drm/i915/execlists: Clear semaphore immediately upon ELSP promotion

There is no significance to our delay before clearing the semaphore the
engine is waiting on, so release it as soon as we acknowledge the CS
update following our preemption request. This should allow the GPU to
resume work earlier, if it was stuck on the semaphore at the end of a
request.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191015093204.25693-1-chris@chris-wilson.co.uk

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

@@ -1910,6 +1910,9 @@ static void process_csb(struct intel_engine_cs *engine)
 		else
 			promote = gen8_csb_parse(execlists, buf + 2 * head);
 		if (promote) {
+			if (!inject_preempt_hang(execlists))
+				ring_set_paused(engine, 0);
+
 			/* cancel old inflight, prepare for switch */
 			trace_ports(execlists, "preempted", execlists->active);
 			while (*execlists->active)
@@ -1926,9 +1929,6 @@ static void process_csb(struct intel_engine_cs *engine)
 			if (enable_timeslice(execlists))
 				mod_timer(&execlists->timer, jiffies + 1);
 
-			if (!inject_preempt_hang(execlists))
-				ring_set_paused(engine, 0);
-
 			WRITE_ONCE(execlists->pending[0], NULL);
 		} else {
 			GEM_BUG_ON(!*execlists->active);