MTU Update

RE: MTU Update

Vidar Berg — Tue, 28 Apr 2020 12:42:46 GMT

Hi,

Can you enable logging in the GATT module to see if MTU requests are sent and received?

Relevant message sequence charts for reference:

- GATTC ATT_MTU Exchange

- GATTS ATT_MTU Exchange

RE: MTU Update

jawadk — Mon, 27 Apr 2020 12:19:17 GMT

Hi,

Please see my reply above, it's edited.

I already tried to remove this function, the central start to work fine, but the effective MTU not changing.

see my ticket above please where I explained what every side( central/peripheral) is responsible of ..

Please help me to fix this problem, I have a 40000 bytes to send over BLE as fast as possible !

RE: MTU Update

Vidar Berg — Mon, 27 Apr 2020 12:09:56 GMT

What I meant to suggest was that you could remove the call to nrf_ble_gatt_att_mtu_central_set() from the gatt_init() function.

RE: MTU Update

jawadk — Mon, 27 Apr 2020 12:05:43 GMT

Hi,

How long should the central wait after the connection until it can request the MTU exchange ?

what do you mean by using gatt_init() and not nrf_ble_gatt_att_mtu_central_set()?

see the implementation of my gatt_init() below:

/**@brief Function for initializing the GATT module.
 */
static void gatt_init(void)
{
    ret_code_t err_code = nrf_ble_gatt_init(&m_gatt, gatt_evt_handler);
    APP_ERROR_CHECK(err_code);
    
    err_code = nrf_ble_gatt_att_mtu_central_set(&m_gatt, NRF_SDH_BLE_GATT_MAX_MTU_SIZE);
    APP_ERROR_CHECK(err_code);
}

something else that maybe is related !

I started my project with the ble_app_multilink_central example, and then added to it the nus service and the USB peripheral, and disabled the lbs_c_init() function that i don't need.

can this example be cause a problem with MTU exchange ?

Jawad

RE: MTU Update

Vidar Berg — Mon, 27 Apr 2020 10:12:03 GMT

Hi,

No, I'm afraid I don't have an explanation for that. Have you tried to only call gatt_init() and not nrf_ble_gatt_att_mtu_central_set()? nrf_ble_gatt_att_mtu_central_set() is redundant when you want to use the max. NRF_SDH_BLE_GATT_MAX_MTU_SIZE anyway.

RE: MTU Update

jawadk — Mon, 27 Apr 2020 10:05:04 GMT

Hi,

this is the weirdest thing I seen !

the desired_mtu is 247

the BLE_GATT_ATT_MTU_DEFAULT is 23

the NRF_SDH_BLE_GATT_MAX_MTU_SIZE is 247

and still this nrf_ble_gatt_att_mtu_central_set function return NRF_ERROR_INVALID_PARAM

Do you have any explenation for that ?

I didn't change the anything in this function !

RE: MTU Update

Vidar Berg — Mon, 27 Apr 2020 07:58:52 GMT

Is BLE_GATT_ATT_MTU_DEFAULT still 23 (it should not be changed)?

Here is the param check inside nrf_ble_gatt_att_mtu_central_set:

if ((desired_mtu < BLE_GATT_ATT_MTU_DEFAULT) || (desired_mtu > NRF_SDH_BLE_GATT_MAX_MTU_SIZE))
{
return NRF_ERROR_INVALID_PARAM;
}

RE: MTU Update

jawadk — Mon, 27 Apr 2020 07:16:59 GMT

Hi,

the function nrf_ble_gatt_att_mtu_central_set(&m_gatt, NRF_SDH_BLE_GATT_MAX_MTU_SIZE);

return error invalid paramiter in the central side, in the gatt_init module, before any connection !

RE: MTU Update

Vidar Berg — Mon, 27 Apr 2020 06:57:40 GMT

Hi,

I would recommend using the GATT Module to manage the MTU negotiation as the other SDK examples do. This module will also handle Data length extension requests. But to answer the question, "invalid parameter" is returned when you reply with an invalid MTU size. You may want to double-check that NRF_SDH_BLE_GATT_MAX_MTU_SIZE is set to 247 on the client-side as well.

From the API documentation (sd_ble_gattc_exchange_mtu_request):

[in]client_rx_mtuClient RX MTU size.

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.

Best regards,

Vidar