[PATCH 4/4] v4l: async: add comment about re-probing to v4l2_async_notifier_unregister()

["Niklas Söderlund" <niklas.soderlund+renesas@ragnatech.se>]

The re-probing of subdevices when unregistering a notifier is tricky to
understand, and implemented somewhat as a hack. Add a comment trying to
explain why the re-probing is needed in the first place and why existing
helper functions can't be used in this situation.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
---
 drivers/media/v4l2-core/v4l2-async.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/drivers/media/v4l2-core/v4l2-async.c b/drivers/media/v4l2-core/v4l2-async.c
index d91ff0a33fd3eaff..a3c5a1f6d4d2ab03 100644
--- a/drivers/media/v4l2-core/v4l2-async.c
+++ b/drivers/media/v4l2-core/v4l2-async.c
@@ -234,6 +234,23 @@ void v4l2_async_notifier_unregister(struct v4l2_async_notifier *notifier)
 
 	mutex_unlock(&list_lock);
 
+	/*
+	 * Try to re-probe the subdevices which where part of the notifier.
+	 * This is done so subdevices which where part of the notifier will
+	 * be re-probed to a pristine state and put back on the global
+	 * list of subdevices so they can once more be found and associated
+	 * with a new notifier.
+	 *
+	 * One might be tempted to use device_reprobe() to handle the re-
+	 * probing. Unfortunately this is not possible since some video
+	 * device drivers call v4l2_async_notifier_unregister() from
+	 * there remove function leading to a dead lock situation on
+	 * device_lock(dev->parent). This lock is held when video device
+	 * drivers remove function is called and device_reprobe() also
+	 * tries to take the same lock, so using it here could lead to a
+	 * dead lock situation.
+	 */
+
 	for (i = 0; i < count; i++) {
 		/* If we handled USB devices, we'd have to lock the parent too */
 		device_release_driver(dev[i]);
-- 
2.13.3