[Bluetooth] Reduce L2CAP MTU for RFCOMM connections

This patch reduces the default L2CAP MTU for all RFCOMM connections
from 1024 to 1013 to improve the interoperability with some broken
RFCOMM implementations. To make this more flexible the L2CAP MTU
becomes also a module parameter and so it can changed at runtime.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

include/net/bluetooth/rfcomm.h

@@ -33,7 +33,7 @@
 #define RFCOMM_DEFAULT_MTU	127
 #define RFCOMM_DEFAULT_CREDITS	7
 
-#define RFCOMM_MAX_L2CAP_MTU	1024
+#define RFCOMM_MAX_L2CAP_MTU	1013
 #define RFCOMM_MAX_CREDITS	40
 
 #define RFCOMM_SKB_HEAD_RESERVE	8

net/bluetooth/rfcomm/core.c

@@ -46,13 +46,15 @@
 #include <net/bluetooth/l2cap.h>
 #include <net/bluetooth/rfcomm.h>
 
-#define VERSION "1.6"
-
 #ifndef CONFIG_BT_RFCOMM_DEBUG
 #undef  BT_DBG
 #define BT_DBG(D...)
 #endif
 
+#define VERSION "1.7"
+
+static unsigned int l2cap_mtu = RFCOMM_MAX_L2CAP_MTU;
+
 static struct task_struct *rfcomm_thread;
 
 static DECLARE_MUTEX(rfcomm_sem);
@@ -623,7 +625,7 @@ static struct rfcomm_session *rfcomm_session_create(bdaddr_t *src, bdaddr_t *dst
 	/* Set L2CAP options */
 	sk = sock->sk;
 	lock_sock(sk);
-	l2cap_pi(sk)->imtu = RFCOMM_MAX_L2CAP_MTU;
+	l2cap_pi(sk)->imtu = l2cap_mtu;
 	release_sock(sk);
 
 	s = rfcomm_session_add(sock, BT_BOUND);
@@ -1868,7 +1870,7 @@ static int rfcomm_add_listener(bdaddr_t *ba)
 	/* Set L2CAP options */
 	sk = sock->sk;
 	lock_sock(sk);
-	l2cap_pi(sk)->imtu = RFCOMM_MAX_L2CAP_MTU;
+	l2cap_pi(sk)->imtu = l2cap_mtu;
 	release_sock(sk);
 
 	/* Start listening on the socket */
@@ -2070,6 +2072,9 @@ static void __exit rfcomm_exit(void)
 module_init(rfcomm_init);
 module_exit(rfcomm_exit);
 
+module_param(l2cap_mtu, uint, 0644);
+MODULE_PARM_DESC(l2cap_mtu, "Default MTU for the L2CAP connection");
+
 MODULE_AUTHOR("Maxim Krasnyansky <maxk@qualcomm.com>, Marcel Holtmann <marcel@holtmann.org>");
 MODULE_DESCRIPTION("Bluetooth RFCOMM ver " VERSION);
 MODULE_VERSION(VERSION);