LESC and FreeRTOS?

[RTMerkel]

Re: nRF52833DK, s113 and nRF5_SDK v16.0.0

It appears that nrf_sdh_freertos.c needs to be modified to support JUST_WORKS and LESC because the softdevice_task() in nrf_sdh_freertos.c never calls nrf_lesc_request_handler() causing attempts to pair and bound to timeout..

After exchanging emails with Daniel Veilleux, we came up the following necessary changes to nrf_sdh_freertos.c:

Two tasks:

• The existing softdevice_task() running at a high priority
• A new lesc_request_task() running at a low priority

The lesc_request_task() is woken by the softdevice_task() to call nrf_ble_lesc_request_handler(), but at a lower priority than the softdevice_task(). Here's an example:

1. Softdevice_task() and lesc_request_task() are blocked
2. SD_EVT_IRQHandler() sends a notification to softdevice_task()
3. Softdevice_task()
   1. Sends a notification to lesc_tequest_task()
   2. Calls nrf_sdh_evts_poll()
   3. Blocks on the next SD_EVT_IRQHandler() notification
4. lesc_request_task()
   1. Calls nrf_ble_lesc_request_handler()
   2. Blocks on the next Softdevice_task()
5. Repeat...

Here's the changes I made to the app_ble_hrs_freertos example to test:

// main.c

...

static void gap_params_init(void) {
    ret_code_t              err_code;
    ble_gap_conn_params_t   gap_conn_params;
    ble_gap_conn_sec_mode_t sec_mode;

    BLE_GAP_CONN_SEC_MODE_SET_ENC_NO_MITM(&sec_mode);

    ...
}

...

static void services_init(void) {
    ...
    
    // Here the sec level for the Heart Rate Service can be changed/increased.
    hrs_init.hrm_cccd_wr_sec = SEC_JUST_WORKS;
    hrs_init.bsl_rd_sec      = SEC_JUST_WORKS;

    ...

    // Here the sec level for the Battery Service can be changed/increased.
    bas_init.bl_rd_sec        = SEC_JUST_WORKS;
    bas_init.bl_cccd_wr_sec   = SEC_JUST_WORKS;
    bas_init.bl_report_rd_sec = SEC_JUST_WORKS;

    ...
}

// nrf_sdh_freertos.c

static void softdevice_task(void* pvParameter) {
    NRF_LOG_DEBUG("Enter softdevice_task.");

    if (m_task_hook != NULL)
        m_task_hook(pvParameter);

    while (true) {
        nrf_sdh_evts_poll();                    /* let the handlers run first, incase the EVENT occured before creating this task */
        xTaskNotifyGive(m_lesc_task);           /* wake the LESC task */

        (void) ulTaskNotifyTake(pdTRUE,         /* Clear the notification value before exiting (equivalent to the binary semaphore). */
                                portMAX_DELAY); /* Block indefinitely (INCLUDE_vTaskSuspend has to be enabled).*/
    }
}

static void lesc_task(void* pvParameter) {
    NRF_LOG_DEBUG("Enter lesc_task.");

    for(;;) {
        (void) ulTaskNotifyTake(pdTRUE, portMAX_DELAY);
        NRF_LOG_INFO("calling  nrf_ble_lesc_request_handler()...")
        nrf_ble_lesc_request_handler();
    }
}

void nrf_sdh_freertos_init(nrf_sdh_freertos_task_hook_t hook_fn, void* p_context) {
    NRF_LOG_DEBUG("Creating a SoftDevice task.");

    m_task_hook = hook_fn;

    BaseType_t xReturned = xTaskCreate( lesc_task, 
                                        "LESC", 
                                        NRF_BLE_FREERTOS_LESC_TASK_STACK, 
                                        p_context, 
                                        tskIDLE_PRIORITY+1, 
                                        &m_lesc_task);
    if (xReturned != pdPASS) {
        NRF_LOG_ERROR("SoftDevice LESC task not created!");
        APP_ERROR_HANDLER(NRF_ERROR_NO_MEM);
    }

    xReturned = xTaskCreate(softdevice_task,
                            "BLE",
                            NRF_BLE_FREERTOS_SDH_TASK_STACK,
                            p_context,
                            2,
                            &m_softdevice_task);
    if (xReturned != pdPASS)     {
        NRF_LOG_ERROR("SoftDevice task not created.");
        APP_ERROR_HANDLER(NRF_ERROR_NO_MEM);
    }
}

This seems to work correctly, but I'd like to have it officially blessed by Nordic before I place it production code.

[CheMax]

I would like to understand why this is necessary. Information on LESK cannot be found in the documentation of a freeRTOS.
The essence of the idea is to dynamically lower the priority of the task of the radio handler?

[RTMerkel]

"LESC" is not part of FreeRTOS, but part of BLE security in the Nordic SDK. The Nordic app_ble_hrs_freertos example is app_ble_hrs ported to FreeRTOS. My initial attempts to modify app_ble_hrs_freertos to support Just-Works pairing and bounding with LESC failed with a connection timeout. The root cause is that the softdevice_task() never calls nrf_ble_lesc_request_handler(). The above code calls nrf_ble_lesc_request_handler() from a second thread.

[Susheel Nuguru]

If LESC task is lower priority than softdevice task, then it looks good in my opinion.

[RTMerkel]

I made it just above the timer task priority. I'd like to ask that this functionality be added to the SDK as MMD is really needed for debugging while BLE is active.

[Anthony Ambuehl]

While following your example, I noticed that the nrf_crypto_rng.c library includes nrf_stack_info.   I suspect the MACROs present in this file are incorrect for freertos.   The function nrf_stack_info_overflowed most likely does not work as intended.  

I plan to disable the stack checking when running in freeRTOS for now to avoid any confusion.