[media] v4l2-device: check if already unregistered

It was possible to unregister an already unregistered v4l2_device struct.
Add a check whether that already happened and just return if that was
the case.
Also refuse to register a v4l2_device if both the dev and name fields are
empty. A warning was already produced in that case, but since the name field
is now used to detect whether or not the v4l2_device was already unregistered
this particular combination should be rejected.
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

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

@@ -44,7 +44,8 @@ int v4l2_device_register(struct device *dev, struct v4l2_device *v4l2_dev)
 	v4l2_dev->dev = dev;
 	if (dev == NULL) {
 		/* If dev == NULL, then name must be filled in by the caller */
-		WARN_ON(!v4l2_dev->name[0]);
+		if (WARN_ON(!v4l2_dev->name[0]))
+			return -EINVAL;
 		return 0;
 	}
 
@@ -105,7 +106,9 @@ void v4l2_device_unregister(struct v4l2_device *v4l2_dev)
 {
 	struct v4l2_subdev *sd, *next;
 
-	if (v4l2_dev == NULL)
+	/* Just return if v4l2_dev is NULL or if it was already
+	 * unregistered before. */
+	if (v4l2_dev == NULL || !v4l2_dev->name[0])
 		return;
 	v4l2_device_disconnect(v4l2_dev);
 
@@ -135,6 +138,8 @@ void v4l2_device_unregister(struct v4l2_device *v4l2_dev)
 		}
 #endif
 	}
+	/* Mark as unregistered, thus preventing duplicate unregistrations */
+	v4l2_dev->name[0] = '\0';
 }
 EXPORT_SYMBOL_GPL(v4l2_device_unregister);