drm/i915/ringbuffer: 2-step restart

We may be simply restarting too fast for the culmudgeonly gen3/gen4 as
we still see missing interrupts following a reset. So let's try
restarting a little slower, first wake up the ring empty and then tell
it about the work it has to perform.

References: https://bugs.freedesktop.org/show_bug.cgi?id=108735Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181126122821.4537-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -546,10 +546,11 @@ static int init_ring_common(struct intel_engine_cs *engine)
 	/* Check that the ring offsets point within the ring! */
 	GEM_BUG_ON(!intel_ring_offset_valid(ring, ring->head));
 	GEM_BUG_ON(!intel_ring_offset_valid(ring, ring->tail));
-
 	intel_ring_update_space(ring);
+
+	/* First wake the ring up to an empty/idle ring */
 	I915_WRITE_HEAD(engine, ring->head);
-	I915_WRITE_TAIL(engine, ring->tail);
+	I915_WRITE_TAIL(engine, ring->head);
 	(void)I915_READ_TAIL(engine);
 
 	I915_WRITE_CTL(engine, RING_CTL_SIZE(ring->size) | RING_VALID);
@@ -574,6 +575,12 @@ static int init_ring_common(struct intel_engine_cs *engine)
 	if (INTEL_GEN(dev_priv) > 2)
 		I915_WRITE_MODE(engine, _MASKED_BIT_DISABLE(STOP_RING));
 
+	/* Now awake, let it get started */
+	if (ring->tail != ring->head) {
+		I915_WRITE_TAIL(engine, ring->tail);
+		(void)I915_READ_TAIL(engine);
+	}
+
 	/* Papering over lost _interrupts_ immediately following the restart */
 	intel_engine_wakeup(engine);
 out: