linux-kernel - Re: [PATCH net v2 2/3] net: phy: smsc: Force predictable MDI-X state on LAN87xx

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Message-ID: <20250704100031.19f95d3e@fedora.home>
Date: Fri, 4 Jul 2025 10:00:31 +0200
From: Maxime Chevallier <maxime.chevallier@...tlin.com>
To: Oleksij Rempel <o.rempel@...gutronix.de>
Cc: Andrew Lunn <andrew@...n.ch>, Heiner Kallweit <hkallweit1@...il.com>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet
 <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni
 <pabeni@...hat.com>, Andre Edich <andre.edich@...rochip.com>,
 kernel@...gutronix.de, linux-kernel@...r.kernel.org, Russell King
 <linux@...linux.org.uk>, netdev@...r.kernel.org, Lukas Wunner
 <lukas@...ner.de>
Subject: Re: [PATCH net v2 2/3] net: phy: smsc: Force predictable MDI-X
 state on LAN87xx

On Thu,  3 Jul 2025 13:49:40 +0200
Oleksij Rempel <o.rempel@...gutronix.de> wrote:

> Override the hardware strap configuration for MDI-X mode to ensure a
> predictable initial state for the driver. The initial mode of the LAN87xx
> PHY is determined by the AUTOMDIX_EN strap pin, but the driver has no
> documented way to read its latched status.
> 
> This unpredictability means the driver cannot know if the PHY has
> initialized with Auto-MDIX enabled or disabled, preventing it from
> providing a reliable interface to the user.
> 
> This patch introduces a `config_init` hook that forces the PHY into a
> known state by explicitly enabling Auto-MDIX.
> 
> Fixes: 05b35e7eb9a1 ("smsc95xx: add phylib support")
> Signed-off-by: Oleksij Rempel <o.rempel@...gutronix.de>
> Cc: Andre Edich <andre.edich@...rochip.com>

The patch looks good to me, but I have a few questions.  As this
overrides some configuration on existing HW, and I'm not utra familiar
with auto-mdix, is there any chance this could cause regressions ?

Especially regarding your patch 3, is there any chance that the PHY is
strapped in a fixed MDIX mode to address the broken autoneg off mode ? 

I'm not saying that strapping is a good solution for that ofc :) it's a
shame we can't read the strap config :/

Maxime