linux-kernel - Re: [PATCH net-next v4 09/13] net: phylink: Use phy_caps

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Message-ID: <Z8mZjabeITVg1Khg@shell.armlinux.org.uk>
Date: Thu, 6 Mar 2025 12:48:13 +0000
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Maxime Chevallier <maxime.chevallier@...tlin.com>
Cc: davem@...emloft.net, Andrew Lunn <andrew@...n.ch>,
	Jakub Kicinski <kuba@...nel.org>,
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
	Heiner Kallweit <hkallweit1@...il.com>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, thomas.petazzoni@...tlin.com,
	linux-arm-kernel@...ts.infradead.org,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	Herve Codina <herve.codina@...tlin.com>,
	Florian Fainelli <f.fainelli@...il.com>,
	Vladimir Oltean <vladimir.oltean@....com>,
	Köry Maincent <kory.maincent@...tlin.com>,
	Oleksij Rempel <o.rempel@...gutronix.de>,
	Simon Horman <horms@...nel.org>,
	Romain Gantois <romain.gantois@...tlin.com>
Subject: Re: [PATCH net-next v4 09/13] net: phylink: Use phy_caps_lookup for
 fixed-link configuration

On Tue, Mar 04, 2025 at 03:43:30PM +0100, Maxime Chevallier wrote:
> On Mon,  3 Mar 2025 10:03:15 +0100
> Maxime Chevallier <maxime.chevallier@...tlin.com> wrote:
> 
> > When phylink creates a fixed-link configuration, it finds a matching
> > linkmode to set as the advertised, lp_advertising and supported modes
> > based on the speed and duplex of the fixed link.
> > 
> > Use the newly introduced phy_caps_lookup to get these modes instead of
> > phy_lookup_settings(). This has the side effect that the matched
> > settings and configured linkmodes may now contain several linkmodes (the
> > intersection of supported linkmodes from the phylink settings and the
> > linkmodes that match speed/duplex) instead of the one from
> > phy_lookup_settings().
> > 
> > Signed-off-by: Maxime Chevallier <maxime.chevallier@...tlin.com>
> > ---
> 
> Maybe before anything goes further with this patch, I'd like to get
> some feedback from it on a particular point. This changes the linkmodes
> that are reported on fixed-link interfaces. Instead of reporting one
> single mode, we report all modes supported by the fixed-link' speed and
> duplex settings.

This is a good question. We have historically only used the baseT link
modes because the software PHY implementation was based around clause
22 baseT PHYs (although that doesn't support >1G of course.)

The real question is... does it matter, to which I'd say I don't know.
One can argue that it shouldn't matter, and I think userspace would be
unlikely to break, but userspace tends to do weird stuff all the time
so there's never any guarantee.

> The fixed-link in question is for the CPU port of a DSA switch.
> 
> In my opinion, this is OK as the linkmodes expressed here don't match
> physical linkmodes on an actual wire, but as this is a user visible
> change, I'd like to make sure this is OK. Any comment here is more than
> welcome.

Maybe Andrew has an opinion, but I suspect like me, it's really a case
that "we don't know".

-- 
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