netdev - Re: [PATCH net-next 14/14] net: stmmac: convert to phylink PCS support

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Message-ID: <aPAYtHPcF5bes7Xi@shell.armlinux.org.uk>
Date: Wed, 15 Oct 2025 22:57:08 +0100
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Andrew Lunn <andrew@...n.ch>
Cc: Heiner Kallweit <hkallweit1@...il.com>,
	Abhishek Chauhan <quic_abchauha@...cinc.com>,
	Alexandre Torgue <alexandre.torgue@...s.st.com>,
	Alexis Lothore <alexis.lothore@...tlin.com>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	Boon Khai Ng <boon.khai.ng@...era.com>,
	Choong Yong Liang <yong.liang.choong@...ux.intel.com>,
	Daniel Machon <daniel.machon@...rochip.com>,
	"David S. Miller" <davem@...emloft.net>,
	Drew Fustini <dfustini@...storrent.com>,
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	Eric Dumazet <edumazet@...gle.com>,
	Faizal Rahim <faizal.abdul.rahim@...ux.intel.com>,
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	Vladimir Oltean <olteanv@...il.com>,
	Vladimir Oltean <vladimir.oltean@....com>,
	Yu-Chun Lin <eleanor15x@...il.com>
Subject: Re: [PATCH net-next 14/14] net: stmmac: convert to phylink PCS
 support

On Wed, Oct 15, 2025 at 11:31:37PM +0200, Andrew Lunn wrote:
> > - create stmmac_pcs.c, which contains the phylink_pcs_ops structure, a
> >   dummy .pcs_get_state() method which always reports link-down
> 
> I've not followed the PCS code too closely. Why always report link
> down? Why is a dummy method needed?

If phylink is put into inband mode, and a PCS is supplied to phylink
where this method left NULL, the kernel will oops.

As the code stands today in mainline, if phylink were to be put into
inband mode with the integrated PCS, then there will be no phylink PCS,
and so phylink_mac_pcs_get_state() will fall into the "else" path of:

        pcs = pl->pcs;
        if (pcs)
                pcs->ops->pcs_get_state(pcs, pl->pcs_neg_mode, state);
        else
                state->link = 0;

and force the link down.

So, adding this method keeps the status quo - not oopsing the kernel
and not allowing the link to come up. No unintended behavioural
change in this regard from how it would behave today. :)

-- 
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