netdev - Re: [PATCH net-next 1/7] net: phy: allow isolating PHY devices

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Message-ID: <7736f0f2-8a99-4329-b290-089454d56e36@gmail.com>
Date: Thu, 12 Sep 2024 11:30:26 -0700
From: Florian Fainelli <f.fainelli@...il.com>
To: Maxime Chevallier <maxime.chevallier@...tlin.com>, davem@...emloft.net
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
 thomas.petazzoni@...tlin.com, Andrew Lunn <andrew@...n.ch>,
 Jakub Kicinski <kuba@...nel.org>, Eric Dumazet <edumazet@...gle.com>,
 Paolo Abeni <pabeni@...hat.com>, Russell King <linux@...linux.org.uk>,
 linux-arm-kernel@...ts.infradead.org,
 Christophe Leroy <christophe.leroy@...roup.eu>,
 Herve Codina <herve.codina@...tlin.com>,
 Heiner Kallweit <hkallweit1@...il.com>,
 Vladimir Oltean <vladimir.oltean@....com>, Marek Behún
 <kabel@...nel.org>, Köry Maincent
 <kory.maincent@...tlin.com>, Oleksij Rempel <o.rempel@...gutronix.de>
Subject: Re: [PATCH net-next 1/7] net: phy: allow isolating PHY devices

On 9/11/24 14:27, Maxime Chevallier wrote:
> The 802.3 specifications describes the isolation mode as setting the
> PHY's MII interface in high-impedance mode, thus isolating the PHY from
> that bus. This effectively breaks the link between the MAC and the PHY,
> but without necessarily disrupting the link between the PHY and the LP.
> 
> This mode can be useful for testing purposes, but also when there are
> multiple PHYs on the same MII bus (a case that the 802.3 specification
> refers to).
> 
> In Isolation mode, the PHY will still continue to respond to MDIO
> commands.
> 
> Introduce a helper to set the phy in an isolated mode.
> 
> Signed-off-by: Maxime Chevallier <maxime.chevallier@...tlin.com>

Not sure where that comment belongs so I will put it here, one thing 
that concerns me is if you have hardware that is not strapped to be 
isolated by default, and the PHY retains the state configured by Linux, 
such that the PHY is in isolation mode. A boot loader that is not 
properly taking the PHY out of isolation mode would be unavailable to 
use it and that would be a bug that Linux would likely be on the hook to 
fix.

Would recommend adding a phy_shutdown() method which is called upon 
reboot/kexec and which, based upon a quirk/flag can ensure that the 
isolation bit is cleared.
-- 
Florian