media: v4l: async: Allow async notifier register call succeed with no subdevs

The information on how many async sub-devices would be bindable to a
notifier is typically dependent on information from platform firmware and
it's not driver's business to be aware of that.

Many V4L2 main drivers are perfectly usable (and useful) without async
sub-devices and so if there aren't any around, just proceed call the
notifier's complete callback immediately without registering the notifier
itself.

If a driver needs to check whether there are async sub-devices available,
it can be done by inspecting the notifier's num_subdevs field which tells
the number of async sub-devices.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/media/v4l2-core/v4l2-async.c

@@ -180,14 +180,22 @@ int v4l2_async_notifier_register(struct v4l2_device *v4l2_dev,
 	int ret;
 	int i;
 
-	if (!v4l2_dev || !notifier->num_subdevs ||
-	    notifier->num_subdevs > V4L2_MAX_SUBDEVS)
+	if (!v4l2_dev || notifier->num_subdevs > V4L2_MAX_SUBDEVS)
 		return -EINVAL;
 
 	notifier->v4l2_dev = v4l2_dev;
 	INIT_LIST_HEAD(&notifier->waiting);
 	INIT_LIST_HEAD(&notifier->done);
 
+	if (!notifier->num_subdevs) {
+		int ret;
+
+		ret = v4l2_async_notifier_call_complete(notifier);
+		notifier->v4l2_dev = NULL;
+
+		return ret;
+	}
+
 	for (i = 0; i < notifier->num_subdevs; i++) {
 		asd = notifier->subdevs[i];