Re: [net-next,v3,01/11] net: phy: phy_port: Correctly recompute the port's linkmodes

[Simon Horman <horms@kernel.org>]

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---
net: phy: phy_port: Correctly recompute the port's linkmodes

This commit fixes phy_port_update_supported() to properly filter linkmodes
instead of incorrectly or'ing medium-specific modes to the existing support.
It moves the medium iteration inside a check for empty port->supported.

Fixes: 589e934d2735 ("net: phy: Introduce PHY ports representation")

> diff --git a/drivers/net/phy/phy_port.c b/drivers/net/phy/phy_port.c
> --- a/drivers/net/phy/phy_port.c
> +++ b/drivers/net/phy/phy_port.c

[ ... ]

> @@ -132,6 +126,19 @@ void phy_port_update_supported(struct phy_port *port)
>  			port->pairs = max_t(int, port->pairs,
>  					    ethtool_linkmode_n_pairs(mode));
>
> +	if (linkmode_empty(port->supported)) {
> +		for_each_set_bit(i, &port->mediums,
> +				 __ETHTOOL_LINK_MEDIUM_LAST) {
> +			__ETHTOOL_DECLARE_LINK_MODE_MASK(med_supported) = {0};
> +
> +			phy_caps_medium_get_supported(med_supported, i,
> +						      port->pairs);
> +			linkmode_or(supported, supported, med_supported);
> +		}
> +
> +		linkmode_and(port->supported, port->supported, supported);
> +	}

Jakub Kicinski raised a concern in v2 about the AND operation when
port->supported starts empty. The v3 fix adds the
'if (linkmode_empty(port->supported))' check, but then still performs:

    linkmode_and(port->supported, port->supported, supported);

inside that block. Since we just confirmed port->supported is empty,
ANDing it with anything will always produce an empty result.

Should this be linkmode_copy() or linkmode_or() instead of linkmode_and()
when port->supported starts empty?

Reference: https://lore.kernel.org/all/20260130040009.707764-1-kuba@kernel.org/