Notification subscription fails

[Annette]

Hi,

I'm using the nRF Connect for VS Code with SDK version 1.8.0. 

Working with one custom PCB with nRF52840 as the peripheral and another custom PCB with nRF52840 as the central. I have also tried with the nRF52840-DK with the same result as custom HW.

Currently the peripheral has one custom service with 6 characteristics. When I, from the nRF Connect app, enable notifications on all the characteristics I get updated data as expected.

Now I want to implement the service client on the central side. I have looked into hrs_client.c and enable notifications with the bt_gatt_subscribe() function as in the following code from the

bt_hrs_client_measurement_subscribe function:

params->ccc_handle = hrs_c->measurement_char.ccc_handle;
params->value_handle = hrs_c->measurement_char.handle;
params->value = BT_GATT_CCC_NOTIFY;
params->notify = on_hrs_measurement_notify;

atomic_set_bit(params->flags, BT_GATT_SUBSCRIBE_FLAG_VOLATILE);

err = bt_gatt_subscribe(hrs_c->conn, params);
if (err) {
	atomic_clear_bit(&hrs_c->state, HRS_MEASUREMENT_NOTIFY_ENABLED);
	LOG_ERR("Subscribe to Heart Rate Measurement characteristic failed");
} else {
		LOG_DBG("Subscribed to Heart Rate Measurement characteristic");
}

When attempting to subscribe to all 6 characteristics in my custom service it fails at the fourth call to bt_gatt_subscribe(). No matter which combination of characteristics I enable notifications for it works for the first 3 and data is also being notified. 

Looking into what fails it seems to be allocation of memory:

bt_gatt_subscribe() -> 
gatt_write_ccc() ->
gatt_req_send() ->
gatt_req_alloc() ->
bt_att_req_alloc() ->
k_mem_slab_alloc()

The last function checks if (slab->free_list != NULL)  and ends in the else function which fails:

	if (slab->free_list != NULL) {
		// CODE REMOVED
	} else if (K_TIMEOUT_EQ(timeout, K_NO_WAIT) ||
		   !IS_ENABLED(CONFIG_MULTITHREADING)) {
		// CODE REMOVED
	} else {
		SYS_PORT_TRACING_OBJ_FUNC_BLOCKING(k_mem_slab, alloc, slab, timeout);

		/* wait for a free block or timeout */
		result = z_pend_curr(&slab->lock, key, &slab->wait_q, timeout);
		if (result == 0) {
			*mem = _current->base.swap_data;
		}

		SYS_PORT_TRACING_OBJ_FUNC_EXIT(k_mem_slab, alloc, slab, timeout, result);

		return result;
	}
	
	

I have tried adjusting the following configs which didn't make it work:

CONFIG_HEAP_MEM_POOL_SIZE

CONFIG_MAIN_STACK_SIZE

CONFIG_SYSTEM_WORKQUEUE_STACK_SIZE

This is only the first service - I have requirements of having notification enabled for several other characteristics in other services.

I hope you have a solution to fix this.

Best regards,

Annette

[Elfving]

Hey Annette!

I will have to get back to you on this. Due to the holidays you might have to expect both limited help and slower responses on DevZone.

Best regards,

Elfving

[Annette]

Hi, 

As the holidays are over and we have a deadline, I would like to ask for a status of this issue.

Best regards,

Annette

[Elfving]

Hello Annette!

The new status is that we are now looking into it, though unfortunately I do not have an answer ready for you yet. Thank you for your patience.

Best regards,

Elfving

[Elfving]

Hello again Annette!

Could you try changing the configuration CONFIG_BT_L2CAP_TX_BUF_COUNT to 6, it is presumably defaulting to 3 in your application.

You could alternatively wait for ATT Responses for each of those subscriptions before triggering another bt_gatt_subscribe(). You can see that the memory is freed from the request slab in att_confirm() ->att_handle_rsp().

Best regards,

Elfving

[Annette]

Hi,

My colleague does not encounter the same issue so I should be able to figure out what the problem is myself - missing committed files or different configurations. I apologize for the inconvenience!

Best regards,

Annette