Timing behavior of GAP SoftDevice calls and events

[puz_md]

Hi everyone,

I was wondering how GAP calls and events are synchronized within the SoftDevice.

Example:

// conn_handle is updated in the CONNECTED and DISCONNECTED event handlers
sd_ble_gap_connect_cancel();
if (conn_handle != BLE_CONN_HANDLE_INVALID)
    sd_ble_gap_disconnect(conn_handle, BLE_HCI_REMOTE_USER_TERMINATED_CONNECTION);

Considering this code is executed without interruption (e.g. on APP_PRIORITY_LOW level).

What happens if before sd_ble_gap_connect_cancel(), a connection has already been established and the connection event is still pending in the SoftDevice event queue? I cannot disconnect before I receive the connection ID in the CONNECTED event handler. So I cannot trigger the disconnect manually from this code location.

I know that the possibility that this scenario occurs is pretty low, but I want to create a implementation that works 100% safe/reliable by design.

Question: Will the sd_ble_gap_connect_cancel() function implicitly detect that a connection has already been established, trigger the disconnect and remove the CONNECTED event from the queue? Or is it possible that I may still receive a connected event after this code section?

[tesc]

Hi,

I do understand your worries, as this kind of corner case can quickly become annoying. I'll try to find someone who knows the details so that you can get an answer.

Regards, Terje

[tesc]

Hi,

sd_ble_gap_connect_cancel() cancels an initiated connection procedure. It will never perform a disconnect or remove connection events, and this is also true in cases where the connection procedure has successfully finished and there is a pending connect event.

Upon success the return value is NRF_SUCCESS. In the case when a connection has been established it will return NRF_ERROR_INVALID_STATE. You will then have to pull out the CONNECTED event and do a regular sd_ble_gap_disconnect(). You will also get NRF_ERROR_INVALID_STATE if you have not started a connection procedure, or if the connection procedure has timed out. See the Central Connection Establishment and Termination Message Sequence Chart for reference.

The race condition between sd_ble_gap_connect_cancel() and the part of the SoftDevice initiating the connection is accounted for by the SoftDevice. (I.e. the return value of sd_ble_gap_connect_cancel() can be safely trusted.)

Regards, Terje

[puz_md]

Thank you very much.