drm/etnaviv: restore ETNA_PREP_NOSYNC behaviour

This reverts commit cd34db4a (drm/etnaviv: Remove manual call to
reservation_object_test_signaled_rcu before wait), as the patch to turn
reservation_object_wait_timeout_rcu() into
reservation_object_test_signaled_rcu() with a 0 timeout has been reverted.
This causes the driver to call into the fence wait, even with a timeout of 0

The etnaviv BO cache depends on ETNA_PREP_NOSYNC to be wait-free, even if
the BO has attached fences, so restore the behaviour for this flag.
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>

drivers/gpu/drm/etnaviv/etnaviv_gem.c

@@ -411,16 +411,20 @@ int etnaviv_gem_cpu_prep(struct drm_gem_object *obj, u32 op,
 	struct etnaviv_gem_object *etnaviv_obj = to_etnaviv_bo(obj);
 	struct drm_device *dev = obj->dev;
 	bool write = !!(op & ETNA_PREP_WRITE);
-	unsigned long remain =
-		op & ETNA_PREP_NOSYNC ? 0 : etnaviv_timeout_to_jiffies(timeout);
-	long lret;
+	int ret;
+
+	if (op & ETNA_PREP_NOSYNC) {
+		if (!reservation_object_test_signaled_rcu(etnaviv_obj->resv,
+							  write))
+			return -EBUSY;
+	} else {
+		unsigned long remain = etnaviv_timeout_to_jiffies(timeout);
 
-	lret = reservation_object_wait_timeout_rcu(etnaviv_obj->resv,
+		ret = reservation_object_wait_timeout_rcu(etnaviv_obj->resv,
 							  write, true, remain);
-	if (lret < 0)
-		return lret;
-	else if (lret == 0)
-		return remain == 0 ? -EBUSY : -ETIMEDOUT;
+		if (ret <= 0)
+			return ret == 0 ? -ETIMEDOUT : ret;
+	}
 
 	if (etnaviv_obj->flags & ETNA_BO_CACHED) {
 		if (!etnaviv_obj->sgt) {