Logical Link Control and Adaptation Protocol (L2CAP)

L2CAP layer enables connection-oriented channels which can be enabled with the configuration option: CONFIG_BT_L2CAP_DYNAMIC_CHANNEL. These channels support segmentation and reassembly transparently, they also support credit based flow control making it suitable for data streams.

The user can also define fixed channels using the BT_L2CAP_FIXED_CHANNEL_DEFINE macro. Fixed channels are initialized upon connection, and do not support segmentation. An example of how to define a fixed channel is shown below.

static struct bt_l2cap_chan fixed_chan[CONFIG_BT_MAX_CONN];

/* Callbacks are assumed to be defined prior. */
static struct bt_l2cap_chan_ops ops = {
    .recv = recv_cb,
    .sent = sent_cb,
    .connected = connected_cb,
    .disconnected = disconnected_cb,
};

static int l2cap_fixed_accept(struct bt_conn *conn, struct bt_l2cap_chan **chan)
{
    uint8_t conn_index = bt_conn_index(conn);

    *chan = &fixed_chan[conn_index];

    **chan = (struct bt_l2cap_chan){
        .ops = &ops,
    };

    return 0;
}

BT_L2CAP_FIXED_CHANNEL_DEFINE(fixed_channel) = {
    .cid = 0x0010,
    .accept = l2cap_fixed_accept,
};

Channels instances are represented by the bt_l2cap_chan struct which contains the callbacks in the bt_l2cap_chan_ops struct to inform when the channel has been connected, disconnected or when the encryption has changed. In addition to that it also contains the recv callback which is called whenever an incoming data has been received. Data received this way can be marked as processed by returning 0 or using bt_l2cap_chan_recv_complete() API if processing is asynchronous.

Note

The recv callback is called directly from RX Thread thus it is not recommended to block for long periods of time.

For sending data the bt_l2cap_chan_send() API can be used noting that it may block if no credits are available, and resuming as soon as more credits are available.

Servers can be registered using bt_l2cap_server_register() API passing the bt_l2cap_server struct which informs what psm it should listen to, the required security level sec_level, and the callback accept which is called to authorize incoming connection requests and allocate channel instances.

Client channels can be initiated with use of bt_l2cap_chan_connect() API and can be disconnected with the bt_l2cap_chan_disconnect() API. Note that the later can also disconnect channel instances created by servers.

API Reference

L2CAP