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.