Bluetooth: Host: missing api: bt_gatt_get_mtu

[alwa-nordic]

As a Bluetooth LE Audio subsystem developer, I need my API to enforce a requirement from the specification: Some audio functions require the UATT MTU to be at least 64. They must return an error otherwise.

Currently, there is no way to get the UATT MTU (in a reliable way) from the Host.

Describe the solution you'd like A new API: uint16_t bt_uatt_get_mtu(struct bt_conn *conn)

Describe alternatives you've considered Not enforcing the ATT MTU at service level. This could cause hard-to-debug errors for the user.

Why the current API does not satisfy my requirements

• bt_gatt_get_mtu: Affected by EATT MTU. I need the UATT MTU.
• bt_gatt_exchange_mtu: The service layer is not the place to initiate the MTU exchange. That is done by the sample/application.
• bt_gatt_cb.att_mtu_updated: Affected by EATT MTU. I need the UATT MTU.

[alwa-nordic]

Relates to: https://github.com/zephyrproject-rtos/zephyr/issues/64172

[ubieda]

@alwa-nordic is this issue up for grabs? I could tackle it as it would enable me coming up with a solution for #72018.

[alwa-nordic]

Yes, issue is up for grabs. Thanks for taking it! :)