Re: [PATCH net v2] net: pcs: xpcs: make supported_interfaces authoritative

[Andrew Lunn <andrew@lunn.ch>]

On Thu, Oct 29, 2026 at 06:27:29PM +0800, Coia Prant wrote:
> pcs.supported_interfaces is the mask of interfaces a given xpcs
> instance actually supports. The compat table describes what the
> Synopsys IP can do in general, but a specific integration may support
> a different subset: the RK3568 XPCS supports 1000BASE-X, SGMII and
> QSGMII in hardware, but does not implement the C73 10G or 2500BASE-X
> modes that the Synopsys compat table advertises.
> 
> The Rockchip glue currently advertises only SGMII, since that is what
> the RK3568 MAC side supports today. 1000BASE-X and QSGMII can be added
> later when the MAC side gains support.
> 
> Without narrowing the mask, stmmac_phylink_setup() merges the PCS
> supported_interfaces into the MAC mask with phy_interface_or(), so the
> XPCS would contribute 1000BASE-X, 2500BASE-X, USXGMII and the 10G/25G
> modes to the combined set. The MAC side forces the phy modes and
> phylink start passes the mode explicitly, so this does not currently
> make phylink switch to an unsupported mode, but the combined mask still
> advertises interfaces the SoC cannot use.

I don't know this code too well, but something does not feel right
here. It might be due to the way stmmac does PCS with phylink, which
is often considered broken.

What i would expect to happen is that each piece of hardware indicates
what it can do, and then phylink finds the common modes.

So maybe stmmac_phylink_setup() is wrong when it ORs them together?

Maybe look at other MAC drivers using pcs and phylink.

      Andrew