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.

[Palmer Huang]

Yes, my output is also like this.

So, why is the MTU_SIZE obtained in discovery_complete() 23 instead of 247? I obviously set 247 in prj.conf

[haakonsh]

haakonsh said:

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. 

The MTU that you've set in the configs will be applied when calling bt_gatt_exchange_mtu() with an empty exchange_params.