drm/i915/execlists: Check the sentinel is alone in the ELSP

We only use sentinel requests for "preempt-to-idle" passes, so assert
that they are the only request in a new submission.
Signed-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/20200302085812.4172450-12-chris@chris-wilson.co.uk

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

@@ -1448,6 +1448,7 @@ assert_pending_valid(const struct intel_engine_execlists *execlists,
 {
 	struct i915_request * const *port, *rq;
 	struct intel_context *ce = NULL;
+	bool sentinel = false;
 
 	trace_ports(execlists, msg, execlists->pending);
 
@@ -1481,6 +1482,26 @@ assert_pending_valid(const struct intel_engine_execlists *execlists,
 		}
 		ce = rq->context;
 
+		/*
+		 * Sentinels are supposed to be lonely so they flush the
+		 * current exection off the HW. Check that they are the
+		 * only request in the pending submission.
+		 */
+		if (sentinel) {
+			GEM_TRACE_ERR("context:%llx after sentinel in pending[%zd]\n",
+				      ce->timeline->fence_context,
+				      port - execlists->pending);
+			return false;
+		}
+
+		sentinel = i915_request_has_sentinel(rq);
+		if (sentinel && port != execlists->pending) {
+			GEM_TRACE_ERR("sentinel context:%llx not in prime position[%zd]\n",
+				      ce->timeline->fence_context,
+				      port - execlists->pending);
+			return false;
+		}
+
 		/* Hold tightly onto the lock to prevent concurrent retires! */
 		if (!spin_trylock_irqsave(&rq->lock, flags))
 			continue;