media: v4l2-async: Document asd allocation requirements

The v4l2_async_notifier_add_subdev() function requires the asd pointer
it receives to be allocated dynamically, but doesn't explicitly say so.
Only one driver out of 13 get its right (atmel-sama5d2-isc.c, but with
memory leaks in the error paths), clearly showing we have an issue.

Update the v4l2_async_notifier_add_subdev() documentation to clearly
state the allocation requirement. Whether this will be enough to avoid
new offending code isn't certain, but it's a good first step
nonetheless.

Fixes: 9ca46531 ("media: v4l: fwnode: Support generic parsing of graph endpoints in a device")
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

include/media/v4l2-async.h

@@ -154,8 +154,9 @@ void v4l2_async_notifier_init(struct v4l2_async_notifier *notifier);
  * @notifier: pointer to &struct v4l2_async_notifier
  * @asd: pointer to &struct v4l2_async_subdev
  *
- * Call this function before registering a notifier to link the
- * provided asd to the notifiers master @asd_list.
+ * Call this function before registering a notifier to link the provided @asd to
+ * the notifiers master @asd_list. The @asd must be allocated with k*alloc() as
+ * it will be freed by the framework when the notifier is destroyed.
  */
 int v4l2_async_notifier_add_subdev(struct v4l2_async_notifier *notifier,
 				   struct v4l2_async_subdev *asd);