nRF5340: MQTT subscribe failed

Question

Hi Support team,

The communication of my net-core of nRF5340 is working well, and I want to try mqtt subscription, but it always fails, could you help give me some hints?

I try to do the subscription in the event handler of MQTT_EVT_CONNACK:

#define SPARKPLUGB_NCMD         "spBv1.0/BDS/NCMD/Node1/#"
#define SPARKPLUGB_DCMD         "spBv1.0/BDS/DCMD/Node1/#"

int sparkb_sub_cmd(struct mqtt_client *const client)
{
    struct mqtt_topic cmdTopic[2];
	cmdTopic[0].topic.utf8 = (uint8_t *)SPARKPLUGB_NCMD;
	cmdTopic[0].topic.size = strlen(SPARKPLUGB_NCMD);
	cmdTopic[0].qos = MQTT_QOS_0_AT_MOST_ONCE;
	cmdTopic[1].topic.utf8 = (uint8_t *)SPARKPLUGB_DCMD;
	cmdTopic[1].topic.size = strlen(SPARKPLUGB_DCMD);
	cmdTopic[1].qos = MQTT_QOS_0_AT_MOST_ONCE;

	struct mqtt_subscription_list sub_param = {
		.list = cmdTopic,
		.list_count = 2,
		.message_id = 0U
	};

	return mqtt_subscribe(client, &sub_param);
}

In void mqtt_evt_handler(struct mqtt_client *const client, const struct mqtt_evt *evt)
{
	int err;

	switch (evt->type) {
		case MQTT_EVT_CONNACK:
			if (evt->result != 0) {
				LOG_ERR("-----[MQTT_EVT_CONNACK] MQTT connect failed %d", evt->result);
				break;
			}

			g_connected = true;
			LOG_INF("-----[MQTT_EVT_CONNACK] MQTT client connected!");
			
			//Upon connect successful, subscribe to sparkplug NCMD and DCMD msgs. A production should handle MQTT connect failures.
			err = sparkb_sub_cmd(client);
			if (err != 0) {
				LOG_ERR("-----[MQTT_EVT_CONNACK] Failed to subscribe sparkplug NCMD and DCMD: %d", err);
			}

			break;

The MQTT connection succeded, but the subscription is failed, the error code is -22 (#define EINVAL 22 /* Invalid argument */), the trace as below:

[00:00:25.672,332] <inf> main: modem_test() successful!
[00:00:25.905,395] <inf> mqtttest: mqtt_connect OK, rc is 0
[00:00:26.146,850] <inf> mqtttest: do zsock_poll() 2s, ret = 1
[00:00:26.146,972] <inf> mqtttest: -----[MQTT_EVT_CONNACK] MQTT client connected!
[00:00:26.147,003] <err> mqtttest: -----[MQTT_EVT_CONNACK] Failed to subscribe sparkplug NCMD and DCMD: -22

The published two topics "spBv1.0/BDS/NCMD/Node1/#" and "spBv1.0/BDS/DCMD/Node1/#" by other mqtt clients, so these two topics have existed on the MQTT server.

Could you help have a look and give some hints as to why subscriptions always fail? Thank you very much.

Best regards,
Yanpeng Wu

Didrik Rokhaug · Accepted Answer

Hi, and sorry for the late reply. 
 Have you tried to send data with QoS1 (or 2)? 
 Are you calling mqtt_input in your application? 
 Or otherwise checking for incoming packets other than the CONNACK?

Didrik Rokhaug · Answer

I am mostly working with the nRF9160, where the socket operations are offloaded to the modem, so I am not supoer familiar with the finer points of Zephyr's IP stack. 
 However, things should mostly be the same, so while this is based on how we do things on the nRF9160, it should be transferable to your modem as well: 
 When you call poll(), the current thread is blocked until something happens on one of the polled sockets, or the call times out. While the thread is blocked, other threads can run, or if there are no other runnable threads, the scheduler will put the CPU in idle. So calling poll() shouldn't cause any extra power consumption, and it also shouldn't affect the performance. 
 For MQTT, what we typically do, is to call poll with the timeout equal to the time until we must send the next keepalive message. If we receive an incomming message before that time, poll() will return, indicating that we have received a message, so we can call mqtt_input() to handle the message. If no message is received before poll() times out, it will indicate that it timed out, and we can send a keepalive message(). 
 You can see an example of that in one of the exercises from our Cellular Fundamentals course on the Nordic DevAcademy: https://github.com/NordicDeveloperAcademy/cell-fund/blob/master/v2.4.0-v2.x.x/lesson4/cellfund_less4_exer1_solution/src/main.c#L140-L170 
 Note that in this case, we call mqtt_live() each time poll() returns, regardless of cause. mqtt_live() will only send a keepalive packet if necessary.

nRF5340: MQTT subscribe failed

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