Re: [PATCH net v2] net: phy: skip EEE advertisement write when autoneg is disabled

[Nicolai Buchwitz]

Hi Nerijus

On 16.5.2026 17:02, Nerijus Bendžiūnas wrote:
> genphy_c45_an_config_eee_aneg() writes the EEE advertisement to the
> auto-negotiation device's MMD register space (MDIO_MMD_AN, register
> MDIO_AN_EEE_ADV).  These registers are read by the link partner only
> during auto-negotiation, so writing them while autoneg is disabled
> cannot influence the link.  On some PHYs (e.g. Broadcom BCM54213PE)
> the write nevertheless reaches the chip and disturbs the receive
> datapath.
>
> Concretely, running
>
>     ethtool -s eth0 speed 100 duplex full autoneg off
>     ethtool --set-eee eth0 eee off
>
> leaves eth0 with TX working and RX completely silent on a
> Raspberry Pi 4 / CM4 board (bcmgenet + BCM54213PE in rgmii-rxid).
> Switching back to autoneg recovers the link.

Can confirm this. A quick look at the BCM54213PE datasheet shows that 
EEE is
only applied if autoneg is enabled (as expected). With autoneg disabled 
it
is undefined behavior, but clearly breaks communication.

> [...]

> diff --git a/drivers/net/phy/phy-c45.c b/drivers/net/phy/phy-c45.c
> index d48aa7231b37..126951741428 100644
> --- a/drivers/net/phy/phy-c45.c
> +++ b/drivers/net/phy/phy-c45.c
> @@ -940,6 +940,14 @@ EXPORT_SYMBOL_GPL(genphy_c45_read_eee_abilities);
>   */
>  int genphy_c45_an_config_eee_aneg(struct phy_device *phydev)
>  {
> +	/* Writing MMD AN advertisements while autoneg is disabled has no
> +	 * effect on link-partner negotiation, but on some PHYs (e.g. the
> +	 * Broadcom BCM54213PE) the write itself disturbs the receive
> +	 * datapath. Skip it.
> +	 */
> +	if (phydev->autoneg == AUTONEG_DISABLE)
> +		return 0;
> +
>  	if (!phydev->eee_cfg.eee_enabled) {
>  		__ETHTOOL_DECLARE_LINK_MODE_MASK(adv) = {};

Reviewed-by: Nicolai Buchwitz <nb@xxxxxxxxxxx>
Tested-by: Nicolai Buchwitz <nb@xxxxxxxxxxx>

Thanks
Nicolai