drm/i915/gt: Delay release of engine-pm after last retirement

Keep the engine-pm awake until the next jiffie, to avoid immediate
ping-pong under moderate load. (Forcing the idle barrier excerbates the
moderate load, dramatically increasing the driver overhead.) On the
other hand, delaying the idle-barrier slightly incurs longer rc6-off
and so more power consumption.

Closes: https://gitlab.freedesktop.org/drm/intel/issues/848Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200323092841.22240-4-chris@chris-wilson.co.uk

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

@@ -350,7 +350,7 @@ void intel_context_enter_engine(struct intel_context *ce)
 void intel_context_exit_engine(struct intel_context *ce)
 {
 	intel_timeline_exit(ce->timeline);
-	intel_engine_pm_put(ce->engine);
+	intel_engine_pm_put_delay(ce->engine, 1);
 }
 
 int intel_context_prepare_remote_request(struct intel_context *ce,

drivers/gpu/drm/i915/i915_active.c

@@ -818,7 +818,7 @@ void i915_active_acquire_barrier(struct i915_active *ref)
 
 		GEM_BUG_ON(!intel_engine_pm_is_awake(engine));
 		llist_add(barrier_to_ll(node), &engine->barrier_tasks);
-		intel_engine_pm_put(engine);
+		intel_engine_pm_put_delay(engine, 1);
 	}
 }