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@xxxxxxxxxxxx>
---
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
