drm/i915: Skip waking the signaler when enabling before request submission

If we are enabling the breadcrumbs signaling prior to submitting the
request, we know that we cannot have missed the interrupt and can
therefore skip immediately waking the signaler to check.

This reduces a significant chunk of the __i915_gem_request_submit()
overhead for inter-engine synchronisation, for example in gem_exec_whisper.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170426080659.28771-1-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

drivers/gpu/drm/i915/intel_breadcrumbs.c

@@ -667,12 +667,13 @@ static int intel_breadcrumbs_signaler(void *arg)
 	return 0;
 }
 
-void intel_engine_enable_signaling(struct drm_i915_gem_request *request)
+void intel_engine_enable_signaling(struct drm_i915_gem_request *request,
+				   bool wakeup)
 {
 	struct intel_engine_cs *engine = request->engine;
 	struct intel_breadcrumbs *b = &engine->breadcrumbs;
 	struct rb_node *parent, **p;
-	bool first, wakeup;
+	bool first;
 	u32 seqno;
 
 	/* Note that we may be called from an interrupt handler on another
@@ -705,7 +706,7 @@ void intel_engine_enable_signaling(struct drm_i915_gem_request *request)
 	 * If we are the oldest waiter, enable the irq (after which we
 	 * must double check that the seqno did not complete).
 	 */
-	wakeup = __intel_engine_add_wait(engine, &request->signaling.wait);
+	wakeup &= __intel_engine_add_wait(engine, &request->signaling.wait);
 
 	/* Now insert ourselves into the retirement ordered list of signals
 	 * on this engine. We track the oldest seqno as that will be the