Bluetooth: RFCOMM: Use MTU auto tune logic

This reuse the L2CAP MTU auto logic to select the MTU used for RFCOMM
channels, this should increase the maximum from 1013 to 1021 when 3-DH5
is supported.

Since it does not set an L2CAP MTU we no longer need a debugfs so that
is removed.
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

include/net/bluetooth/rfcomm.h

@@ -34,7 +34,6 @@
 #define RFCOMM_DEFAULT_MTU	127
 #define RFCOMM_DEFAULT_CREDITS	7
 
-#define RFCOMM_MAX_L2CAP_MTU	1013
 #define RFCOMM_MAX_CREDITS	40
 
 #define RFCOMM_SKB_HEAD_RESERVE	8

net/bluetooth/rfcomm/core.c

@@ -40,7 +40,6 @@
 static bool disable_cfc;
 static bool l2cap_ertm;
 static int channel_mtu = -1;
-static unsigned int l2cap_mtu = RFCOMM_MAX_L2CAP_MTU;
 
 static struct task_struct *rfcomm_thread;
 
@@ -749,7 +748,8 @@ static struct rfcomm_session *rfcomm_session_create(bdaddr_t *src,
 	/* Set L2CAP options */
 	sk = sock->sk;
 	lock_sock(sk);
-	l2cap_pi(sk)->chan->imtu = l2cap_mtu;
+	/* Set MTU to 0 so L2CAP can auto select the MTU */
+	l2cap_pi(sk)->chan->imtu = 0;
 	l2cap_pi(sk)->chan->sec_level = sec_level;
 	if (l2cap_ertm)
 		l2cap_pi(sk)->chan->mode = L2CAP_MODE_ERTM;
@@ -2036,7 +2036,8 @@ static int rfcomm_add_listener(bdaddr_t *ba)
 	/* Set L2CAP options */
 	sk = sock->sk;
 	lock_sock(sk);
-	l2cap_pi(sk)->chan->imtu = l2cap_mtu;
+	/* Set MTU to 0 so L2CAP can auto select the MTU */
+	l2cap_pi(sk)->chan->imtu = 0;
 	release_sock(sk);
 
 	/* Start listening on the socket */
@@ -2234,9 +2235,6 @@ MODULE_PARM_DESC(disable_cfc, "Disable credit based flow control");
 module_param(channel_mtu, int, 0644);
 MODULE_PARM_DESC(channel_mtu, "Default MTU for the RFCOMM channel");
 
-module_param(l2cap_mtu, uint, 0644);
-MODULE_PARM_DESC(l2cap_mtu, "Default MTU for the L2CAP connection");
-
 module_param(l2cap_ertm, bool, 0644);
 MODULE_PARM_DESC(l2cap_ertm, "Use L2CAP ERTM mode for connection");