Subject: Re: [PATCH net-next] net: phy: fixed_phy: use genphy_read_abilities to simplify the code

From: "Russell King (Oracle)" <[email-masked]>
Date: Mon, 10 Nov 2025 21:57:01 +0000

On Mon, Nov 10, 2025 at 10:11:24PM +0100, Heiner Kallweit wrote:
> Populating phy->supported can be achieved easier by using
> genphy_read_abilities().

Are you sure about that?

> -	switch (status->speed) {
> -	case SPEED_1000:
> -		linkmode_set_bit(ETHTOOL_LINK_MODE_1000baseT_Half_BIT,
> -				 phy->supported);
> -		linkmode_set_bit(ETHTOOL_LINK_MODE_1000baseT_Full_BIT,
> -				 phy->supported);
> -		fallthrough;
> -	case SPEED_100:
> -		linkmode_set_bit(ETHTOOL_LINK_MODE_100baseT_Half_BIT,
> -				 phy->supported);
> -		linkmode_set_bit(ETHTOOL_LINK_MODE_100baseT_Full_BIT,
> -				 phy->supported);
> -		fallthrough;
> -	case SPEED_10:
> -	default:
> -		linkmode_set_bit(ETHTOOL_LINK_MODE_10baseT_Half_BIT,
> -				 phy->supported);
> -		linkmode_set_bit(ETHTOOL_LINK_MODE_10baseT_Full_BIT,
> -				 phy->supported);

This code sets both HD and FD for each speed, and if at 1G it sets
100M and 10M as well, if at 100M, it sets 10M as well.

However, swphy emulation (including what was reported through BMSR
and ESTAT) has only ever indicated one speed and duplex supported
via the normal ability bits in these registers. So, "simplifying"
the code introduces user visible changes. This needs to be mentioned
in the commit message.

The next questions are:
1. does this difference matter?
2. is it a bug fix?
3. is swphy wrong?

-- 
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