drm/i915: Flush the tasklet when checking for idle

In order to reduce latency when checking for idle we kick the tasklet
directly. Sometimes this is not enough as it is queued on another cpu
and so to improve the accuracy of this idle-check (and so to reduce
latency overall by avoiding another pass, or worse declaring a timeout!)
wait for the tasklet to complete.

References: https://bugs.freedesktop.org/show_bug.cgi?id=107916


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180914080017.30308-2-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_engine_cs.c

@@ -990,6 +990,9 @@ bool intel_engine_is_idle(struct intel_engine_cs *engine)
 		}
 		local_bh_enable();
 
+		/* Otherwise flush the tasklet if it was on another cpu */
+		tasklet_unlock_wait(t);
+
 		if (READ_ONCE(engine->execlists.active))
 			return false;
 	}