is empty. So double
+			 * check before giving up.
+			 */
+			if (I915_READ_HEAD(ring) != I915_READ_TAIL(ring))
+				return false;
 		}
 	}
 
-- 
2.1.0.rc1