drm/i915: Skip execlists_dequeue() early if the list is empty

Do an early read of the execlists' queue before we take the spinlock and
start checking. This is safe as the first writer to the execlists queue
will cause the tasklet to be run again after a memory barrier.

v2: Keep guc in sync with execlists queue changes
v3: Explain the mb between the tasklet running on one cpu and the
execlist_first update and schedule from a second cpu.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170317120716.17191-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_guc_submission.c

@@ -577,6 +577,18 @@ static bool i915_guc_dequeue(struct intel_engine_cs *engine)
 	struct rb_node *rb;
 	bool submit = false;
 
+	/* After execlist_first is updated, the tasklet will be rescheduled.
+	 *
+	 * If we are currently running (inside the tasklet) and a third
+	 * party queues a request and so updates engine->execlist_first under
+	 * the spinlock (which we have elided), it will atomically set the
+	 * TASKLET_SCHED flag causing the us to be re-executed and pick up
+	 * the change in state (the update to TASKLET_SCHED incurs a memory
+	 * barrier making this cross-cpu checking safe).
+	 */
+	if (!READ_ONCE(engine->execlist_first))
+		return false;
+
 	spin_lock_irqsave(&engine->timeline->lock, flags);
 	rb = engine->execlist_first;
 	while (rb) {

drivers/gpu/drm/i915/intel_lrc.c

@@ -403,6 +403,18 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
 	struct rb_node *rb;
 	bool submit = false;
 
+	/* After execlist_first is updated, the tasklet will be rescheduled.
+	 *
+	 * If we are currently running (inside the tasklet) and a third
+	 * party queues a request and so updates engine->execlist_first under
+	 * the spinlock (which we have elided), it will atomically set the
+	 * TASKLET_SCHED flag causing the us to be re-executed and pick up
+	 * the change in state (the update to TASKLET_SCHED incurs a memory
+	 * barrier making this cross-cpu checking safe).
+	 */
+	if (!READ_ONCE(engine->execlist_first))
+		return;
+
 	last = port->request;
 	if (last)
 		/* WaIdleLiteRestore:bdw,skl