drm/i915: Double check ring is idle before declaring the GPU wedged

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: NChris Wilson <chris@chris-wilson.co.uk>
Cc: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: NDamien Lespiau <damien.lespiau@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -478,7 +478,12 @@ 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;
 		}
 	}