Performing loopback test

[DevBear]

Hello!

I am currently developing my own custom service with two nrf 52840 development kits where one acts as the Client and one acts as the Server.

I have managed to set up the service and the service is discoverable and the client can subscribe to notifications from the server.
The next step is to perform a loopback test where I want to take the data sent from the Server ---> Client and send it back with my Write without response attributes.

So my questions are:

1: Is there some sample code on how to write from Client --> Server? What do I need to do on both ends? My Client code is inserted below.

2: Once the writing is set up properly, is it possible to just loop the data back to the Server or is there something I need to think about?

3: When the loopback test is set up, is the nrf sniffer a viable way of testing throughput or are there some pot holes I should be careful about?

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#include <stdio.h>

#include <stddef.h>

#include <errno.h>

#include <zephyr/kernel.h>

#include <zephyr/sys/printk.h>

#include <zephyr/sys/byteorder.h>

#include <zephyr/bluetooth/bluetooth.h>

#include <zephyr/bluetooth/hci.h>

#include <zephyr/bluetooth/conn.h>

#include <zephyr/bluetooth/uuid.h>

#include <zephyr/bluetooth/gatt.h>

/* --------------------- START OF FROG UUID:s ---------------- */

#define FROG_SERVICE BT_UUID_128_ENCODE(0x2016ec20, 0xb2eb, 0x11e5, 0xa307, 0x0002a5d5c51b)

// Notify characteristics

#define FROG_TXDATA_UUID BT_UUID_128_ENCODE(0x388706a0, 0xb2eb, 0x11e5, 0xa790, 0x0002a5d5c51b)

#define FROG_RXCREDIT BT_UUID_128_ENCODE(0x43802b40, 0xb2eb, 0x11e5, 0xb6e5, 0x0002a5d5c51b)

XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

#include <stdio.h>
#include <stddef.h>
#include <errno.h>

#include <zephyr/kernel.h>
#include <zephyr/sys/printk.h>
#include <zephyr/sys/byteorder.h>

#include <zephyr/bluetooth/bluetooth.h>
#include <zephyr/bluetooth/hci.h>
#include <zephyr/bluetooth/conn.h>
#include <zephyr/bluetooth/uuid.h>
#include <zephyr/bluetooth/gatt.h>

/* --------------------- START OF FROG UUID:s ---------------- */

#define FROG_SERVICE BT_UUID_128_ENCODE(0x2016ec20, 0xb2eb, 0x11e5, 0xa307, 0x0002a5d5c51b)
// Notify characteristics
#define FROG_TXDATA_UUID BT_UUID_128_ENCODE(0x388706a0, 0xb2eb, 0x11e5, 0xa790, 0x0002a5d5c51b)
#define FROG_RXCREDIT BT_UUID_128_ENCODE(0x43802b40, 0xb2eb, 0x11e5, 0xb6e5, 0x0002a5d5c51b)

// Write without response characteristics


#define BT_UUID_FROG BT_UUID_DECLARE_128(FROG_SERVICE)
#define BT_UUID_TXDATA BT_UUID_DECLARE_128(FROG_TXDATA_UUID)
#define BT_UUID_RXCREDIT BT_UUID_DECLARE_128(FROG_RXCREDIT)

/* ----------------------------- END ------------------------- */


static int scan_start(void);

static struct bt_conn *default_conn;
static struct bt_uuid_16 frog_ccc_discover_uuid = BT_UUID_INIT_16(0);
static struct bt_uuid_128 frog_discover_uuid = BT_UUID_INIT_128(0);
static const struct bt_uuid *ccc_uuid = BT_UUID_GATT_CCC;
//static const struct bt_uuid *ccc_uuid2 = BT_UUID_GATT_CCC;
static const struct bt_uuid *ccc_uuid2;
static struct bt_gatt_discover_params discover_params;
static struct bt_gatt_subscribe_params subscribe_params_txdata;
static struct bt_gatt_subscribe_params subscribe_params_rxcredit;

