Set MTU size in Zephyr

[Palmer Huang]

Hello,

I ran the throughput example ,but I found that setting the MTU size actually requires calling the bt_gatt_exchange_mtu function, which is different from sdk16.0 .

In sdk16.0 or other versions, I can set the MTU size by calling nrf_ble_gatt_att_mtu_periph_set()  and nrf_ble_gatt_att_mtu_central_set() .

Is there any way to automatically set the MTU size in Zephyr?

Best Regards,

Gray

[haakonsh]

What do you mean by automatically?

[Palmer Huang]

Hello, haakonsh

Sorry, my description is not clear enough.
What I mean is how to set the MTU before the connection, without calling bt_gatt_exchange_mtu() to set it after the connection.

Best Regards,

Gray

[haakonsh]

You can't set it before a connection as the MTU exchange is a negotiation between the Central and Peripheral (where the Central ultimately decides what value to use). You'll have to initiate such an exchange for each established connection. 

[Palmer Huang]

Hi,haakonsh

My doubt is why the Central does not choose to set MTU_SIZE?

After connecting, I called bt_gatt_get_mtu() and the read size was 23 instead of the set size.

https://github.com/nrfconnect/sdk-nrf/blob/master/samples/bluetooth/throughput/prj.conf

CONFIG_BT_BUF_ACL_RX_SIZE=251
CONFIG_BT_GATT_CLIENT=y
CONFIG_BT_ATT_PREPARE_COUNT=2
CONFIG_BT_CONN_TX_MAX=10
CONFIG_BT_L2CAP_TX_BUF_COUNT=10
CONFIG_BT_L2CAP_TX_MTU=247
CONFIG_BT_L2CAP_DYNAMIC_CHANNEL=y
CONFIG_BT_CTLR_PHY_2M=y
CONFIG_BT_CTLR_RX_BUFFERS=2
CONFIG_BT_BUF_ACL_TX_COUNT=10
CONFIG_BT_BUF_ACL_TX_SIZE=251
CONFIG_BT_CTLR_DATA_LENGTH_MAX=251

[haakonsh]

Hey Gray,

You configs looks good.

We need more information however. I suggest you start by enabling debug logs for the BT stack and share that. A sniffer trace would also help. 

[Palmer Huang]

Hello, haakonsh

The demo I ran was provided by Nordic( throughput example). I didn’t change the code much. I only called get_mtu_size() in the connected() and exchange_func() functions.

[haakonsh]

Hey again Grey,

I added the bt_gatt_get_mtu functions to the exchange and discovery complete functions:

static void exchange_func(struct bt_conn *conn, uint8_t att_err,
			  struct bt_gatt_exchange_params *params)
{
	struct bt_conn_info info = {0};
	int err;

	printk("MTU exchange %s\n", att_err == 0 ? "successful" : "failed");
	printk("MTU size is: %d\n", bt_gatt_get_mtu(conn));

	err = bt_conn_get_info(conn, &info);
	if (err) {
		printk("Failed to get connection info %d\n", err);
		return;
	}

	if (info.role == BT_CONN_ROLE_MASTER) {
		instruction_print();
		test_ready = true;
	}
}

static void discovery_complete(struct bt_gatt_dm *dm,
			       void *context)
{
	int err;
	struct bt_throughput *throughput = context;

	printk("Service discovery completed\n");

	bt_gatt_dm_data_print(dm);
	bt_throughput_handles_assign(dm, throughput);
	bt_gatt_dm_data_release(dm);

	exchange_params.func = exchange_func;

	printk("MTU size is: %d\n", bt_gatt_get_mtu(default_conn));

	err = bt_gatt_exchange_mtu(default_conn, &exchange_params);
	if (err) {
		printk("MTU exchange failed (err %d)\n", err);
	} else {
		printk("MTU exchange pending\n");
	}
}

This was my output: 

Connected as master
Conn. interval is 320 units
Service discovery completed
MTU size is: 23
MTU exchange pending
MTU exchange successful
MTU size is: 247

I'm on NCS 1.7.1 btw.