media: v4l2-async: Add notifier operation to destroy asd instances

Drivers typically extend the v4l2_async_subdev structure by embedding it
in a driver-specific structure, to store per-subdev custom data. The
v4l2_async_subdev instances are freed by the v4l2-async framework, which
makes this mechanism cumbersome to use safely when custom data needs
special treatment to be destroyed (such as freeing additional memory, or
releasing references to kernel objects).

To ease this, add a .destroy() operation to the
v4l2_async_notifier_operations structure. The operation is called right
before the v4l2_async_subdev is freed, giving drivers a chance to
destroy data if needed.

Signed-off-by: Laurent Pinchart <[e-mail]>
Reviewed-by: Hans Verkuil <[e-mail]>
Acked-by: Sakari Ailus <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

Documentation/driver-api/media/v4l2-subdev.rst

···
 .complete() callback is called. When a subdevice is removed from the
 system the .unbind() method is called. All three callbacks are optional.
 
+Drivers can store any type of custom data in their driver-specific
+:c:type:`v4l2_async_subdev` wrapper. If any of that data requires special
+handling when the structure is freed, drivers must implement the ``.destroy()``
+notifier callback. The framework will call it right before freeing the
+:c:type:`v4l2_async_subdev`.
+
 Calling subdev operations
 ~~~~~~~~~~~~~~~~~~~~~~~~~
 

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

···
 	return n->ops->complete(n);
 }
 
+static void v4l2_async_nf_call_destroy(struct v4l2_async_notifier *n,
+				       struct v4l2_async_subdev *asd)
+{
+	if (!n->ops || !n->ops->destroy)
+		return;
+
+	n->ops->destroy(asd);
+}
+
 static bool match_i2c(struct v4l2_async_notifier *notifier,
 		      struct v4l2_subdev *sd, struct v4l2_async_subdev *asd)
 {
···
 		}
 
 		list_del(&asd->asd_list);
+		v4l2_async_nf_call_destroy(notifier, asd);
 		kfree(asd);
 	}
 }

include/media/v4l2-async.h

···
  * @complete:	All subdevices have been probed successfully. The complete
  *		callback is only executed for the root notifier.
  * @unbind:	a subdevice is leaving
+ * @destroy:	the asd is about to be freed
  */
 struct v4l2_async_notifier_operations {
 	int (*bound)(struct v4l2_async_notifier *notifier,
···
 	void (*unbind)(struct v4l2_async_notifier *notifier,
 		       struct v4l2_subdev *subdev,
 		       struct v4l2_async_subdev *asd);
+	void (*destroy)(struct v4l2_async_subdev *asd);
 };
 
 /**