Not able to keep PHY at 1M when connecting with iPhone

[mahori-ravinder]

Describe the bug

I (Peripheral) wants to connect with iPhone(Central) and make sure connection PHY is 1M.
Please note that my device (peripheral) supports 2M, i just want to use 1M.
I tried the recommended settings (gathered by reading zephyr doc + google search), but the iPhone moves the PHY to 2M after connection.

To Reproduce
To make this happen i set following config params

CONFIG_BT_CTLR_PHY_2M=n
CONFIG_BT_CTLR_PHY_CODED=n

CONFIG_BT_USER_PHY_UPDATE=y

CONFIG_BT_PHY_UPDATE=n
//https://devzone.nordicsemi.com/f/nordic-q-a/119628/how-to-set-cis-phy-to-1m-in-le-audio
CONFIG_BT_AUTO_PHY_UPDATE=n

The expectation was that connection will happen at 1M.

The initial connection happens at 1M. After that iPhone sends a phy request (preferring 2M), my device (peripheral) responds with PHY response (preferring 2M). This leads to PHY moving to 2M.

See below screen grab of OTA.

Expected behavior
iPhone ends phy request (preferring 2M) and my device (peripheral) responds with PHY response (preferring 1M).

Please note: as a work around i created a fork of zephyr (#87810) . This makes use of LE_SET_DEFAULT_PHY HCI commands. This enables me to achieve right behaviour (See OTA below)

Environment (please complete the following information):
Zephyr OS build ba3251d

What configuration i need to do to achieve the desired behaviour?

[dkalowsk]

@mahori-ravinder it's not clear from the issue which device this is on. Is it a Nordic device or another?

[cvinayak]

The preferred PHY is hard-coded to 2M when using CONFIG_BT_AUTO_PHY_UPDATE=y in the Zephyr Host implementation:

zephyr/subsys/bluetooth/host/conn.c

Lines 1800 to 1810 in 3c7e43f

	if (IS_ENABLED(CONFIG_BT_AUTO_PHY_UPDATE) && BT_FEAT_LE_PHY_2M(bt_dev.le.features) &&
	!uses_symmetric_2mbit_phy(conn)) {
	err = bt_le_set_phy(conn, 0U, BT_HCI_LE_PHY_PREFER_2M, BT_HCI_LE_PHY_PREFER_2M,
	BT_HCI_LE_PHY_CODED_ANY);
	if (err) {
	LOG_ERR("Failed LE Set PHY (%d)", err);
	}
	if (conn->state != BT_CONN_CONNECTED) {
	return;
	}
	}

But when CONFIG_BT_AUTO_PHY_UPDATE=n then the Zephyr Controller (nRF and may be OpenISA SoC variants) power-on defaults of using preferred 2M PHY is used. There is no Zephyr Host support to update the Controller defaults using HCI LE Set Default Phy Command, this will be a enhancement request to Zephyr Host. Thank you for sending #87810.

CC @jhedberg @alwa-nordic

Finally, as a intermediate solution, with CONFIG_BT_USER_PHY_UPDATE=y you can use bt_conn_le_phy_update() in your application to switch to any supported PHY on the connection at runtime, like in this sample:

zephyr/samples/bluetooth/central_gatt_write/src/gatt_write_common.c

Lines 113 to 134 in 3c7e43f

	struct bt_conn_le_phy_param conn_phy_param;
	if (conn_info.role == BT_CONN_ROLE_CENTRAL) {
	conn_phy_param.options = phy_param[phy_param_idx].options;
	conn_phy_param.pref_tx_phy = phy_param[phy_param_idx].pref_tx_phy;
	conn_phy_param.pref_rx_phy = phy_param[phy_param_idx].pref_rx_phy;
	} else {
	conn_phy_param.options = phy_param[phy_param_idx].options;
	conn_phy_param.pref_tx_phy = phy_param[phy_param_idx].pref_rx_phy;
	conn_phy_param.pref_rx_phy = phy_param[phy_param_idx].pref_tx_phy;
	}
	printk("%s: PHY Update requested %u %u (%u)\n", __func__,
	conn_phy_param.pref_tx_phy,
	conn_phy_param.pref_rx_phy,
	conn_phy_param.options);
	err = bt_conn_le_phy_update(conn, &conn_phy_param);
	if (err) {
	printk("Failed to update PHY (%d).\n", err);
	return;
	}

[mahori-ravinder]

@mahori-ravinder it's not clear from the issue which device this is on. Is it a Nordic device or another?

This is Infineon 20829.