Bluetooth: l2cap: add l2cap_user sub-modules

Several sub-modules like HIDP, rfcomm, ... need to track l2cap
connections. The l2cap_conn->hcon->dev object is used as parent for sysfs
devices so the sub-modules need to be notified when the hci_conn object is
removed from sysfs.

As submodules normally use the l2cap layer, the l2cap_user objects are
registered there instead of on the underlying hci_conn object. This avoids
any direct dependency on the HCI layer and lets the l2cap core handle any
specifics.

This patch introduces l2cap_user objects which contain a "probe" and
"remove" callback. You can register them on any l2cap_conn object and if
it is active, the "probe" callback will get called. Otherwise, an error is
returned.

The l2cap_conn object will call your "remove" callback directly before it
is removed from user-space. This allows you to remove your submodules
_before_ the parent l2cap_conn and hci_conn object is removed.

At any time you can asynchronously unregister your l2cap_user object if
your submodule vanishes before the l2cap_conn object does.

There is no way around l2cap_user. If we want wire-protocols in the
kernel, we always want the hci_conn object as parent in the sysfs tree. We
cannot use a channel here since we might need multiple channels for a
single protocol.
But the problem is, we _must_ get notified when an l2cap_conn object is
removed. We cannot use reference-counting for object-removal! This is not
how it works. If a hardware is removed, we should immediately remove the
object from sysfs. Any other behavior would be inconsistent with the rest
of the system. Also note that device_del() might sleep, but it doesn't
wait for user-space or block very long. It only _unlinks_ the object from
sysfs and the whole device-tree. Everything else is handled by ref-counts!
This is exactly what the other sub-modules must do: unlink their devices
when the "remove" l2cap_user callback is called. They should not do any
cleanup or synchronous shutdowns.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>

1 files changed, 10 insertions, 0 deletions

diff --git a/include/net/bluetooth/l2cap.h b/include/net/bluetooth/l2cap.h
index 7b4cc5b98535..fb94cf13c777 100644
--- a/include/net/bluetooth/l2cap.h
+++ b/include/net/bluetooth/l2cap.h
@@ -584,6 +584,13 @@ struct l2cap_conn {
         struct list_head        chan_l;
         struct mutex            chan_lock;
         struct kref             ref;
+        struct list_head        users;
+};
+
+struct l2cap_user {
+        struct list_head list;
+        int (*probe) (struct l2cap_conn *conn, struct l2cap_user *user);
+        void (*remove) (struct l2cap_conn *conn, struct l2cap_user *user);
 };
 
 #define L2CAP_INFO_CL_MTU_REQ_SENT      0x01
@@ -817,4 +824,7 @@ void __l2cap_physical_cfm(struct l2cap_chan *chan, int result);
 void l2cap_conn_get(struct l2cap_conn *conn);
 void l2cap_conn_put(struct l2cap_conn *conn);
 
+int l2cap_register_user(struct l2cap_conn *conn, struct l2cap_user *user);
+void l2cap_unregister_user(struct l2cap_conn *conn, struct l2cap_user *user);
+
 #endif /* __L2CAP_H */