sd_ble_gatts_exchange_mtu_reply() returns NRF_ERROR_INVALID_PARAM given ATT MTU of 247

[Eliot Stock]

Hi. The same issue I have is described here, but I've been asked to post a new question:

https://devzone.nordicsemi.com/f/nordic-q-a/29084/why-sd_ble_gatts_exchange_mtu_reply-returns-nrf_error_invalid_param

I've tried to copy the throughput example in SDK 14.0.0 to increase the ATT MTU. Here's my handling of the BLE_GATTS_EVT_EXCHANGE_MTU_REQUEST event (the peer is requesting 230):

case BLE_GATTS_EVT_EXCHANGE_MTU_REQUEST:

NRF_LOG_DEBUG("BLE_GATTS_EVT_EXCHANGE_MTU_REQUEST, client_rx_mtu: %d", p_ble_evt->evt.gatts_evt.params.exchange_mtu_request.client_rx_mtu);

err_code = sd_ble_gatts_exchange_mtu_reply(p_ble_evt->evt.gatts_evt.conn_handle, BLE_ATT_MTU);

APP_ERROR_CHECK(err_code);

break;

In app_config.h:

#define NRF_BLE_GATT_MAX_MTU_SIZE 247

Before enabling the BLE stack:

ble_cfg.conn_cfg.params.gatt_conn_cfg.att_mtu = NRF_BLE_GATT_MAX_MTU_SIZE;

NRF_ERROR_INVALID_PARAM here means "Invalid Server RX MTU size supplied". How is it invalid?

[Edit]

I'm using the advertising module and setting the tag as follows:

#define APP_BLE_CONN_CFG_TAG 1

ble_advertising_conn_cfg_tag_set(&m_advertising, APP_BLE_CONN_CFG_TAG);

err_code = ble_advertising_start(&m_advertising, BLE_ADV_MODE_SLOW);

APP_ERROR_CHECK(err_code);

[Vidar Berg]

Hi,

Assuming you are using the advertising module, did you use ble_advertising_conn_cfg_tag_set() to update the connection settings tag to the same tag that you used for configuring the softdevice? This error may occur if the wrong tag number is passed to "advertising start". 

Note that you can use the GATT module to handle MTU requests as well: http://infocenter.nordicsemi.com/topic/com.nordic.infocenter.sdk5.v14.2.0/lib_ble_gatt.html?cp=4_0_0_3_2_10 

[Eliot Stock]

Thanks. Have added an edit above with what I'm doing for the tag, but it's the same as the HRS example. I'm not passing any tag number in when starting advertising. It's not at all clear from the SDK docs how the tags work, sorry. Yes, I'm using the GATT module and have tried to follow the MTU throughput example.

[Vidar Berg]

Is the BLE_GATTS_EVT_EXCHANGE_MTU_REQUEST event handled in main.c or by the GATT module? It's important that event handling isn't duplicated.

The following line needs to be added in HRS example in order for the GATT module to request long MTU: 

--- a/examples/ble_peripheral/ble_app_hrs/main.c
+++ b/examples/ble_peripheral/ble_app_hrs/main.c
@@ -533,6 +533,9 @@ 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_periph_set(&m_gatt, 247);
+ APP_ERROR_CHECK(err_code);
}

[Eliot Stock]

But nrf_ble_gatt_att_mtu_periph_set() returns NRF_ERROR_INVALID_PARAM when passed 247.

[Vidar Berg]

Is NRF_SDH_BLE_GATT_MAX_MTU_SIZE set to '247' in your code?