[Intel-gfx] [PATCH] drm/i915: correctly order the ring init sequence

[Daniel Vetter]

We may only start to set up the new register values after having
confirmed that the ring is truely off. Otherwise the hw might lose the
newly written register values. This is caught later on in the init
sequence, when we check whether the register writes have stuck.

Cc: stable at vger.kernel.org
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=50522
Tested-by: Yang Guang <guang.a.yang at intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>
---
 drivers/gpu/drm/i915/intel_ringbuffer.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
index bf0195a..5b19917 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -289,8 +289,6 @@ static int init_ring_common(struct intel_ring_buffer *ring)
 	I915_WRITE_HEAD(ring, 0);
 	ring->write_tail(ring, 0);
 
-	/* Initialize the ring. */
-	I915_WRITE_START(ring, obj->gtt_offset);
 	head = I915_READ_HEAD(ring) & HEAD_ADDR;
 
 	/* G45 ring initialization fails to reset head to zero */
@@ -316,6 +314,10 @@ static int init_ring_common(struct intel_ring_buffer *ring)
 		}
 	}
 
+	/* Initialize the ring. This must happen _after_ we have confirmed that
+	 * the ring is off (with the above head == 0 check), otherwise the hw
+	 * might lose the new ring register values. */
+	I915_WRITE_START(ring, obj->gtt_offset);
 	I915_WRITE_CTL(ring,
 			((ring->size - PAGE_SIZE) & RING_NR_PAGES)
 			| RING_VALID);
-- 
1.7.10.4