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

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;
 }
 

drivers/gpu/drm/i915/intel_uncore.c

@@ -1387,10 +1387,14 @@ static void gen3_stop_engine(struct intel_engine_cs *engine)
 		DRM_DEBUG_DRIVER("%s: timed out on STOP_RING\n",
 				 engine->name);
 
-	I915_WRITE_FW(RING_CTL(base), 0);
+	I915_WRITE_FW(RING_HEAD(base), I915_READ_FW(RING_TAIL(base)));
+
 	I915_WRITE_FW(RING_HEAD(base), 0);
 	I915_WRITE_FW(RING_TAIL(base), 0);
 
+	/* The ring must be empty before it is disabled */
+	I915_WRITE_FW(RING_CTL(base), 0);
+
 	/* Check acts as a post */
 	if (I915_READ_FW(RING_HEAD(base)) != 0)
 		DRM_DEBUG_DRIVER("%s: ring head not parked\n",