Re: [PATCHv1 16/17] Bluetooth: A2MP: Handling fixed channels

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On Tue, 22 May 2012, Andrei Emeltchenko wrote:

Hi Mat,

Thanks for review,

On Mon, May 21, 2012 at 03:28:57PM -0700, Mat Martineau wrote:
@@ -480,7 +487,7 @@ static void l2cap_chan_del(struct l2cap_chan *chan, int err)
	struct sock *sk = chan->sk;
	struct l2cap_conn *conn = chan->conn;
-	struct sock *parent = bt_sk(sk)->parent;
+	struct sock *parent;


@@ -496,6 +503,9 @@ static void l2cap_chan_del(struct l2cap_chan *chan, int err)

+	if (chan->chan_type == L2CAP_CHAN_CONN_FIX_A2MP)
+		goto clean;

Since this has more to do with socketless channels than A2MP, it
might be better to do something more generic here like add a
callback to l2cap_ops so this socket code can be removed from
l2cap_core rather than worked around.

This can be done. What would be good function name? l2cap_chan_del_cb,
l2cap_chan_unlink ?

The downside is that the code will be used only once ...

I think the long term goal is to get all socket code out of l2cap_core.c and rework RFCOMM to directly use l2cap_chan instead of sockets, so the callback would be used elsewhere.


	__l2cap_state_change(chan, BT_CLOSED);
@@ -504,6 +514,7 @@ static void l2cap_chan_del(struct l2cap_chan *chan, int err)
	if (err)
		__l2cap_chan_set_err(chan, err);

+	parent = bt_sk(sk)->parent;
	if (parent) {
		parent->sk_data_ready(parent, 0);
@@ -515,6 +526,7 @@ static void l2cap_chan_del(struct l2cap_chan *chan, int err)
	if (test_bit(CONF_NOT_COMPLETE, &chan->conf_state))


	if (chan->mode == L2CAP_MODE_ERTM) {
@@ -992,6 +1004,9 @@ static void l2cap_send_disconn_req(struct l2cap_conn *conn, struct l2cap_chan *c
	if (!conn)

+	if (chan->chan_type == L2CAP_CHAN_CONN_FIX_A2MP)
+		return;

There are places in the ERTM state machine that call
l2cap_send_disconn_req() to tear down an unrecoverable ERTM
connection.  If the connection remained up, I'm not sure how ERTM
will behave.

I can rework this part so that in a case of fixed channel we do not send
Disc Req and do not lock sk. So this would adds clearing timers and then


I don't know that the spec is very clear about what to do when ERTM
must disconnect the channel, but the channel is fixed.

I think we shall do nothing.

I think it would be best to tell the owner of the channel (A2MP in
this case) that the channel has been disconnected using the
state_change l2cap op, even if no disconnect request is sent to the
remote device.  This way a new A2MP fixed channel can be started up
so there is some chance that the two sides of the A2MP ERTM
connection can sync up again.

I can add state_change and the code would look like:

static void l2cap_send_disconn_req(struct l2cap_conn *conn, struct
l2cap_chan *chan, int err)
	struct sock *sk = chan->sk;
	struct l2cap_disconn_req req;

	if (!conn)

	if (chan->mode == L2CAP_MODE_ERTM) {

	if (chan->chan_type == L2CAP_CHAN_CONN_FIX_A2MP) {
		__l2cap_state_change(chan, BT_DISCONN);

	req.dcid = cpu_to_le16(chan->dcid);
	req.scid = cpu_to_le16(chan->scid);
	l2cap_send_cmd(conn, l2cap_get_ident(conn),
			L2CAP_DISCONN_REQ, sizeof(req), &req);

	__l2cap_state_change(chan, BT_DISCONN);
	__l2cap_chan_set_err(chan, err);

Looks fine to me. The A2MP fixed channel is a special case in this regard.

Then function might be renamed to l2cap_disconnect_chan()

	if (chan->mode == L2CAP_MODE_ERTM) {
@@ -1201,6 +1216,11 @@ static void l2cap_conn_ready(struct l2cap_conn *conn)


+		if (chan->chan_type == L2CAP_CHAN_CONN_FIX_A2MP) {
+			l2cap_chan_unlock(chan);
+			continue;
+		}

This bypasses the socket code in that loop, but also means the state
change is not sent to the AMP manager.

Yes, but state is already BT_CONNECTED for fixed channel so it does not
need to change.

Since the ERTM fixed channel can be created synchronously now (unlike the older Code Aurora Forum code), I think you're right that the state change notification isn't needed here any more.

Is there a way to fix up the
state change code in l2cap_ops so the socket code can be removed
from l2cap_conn_ready and other socketless L2CAP channels will also
work without special case code?

If we would invoke sk->sk_state_change(sk) each time we call state_change
it would be easily done. Otherwise we might need special ops for

Right. I'm not sure why they need to be separate - there may be some good reason, or maybe it's just hanging around from older code.

		if (conn->hcon->type == LE_LINK) {
			if (smp_conn_security(conn, chan->sec_level))
@@ -1581,7 +1601,8 @@ static void l2cap_monitor_timeout(struct work_struct *work)

	if (chan->retry_count >= chan->remote_max_tx) {
-		l2cap_send_disconn_req(chan->conn, chan, ECONNABORTED);
+		if (chan->chan_type != L2CAP_CHAN_CONN_FIX_A2MP)
+			l2cap_send_disconn_req(chan->conn, chan, ECONNABORTED);

The spec defines the MaxTransmit value as 0xff for the A2MP channel,
not 0x00 (for infinite retries).  There is therefore an expectation
that the ERTM connection will reset after too many retries - and I
don't think it is ok to skip the disconnect here.

I will delete this chunk at all since we already check for fixed channel
in l2cap_send_disconn_req

Mat Martineau
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum

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