关于MTU设置：

1、不配置MTU时，协议栈使用的默认值是MTU支持的最大值512；

2、在广播，扫描前前用ble_gap_l2cap_params_set设置MTU的长度，调用ble_gatt_mtu_set无用。参考ble_app_uart MTU 的配置；

static void gap_params_init(void)
{
    sdk_err_t   error_code;

    error_code = ble_gap_device_name_set(BLE_GAP_WRITE_PERM_DISABLE, (uint8_t *)DEVICE_NAME, strlen(DEVICE_NAME));

    s_gap_adv_param.adv_intv_max = APP_ADV_SLOW_MAX_INTERVAL;
    s_gap_adv_param.adv_intv_min = APP_ADV_FAST_MIN_INTERVAL;
    s_gap_adv_param.adv_mode     = BLE_GAP_ADV_TYPE_ADV_IND;
    s_gap_adv_param.chnl_map     = BLE_GAP_ADV_CHANNEL_37_38_39;
    s_gap_adv_param.disc_mode    = BLE_GAP_DISC_MODE_GEN_DISCOVERABLE;
    s_gap_adv_param.filter_pol   = BLE_GAP_ADV_ALLOW_SCAN_ANY_CON_ANY;

    error_code = ble_gap_adv_param_set(0, BLE_GAP_OWN_ADDR_STATIC, &s_gap_adv_param);
    APP_ERROR_CHECK(error_code);

    error_code = ble_gap_adv_data_set(0, BLE_GAP_ADV_DATA_TYPE_DATA, s_adv_data_set, sizeof(s_adv_data_set));
    APP_ERROR_CHECK(error_code);

    error_code = ble_gap_adv_data_set(0, BLE_GAP_ADV_DATA_TYPE_SCAN_RSP, s_adv_rsp_data_set, sizeof(s_adv_rsp_data_set));
    APP_ERROR_CHECK(error_code);

    s_gap_adv_time_param.duration    = 0;
    s_gap_adv_time_param.max_adv_evt = 0;

    error_code = ble_gap_l2cap_params_set(MAX_MTU_DEFUALT, MAX_MPS_DEFUALT, MAX_NB_LECB_DEFUALT);
    APP_ERROR_CHECK(error_code);

    error_code = ble_gap_data_length_set(MAX_TX_OCTET_DEFUALT, MAX_TX_TIME_DEFUALT);
    APP_ERROR_CHECK(error_code);

    ble_gap_pref_phy_set(BLE_GAP_PHY_ANY, BLE_GAP_PHY_ANY);
}

3、在连接后，需要用下面两个函数配合，进行MTU的更改。注意，ble_gatt_mtu_set只是配置MTU的值，需要调用ble_gattc_mtu_exchange进行MTU交换才能生效；

sdk_err_t error_code = ble_gatt_mtu_set(mtu);
APP_ERROR_CHECK(error_code);

error_code = ble_gattc_mtu_exchange(conn_idx);
APP_ERROR_CHECK(error_code);

4、本端在调用ble_gap_l2cap_params_set和ble_gatt_mtu_set只是设置本端希望的MTU配置，不一定是最终的MTU，双方会协商出适合双方设备的MTU，最终协商的值会通过BLE_GATT_COMMON_EVT_MTU_EXCHANGE事件告知上层应用；