drm/i915: Wait until the intel_wakeref idle callback is complete

When waiting for idle, serialise with any ongoing callback so that it
will have completed before completing the wait.
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/20191118230254.2615942-12-chris@chris-wilson.co.uk
(cherry picked from commit f4ba0707)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>

drivers/gpu/drm/i915/intel_wakeref.c

@@ -109,8 +109,17 @@ void __intel_wakeref_init(struct intel_wakeref *wf,
 
 int intel_wakeref_wait_for_idle(struct intel_wakeref *wf)
 {
-	return wait_var_event_killable(&wf->wakeref,
+	int err;
+
+	might_sleep();
+
+	err = wait_var_event_killable(&wf->wakeref,
 				      !intel_wakeref_is_active(wf));
+	if (err)
+		return err;
+
+	intel_wakeref_unlock_wait(wf);
+	return 0;
 }
 
 static void wakeref_auto_timeout(struct timer_list *t)

drivers/gpu/drm/i915/intel_wakeref.h

@@ -151,6 +151,21 @@ intel_wakeref_unlock(struct intel_wakeref *wf)
 	mutex_unlock(&wf->mutex);
 }
 
+/**
+ * intel_wakeref_unlock_wait: Wait until the active callback is complete
+ * @wf: the wakeref
+ *
+ * Waits for the active callback (under the @wf->mutex or another CPU) is
+ * complete.
+ */
+static inline void
+intel_wakeref_unlock_wait(struct intel_wakeref *wf)
+{
+	mutex_lock(&wf->mutex);
+	mutex_unlock(&wf->mutex);
+	flush_work(&wf->work);
+}
+
 /**
  * intel_wakeref_is_active: Query whether the wakeref is currently held
  * @wf: the wakeref