Bluetooth: Auto tune if input MTU is set to 0

This enables the code to set the input MTU using the underline link
packet types when set to 0, previously this would likely be rejected by
the remote peer since it would be bellow the minimal of 48 for BR/EDR
or 23 for LE, that way it shall be safe to use 0 without causing any
side effects.

This is convenient for the likes of A2DP transport, see:

https://habr.com/en/post/456182/Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/bluetooth/l2cap_core.c

@@ -1289,6 +1289,9 @@ static void l2cap_le_connect(struct l2cap_chan *chan)
 	if (test_and_set_bit(FLAG_LE_CONN_REQ_SENT, &chan->flags))
 		return;
 
+	if (!chan->imtu)
+		chan->imtu = chan->conn->mtu;
+
 	l2cap_le_flowctl_init(chan, 0);
 
 	req.psm     = chan->psm;
@@ -3226,6 +3229,49 @@ static inline void l2cap_txwin_setup(struct l2cap_chan *chan)
 	chan->ack_win = chan->tx_win;
 }
 
+static void l2cap_mtu_auto(struct l2cap_chan *chan)
+{
+	struct hci_conn *conn = chan->conn->hcon;
+
+	chan->imtu = L2CAP_DEFAULT_MIN_MTU;
+
+	/* The 2-DH1 packet has between 2 and 56 information bytes
+	 * (including the 2-byte payload header)
+	 */
+	if (!(conn->pkt_type & HCI_2DH1))
+		chan->imtu = 54;
+
+	/* The 3-DH1 packet has between 2 and 85 information bytes
+	 * (including the 2-byte payload header)
+	 */
+	if (!(conn->pkt_type & HCI_3DH1))
+		chan->imtu = 83;
+
+	/* The 2-DH3 packet has between 2 and 369 information bytes
+	 * (including the 2-byte payload header)
+	 */
+	if (!(conn->pkt_type & HCI_2DH3))
+		chan->imtu = 367;
+
+	/* The 3-DH3 packet has between 2 and 554 information bytes
+	 * (including the 2-byte payload header)
+	 */
+	if (!(conn->pkt_type & HCI_3DH3))
+		chan->imtu = 552;
+
+	/* The 2-DH5 packet has between 2 and 681 information bytes
+	 * (including the 2-byte payload header)
+	 */
+	if (!(conn->pkt_type & HCI_2DH5))
+		chan->imtu = 679;
+
+	/* The 3-DH5 packet has between 2 and 1023 information bytes
+	 * (including the 2-byte payload header)
+	 */
+	if (!(conn->pkt_type & HCI_3DH5))
+		chan->imtu = 1021;
+}
+
 static int l2cap_build_conf_req(struct l2cap_chan *chan, void *data, size_t data_size)
 {
 	struct l2cap_conf_req *req = data;
@@ -3255,8 +3301,12 @@ static int l2cap_build_conf_req(struct l2cap_chan *chan, void *data, size_t data
 	}
 
 done:
-	if (chan->imtu != L2CAP_DEFAULT_MTU)
-		l2cap_add_conf_opt(&ptr, L2CAP_CONF_MTU, 2, chan->imtu, endptr - ptr);
+	if (chan->imtu != L2CAP_DEFAULT_MTU) {
+		if (!chan->imtu)
+			l2cap_mtu_auto(chan);
+		l2cap_add_conf_opt(&ptr, L2CAP_CONF_MTU, 2, chan->imtu,
+				   endptr - ptr);
+	}
 
 	switch (chan->mode) {
 	case L2CAP_MODE_BASIC: