drm/i915/ringbuffer: Flush writes before RING_TAIL update

Be paranoid and make sure we flush any and all writes out of the WCB
before performing the UC mmio to update the RING_TAIL. (An UC write
should itself be enough to do the flush, hence the paranoia here.) Quite
infrequently, we see problems where the GPU seems to overshoot the
RING_TAIL and so executes garbage, hence the speculation.

References: https://bugs.freedesktop.org/show_bug.cgi?id=111598
References: https://bugs.freedesktop.org/show_bug.cgi?id=111417
References: https://bugs.freedesktop.org/show_bug.cgi?id=111034Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190909113018.13300-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/intel_ringbuffer.c

@@ -930,6 +930,7 @@ static void cancel_requests(struct intel_engine_cs *engine)
 static void i9xx_submit_request(struct i915_request *request)
 {
 	i915_request_submit(request);
+	wmb(); /* paranoid flush writes out of the WCB before mmio */
 
 	ENGINE_WRITE(request->engine, RING_TAIL,
 		     intel_ring_set_tail(request->ring, request->tail));