static uint8_t notify_func(struct bt_conn *conn,
			   struct bt_gatt_subscribe_params *params,
			   const void *data, uint16_t length)
{

	if (!data) {
		printk("[UNSUBSCRIBED]\n");
		params->value_handle = 0U;
		return BT_GATT_ITER_STOP;
	}


	printf("[NOTIFICATION]: Data from server %p.\n", data);
	uint8_t* uint8data = (uint8_t *)data;
	printf("[NOTIFICATION]: Data from server");
	int i;
	for (i = 0; i < length; ++i)
	{
		printf(" 0x%02x", uint8data[i]);
	}
	printf("\n");	

	return BT_GATT_ITER_CONTINUE;
}

static uint8_t discover_func(struct bt_conn *conn,
			     const struct bt_gatt_attr *attr,
			     struct bt_gatt_discover_params *params)
{
	int err;

	if (!attr) {
		printk("Discover complete\n");
		(void)memset(params, 0, sizeof(*params));
		return BT_GATT_ITER_STOP;
	}

	printk("[ATTRIBUTE] handle %u\n", attr->handle);

	if (!bt_uuid_cmp(discover_params.uuid, BT_UUID_FROG)) {
		printk("Discovering TxDATA\n");
		memcpy(&frog_discover_uuid, BT_UUID_TXDATA, sizeof(frog_discover_uuid));
		discover_params.uuid = &frog_discover_uuid.uuid;
		discover_params.start_handle = attr->handle + 1;
		discover_params.type = BT_GATT_DISCOVER_CHARACTERISTIC;

		err = bt_gatt_discover(conn, &discover_params);
		if (err) {
			printk("Discover failed (err %d)\n", err);
		}
	} else if (!bt_uuid_cmp(discover_params.uuid, BT_UUID_TXDATA)) {
		printk("Discovering CCC TxData\n");
	
		memcpy(&frog_ccc_discover_uuid, BT_UUID_GATT_CCC, sizeof(frog_ccc_discover_uuid));
		discover_params.uuid = &frog_ccc_discover_uuid.uuid;
		discover_params.start_handle = bt_gatt_attr_value_handle(attr);//attr->handle + 1;
		discover_params.type = BT_GATT_DISCOVER_DESCRIPTOR;

		subscribe_params_txdata.value_handle = bt_gatt_attr_value_handle(attr);

		err = bt_gatt_discover(conn, &discover_params);
		if (err) {
			printk("Discover failed (err %d)\n", err);
		}
	} else if (!bt_uuid_cmp(discover_params.uuid, BT_UUID_RXCREDIT)) {
		
		printk("[CHARACTERISTIC] RxCredit handle %u\n", attr->handle);
		printk("Discovering CCC RxCredit\n");
		//memcpy(&frog_discover_uuid, BT_UUID_GATT_CCC, sizeof(frog_discover_uuid));

		memcpy(&frog_ccc_discover_uuid, BT_UUID_GATT_CCC, sizeof(frog_ccc_discover_uuid));
		discover_params.uuid = &frog_ccc_discover_uuid.uuid;

		discover_params.start_handle = bt_gatt_attr_value_handle(attr);
		discover_params.type = BT_GATT_DISCOVER_DESCRIPTOR;

		printk("Value handle RxCredit: %d \n", bt_gatt_attr_value_handle(attr));

		subscribe_params_rxcredit.value_handle = bt_gatt_attr_value_handle(attr);

		err = bt_gatt_discover(conn, &discover_params);
		if (err) {
			printk("Discover failed (err %d)\n", err);
		}
	} else if (!bt_uuid_cmp(discover_params.uuid, BT_UUID_GATT_CCC)){
		printk("Discover params 1:%d\n", discover_params.start_handle);
		printk("Discover params 2:%d\n", subscribe_params_txdata.value_handle);
		printk("Discovered CCCD\n");

