Data length extension causes NRF_ERROR_NO_MEM on services init

[Eliot Stock]

Using SDK 14.0.0 with an nRF52832, I'm trying to use the data length extension, following some sample code found in this post:

https://devzone.nordicsemi.com/f/nordic-q-a/24840/using-dle-and-nrf_ble_gatt_max_mtu_size-23

Here's how I init the BLE stack.

static void ble_stack_init(void)
{
    ret_code_t err_code;

    err_code = nrf_sdh_enable_request();
    APP_ERROR_CHECK(err_code);

    // Configure the BLE stack using the default settings. Fetch the start address of the application RAM.
    uint32_t ram_start = 0;
    err_code = nrf_sdh_ble_default_cfg_set(APP_BLE_CONN_CFG_TAG, &ram_start);
    APP_ERROR_CHECK(err_code);

    // Overwrite some of the default configurations for the BLE stack.
    ble_cfg_t ble_cfg;
    memset(&ble_cfg, 0, sizeof(ble_cfg));

	// Vendor-specific UUIDs. Count these in services_init() below.
    ble_cfg.common_cfg.vs_uuid_cfg.vs_uuid_count = 3;

    // Maximum transmission unit. 
    ble_cfg.conn_cfg.params.gatt_conn_cfg.att_mtu = NRF_SDH_BLE_GATT_MAX_MTU_SIZE;

    // Data length extension.
    ble_cfg.conn_cfg.conn_cfg_tag = APP_BLE_CONN_CFG_TAG;

    uint8_t data_length = NRF_SDH_BLE_GATT_MAX_MTU_SIZE + L2CAP_HDR_LEN;
    err_code = nrf_ble_gatt_data_length_set(&m_gatt, BLE_CONN_HANDLE_INVALID, data_length);
    APP_ERROR_CHECK(err_code);

    err_code = sd_ble_cfg_set(BLE_COMMON_CFG_VS_UUID, &ble_cfg, ram_start);
    APP_ERROR_CHECK(err_code);

    // Enable BLE stack.
    err_code = nrf_sdh_ble_enable(&ram_start);
    APP_ERROR_CHECK(err_code);

    // Register a handler for BLE events.
    NRF_SDH_BLE_OBSERVER(m_ble_observer, APP_BLE_OBSERVER_PRIO, ble_evt_handler, NULL);

    // Connection event length extension. Must be done *after* BLE is enabled.
    ble_opt_t opt;
    memset(&opt, 0x00, sizeof(opt));
    opt.common_opt.conn_evt_ext.enable = 1;
    err_code = sd_ble_opt_set(BLE_COMMON_OPT_CONN_EVT_EXT, &opt);
    APP_ERROR_CHECK(err_code);
}

This causes NRF_ERROR_NO_MEM at the point where I init the third of four services (three of which are vendor specific) in my application. My linker script looks like this.

MEMORY
{
  FLASH (rx) : ORIGIN = 0x23000, LENGTH = 0x2F000

  RAM (rwx) :  ORIGIN = 0x20002760, LENGTH = 0xB4A0

  NOINIT (rwx) :  ORIGIN = 0x2000DC00, LENGTH = 0x2400
}

I'm using the exact RAM start and size that the warnings I get at runtime ask for.

Removing the data length extension stuff fixes the NRF_ERROR_NO_MEM error (and produces new warnings about RAM size).

[Vidar Berg]

Hi Eliot,

The maximum MTU size will typically   define the max. attribute data length for a BLE characteristic. Thus, a longer MTU may actually increase the size of the ATT table that is stored in SD memory.  You can increase the ATT table size by changing  the NRF_SDH_BLE_GATTS_ATTR_TAB_SIZE define in sdk_config.

[Eliot Stock]

OK thanks, but what do I set it to? How do I calculate the required size?

[Vidar Berg]

Unfortunately, there is no easy way to calculate the required size (the internal table structure is not documented). We actually recommend to determine the required size by trial and error. I.e., increase the ATT table size until you no longer get the NRF_ERROR_NO_MEM error. Note that the size must be a multiple of 4. 

[Eliot Stock]

OK, does this get safer if I do it on a later SDK version? Say, 14.1 or 14.2 or even 15.0 once it ships? If my test coverage is not good, am I risking hitting an NRF_ERROR_NO_MEM at runtime in the field?

[Vidar Berg]

the size requirement is static as long as long as you always initialize your services with the same parameters . If not, you should account for the worst case (max. number of characteristics, max attribute length, etc).