[Intel-gfx] [PATCH] drm/i915: Perform a posting read of the ring start before enabling

[Chris Wilson]

Mostly paranoia as the writes to the ring registers should not be
reordered, but it should function well as an addition explicit check in
a mysteriously fragile bit of code.

References: https://bugs.freedesktop.org/show_bug.cgi?id=54575
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/intel_ringbuffer.c |    3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
index 53edce0..d5d5177 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -393,6 +393,9 @@ static int init_ring_common(struct intel_ring_buffer *ring)
 	 * also enforces ordering), otherwise the hw might lose the new ring
 	 * register values. */
 	I915_WRITE_START(ring, obj->gtt_offset);
+	if (I915_READ_START(ring) != obj->gtt_offset)
+		DRM_ERROR("failed to set %s start to GTT offset\n", ring->name);
+
 	I915_WRITE_CTL(ring,
 			((ring->size - PAGE_SIZE) & RING_NR_PAGES)
 			| RING_VALID);
-- 
1.7.10.4