		if (attr->handle-1 == subscribe_params_txdata.value_handle)
		{
			printk("[SUBSCRIBE PARAMS]\n");
			subscribe_params_txdata.notify = notify_func;
			subscribe_params_txdata.value = BT_GATT_CCC_NOTIFY;
			subscribe_params_txdata.ccc_handle = attr->handle;
	
			err = bt_gatt_subscribe(conn, &subscribe_params_txdata);
			if (err && err != -EALREADY) {
				printk("Subscribe failed (err %d)\n", err);
			} else {
				printk("[SUBSCRIBED]\n");
			}

			printk("Discovering RxCredit\n");
			//memcpy(&frog_discover_uuid, BT_UUID_GATT_CCC, sizeof(frog_discover_uuid));
			memcpy(&frog_discover_uuid, BT_UUID_RXCREDIT, sizeof(frog_discover_uuid));
			discover_params.uuid = &frog_discover_uuid.uuid;
			discover_params.start_handle = attr->handle + 1;
			discover_params.type = BT_GATT_DISCOVER_CHARACTERISTIC;	
		} 

		else if (attr->handle-1 == subscribe_params_rxcredit.value_handle)
		{
			printk("[SUBSCRIBE PARAMS]\n");
			subscribe_params_rxcredit.notify = notify_func;
			subscribe_params_rxcredit.value = BT_GATT_CCC_NOTIFY;
			subscribe_params_rxcredit.ccc_handle = attr->handle;
	
			err = bt_gatt_subscribe(conn, &subscribe_params_rxcredit);
			if (err && err != -EALREADY) {
				printk("Subscribe failed (err %d)\n", err);
			} else {
				printk("[SUBSCRIBED]\n");
			}
			
			return BT_GATT_ITER_STOP;
		}
		
		//subscribe_params_rxcredit.value_handle = bt_gatt_attr_value_handle(attr); 
		

		err = bt_gatt_discover(conn, &discover_params);
		if (err) {
			printk("Discover failed (err %d)\n", err);
		}
	}

	return BT_GATT_ITER_STOP;
}

static void connected(struct bt_conn *conn, uint8_t conn_err)
{
	char addr[BT_ADDR_LE_STR_LEN];
	int err;

	bt_addr_le_to_str(bt_conn_get_dst(conn), addr, sizeof(addr));

	if (conn_err) {
		printk("Failed to connect to %s (%u)\n", addr, conn_err);

		bt_conn_unref(default_conn);
		default_conn = NULL;

		scan_start();
		return;
	}

	printk("Connected: %s\n", addr);

	if (conn == default_conn) {
		memcpy(&frog_discover_uuid, BT_UUID_FROG, sizeof(frog_discover_uuid));
		discover_params.uuid = &frog_discover_uuid.uuid;
		discover_params.func = discover_func;
		discover_params.start_handle = BT_ATT_FIRST_ATTRIBUTE_HANDLE;
		discover_params.end_handle = BT_ATT_LAST_ATTRIBUTE_HANDLE;
		discover_params.type = BT_GATT_DISCOVER_PRIMARY;

		err = bt_gatt_discover(default_conn, &discover_params);
		if (err) {
			printk("Discover failed(err %d)\n", err);
			return;
		}
	}
}

static bool eir_found(struct bt_data *data, void *user_data)
{
	bt_addr_le_t *addr = user_data;
	int i;

	printk("[AD]: %u data_len %u\n", data->type, data->data_len);
	switch (data->type) {
	case BT_DATA_UUID128_SOME:
	case BT_DATA_UUID128_ALL:
		if (data->data_len % BT_UUID_SIZE_128 != 0U) {
			printk("AD malformed\n");
			return true;
		}
		
		printk("INSIDE SWITCH\n");

