Unable to change MTU size pc-ble-driver-py

[sijohans]

Hello,

I am playing around with pc-ble-driver-py to make a connection towards a BLE peripheral. The peripheral is using a custom service very similar to the Nordic UART Service. I have managed to get it working but I am unable to exchange the MTU size and is stuck with the 23 bytes default MTU.

I have been looking into other code examples and this is what I'm trying to do:

CFG_TAG = 1
nrf_sd_ble_api_ver = config.sd_api_ver_get()
assert nrf_sd_ble_api_ver == 5

# Setup BLEDriver and BLEAdapter
driver = BLEDriver(
    serial_port='/dev/tty.usbmodem0006826680971',
    auto_flash=False,
    baud_rate=1000000,
    log_severity_level="info"
)
adapter = BLEAdapter(driver)

# Later in code
cfg = BLEConfigGapRoleCount(central_role_count=1, periph_role_count=0, central_sec_count=0)
cfg.conn_cfg_tag = CFG_TAG
self.adapter.driver.ble_cfg_set(BLEConfig.role_count, cfg)

cfg = BLEConfigConnGatt(att_mtu=Central.LOCAL_ATT_MTU) # Central.LOCAL_ATT_MTU = 247
cfg.conn_cfg_tag = CFG_TAG
self.adapter.driver.ble_cfg_set(BLEConfig.conn_gatt, cfg)

cfg = BLEConfigConnGap(event_length=320)
cfg.conn_cfg_tag = CFG_TAG
self.adapter.driver.ble_cfg_set(BLEConfig.conn_gap, cfg)

self.adapter.driver.ble_enable()
self.adapter.driver.ble_vs_uuid_add(Central.BASE_UUID)


# Start scan, find device and connect
self.adapter.connect(peer_addr, tag=CFG_TAG)


logging.info('MTU Size: {}'.format(self.adapter.db_conns[self.conn_handle].att_mtu))    # Prints 23
if Central.LOCAL_ATT_MTU > ATT_MTU_DEFAULT:
    logging.info('Requesting MTU exchange')
    self.att_mtu = self.adapter.att_mtu_exchange(self.conn_handle, Central.LOCAL_ATT_MTU)

    max_data_length = 251
    data_length = self.att_mtu + 4
    if data_length > max_data_length:
        data_length = max_data_length
    self.adapter.data_length_update(self.conn_handle, data_length)

Here i get error message:

2020-11-11 10:03:09,265 root         INFO     Requesting MTU exchange
Traceback (most recent call last):
  File "/test/venv/lib/python3.7/site-packages/pc_ble_driver_py/ble_adapter.py", line 220, in att_mtu_exchange
    self.driver.ble_gattc_exchange_mtu_req(conn_handle, mtu)
  File "/test/venv/lib/python3.7/site-packages/pc_ble_driver_py/ble_driver.py", line 107, in wrapper
    error_code=err_code,
pc_ble_driver_py.exceptions.NordicSemiException: Failed to ble_gattc_exchange_mtu_req. Error code: NRF_ERROR_INVALID_PARAM

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/test/test.py", line 227, in <module>
    central.start()
  File "/test/test.py", line 133, in start
    self.att_mtu = self.adapter.att_mtu_exchange(self.conn_handle, Central.LOCAL_ATT_MTU)
  File "/test/venv/lib/python3.7/site-packages/pc_ble_driver_py/ble_adapter.py", line 225, in att_mtu_exchange
    "different config tags used in ble_cfg_set and connect.") from ex
pc_ble_driver_py.exceptions.NordicSemiException: MTU exchange request failed. Common causes are: missing att_mtu setting in ble_cfg_set, different config tags used in ble_cfg_set and connect.

Any suggestion how to proceed with this?

[Jørgen Holmefjord]

Hi,

As described in the API documentation for sd_ble_gattc_exchange_mtu_request(), NRF_ERROR_INVALID_PARAM is thrown when "Invalid Client RX MTU size supplied". Further, the valid Client RX MTU sizes are:

• The minimum value is BLE_GATT_ATT_MTU_DEFAULT.
• The maximum value is ble_gatt_conn_cfg_t::att_mtu in the connection configuration used for this connection.
• The value must be equal to Server RX MTU size given in sd_ble_gatts_exchange_mtu_reply if an ATT_MTU exchange has already been performed in the other direction.

The two first seems to match your code. Have you captured a sniffer trace to see if the server side initiates a MTU exchange first?

Best regards,
Jørgen

[sijohans]

Hello and thanks for answer, it seems like this is initiated by the server. I put a breakpoint in ble_driver.py in function on_gatts_evt_exchange_mtu_request and that did occur. The BLEAdapter.att_mtu_exchange does update the MTU on successful response (self.db_conns[conn_handle].att_mtu = new_mtu) but the callback on_gatts_evt_exchange_mtu_request does not. I need to register an observer to this callback to update it locally.