drm/i915: Empty the ring before disabling

An interesting snippet from Sandybridge's prm:

"Although a Ring Buffer can be enabled in the non-empty state, it must
not be disabled unless it is empty. Attempting to disable a Ring Buffer
in the non-empty state is UNDEFINED."

Let's avoid the undefined behaviour as we disable the rings prior to
reset and resume.

v2: Tell HEAD to catch up to TAIL (empty ring) first, then reset both to
0 (supposedly while stopped).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171027094311.30380-1-chris@chris-wilson.co.uk

1 files changed, 5 insertions, 1 deletions

diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
index 05e01446b00b..47fadf8da84e 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -480,10 +480,14 @@ static bool stop_ring(struct intel_engine_cs *engine)
                 }
         }
 
-        I915_WRITE_CTL(engine, 0);
+        I915_WRITE_HEAD(engine, I915_READ_TAIL(engine));
+
         I915_WRITE_HEAD(engine, 0);
         I915_WRITE_TAIL(engine, 0);
 
+        /* The ring must be empty before it is disabled */
+        I915_WRITE_CTL(engine, 0);
+
         return (I915_READ_HEAD(engine) & HEAD_ADDR) == 0;
 }