		for (i = 0; i < data->data_len; i += BT_UUID_SIZE_128) {
			struct bt_uuid *uuid128;
			uint8_t btadvData[16];
			memcpy(btadvData, data->data, 16);
			uuid128 = (struct bt_uuid *)btadvData;
			int err;
			printk("INSIDE FOR LOOP\n");


			if (bt_uuid_cmp(uuid128, BT_UUID_TXDATA)) {
				//printk("COMPARISON OK!\n");
				printk("uuid128 == TXDATA\n");				
					err = bt_le_scan_stop();
					if(err) {
						printk("Stop LE scan failed (err %d)\n", err);					
					}
				printk("make connection\n");

				err = bt_conn_le_create(addr, BT_CONN_LE_CREATE_CONN,
						BT_LE_CONN_PARAM_DEFAULT,
						&default_conn);

				printk("err: %d\n", err);

				if (err) {
					printk("Create connection failed (err %d)\n",
						err);
				}
				//continue;
				return false;
			} if (bt_uuid_cmp(uuid128, BT_UUID_RXCREDIT)) {
				//printk("COMPARISON OK!\n");
				printk("uuid128 == RXCREDIT\n");				
					err = bt_le_scan_stop();
					if(err) {
						printk("Stop LE scan failed (err %d)\n", err);					
					}
				printk("make connection");

				err = bt_conn_le_create(addr, BT_CONN_LE_CREATE_CONN,
						BT_LE_CONN_PARAM_DEFAULT,
						&default_conn);

				printk("err: %d\n", err);

				if (err) {
					printk("Create connection failed (err %d)\n",
						err);
				}
				//continue;
				return false;				
			}
		}
	}
	return true;
}

static void device_found(const bt_addr_le_t *addr, int8_t rssi, uint8_t type,
			 struct net_buf_simple *ad)
{
	char dev[BT_ADDR_LE_STR_LEN];

	bt_addr_le_to_str(addr, dev, sizeof(dev));
	printk("[DEVICE]: %s, AD evt type %u, AD data len %u, RSSI %i\n",
	       dev, type, ad->len, rssi);

	/* We're only interested in connectable events */
	if (type == BT_HCI_ADV_IND || type == BT_HCI_ADV_DIRECT_IND) {
		bt_data_parse(ad, eir_found, (void *)addr);
	}
}

static int scan_start(void)
{
	/* Use active scanning and disable duplicate filtering to handle any
	 * devices that might update their advertising data at runtime.
	 */
	struct bt_le_scan_param scan_param = {
		.type       = BT_LE_SCAN_TYPE_ACTIVE,
		.options    = BT_LE_SCAN_OPT_NONE,
		.interval   = BT_GAP_SCAN_FAST_INTERVAL,
		.window     = BT_GAP_SCAN_FAST_WINDOW,
	};

	return bt_le_scan_start(&scan_param, device_found);
}

static void disconnected(struct bt_conn *conn, uint8_t reason)
{
	char addr[BT_ADDR_LE_STR_LEN];
	int err;

	bt_addr_le_to_str(bt_conn_get_dst(conn), addr, sizeof(addr));

	printk("Disconnected: %s (reason 0x%02x)\n", addr, reason);

	if (default_conn != conn) {
		return;
	}

	bt_conn_unref(default_conn);
	default_conn = NULL;

	err = scan_start();
	if (err) {
		printk("Scanning failed to start (err %d)\n", err);
	}
}

BT_CONN_CB_DEFINE(conn_callbacks) = {
	.connected = connected,
	.disconnected = disconnected,
};

int main(void)
{
	int err;

	err = bt_enable(NULL);
	if (err) {
		printk("Bluetooth init failed (err %d)\n", err);
		return 0;
	}

	printk("Bluetooth initialized\n");

	err = scan_start();

	if (err) {
		printk("Scanning failed to start (err %d)\n", err);
		return 0;
	}

	printk("Scanning successfully started\n");
	return 0;
}

Regards,
B