linux-kernel - Re: [PATCH] PHY: Fix no autoneg corner case

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Message-ID: <b9270400-4081-4df9-a644-0c6d22a1973e@gmail.com>
Date: Wed, 19 Mar 2025 22:00:34 +0100
From: Heiner Kallweit <hkallweit1@...il.com>
To: Krzysztof Hałasa <khalasa@...p.pl>
Cc: Andrew Lunn <andrew@...n.ch>, netdev <netdev@...r.kernel.org>,
 Oliver Neukum <oneukum@...e.com>, Andrew Lunn <andrew+netdev@...n.ch>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
 Jose Ignacio Tornos Martinez <jtornosm@...hat.com>,
 Ming Lei <ming.lei@...hat.com>
Subject: Re: [PATCH] PHY: Fix no autoneg corner case

On 02.12.2024 08:57, Krzysztof Hałasa wrote:
> Heiner,
> 
> Heiner Kallweit <hkallweit1@...il.com> writes:
> 
>> If autoneg is supported, then phylib defaults to enable it. I don't see
>> anything wrong with this. BaseT modes from 1000Mbps on require autoneg.
>> Your original commit message seems to refer to a use case where a certain
>> operation mode of the PHY doesn't support autoneg. Then the PHY driver
>> should detect this operation mode and clear the autoneg-supported bit.
> 
> I'm not sure about it, but if there is consensus it should stay this
> way, no problem.
> 
> WRT specific case, It seems the SFP port doesn't support autoneg on
> AX88772BL (I don't have any SFP copper 10/100 module).
> 
> The PHY registers (100BASE-FX, no link currently):
>           x0   x1   x2   x3   x4   x5   x6   x7   x8
>   0000: 2100 7809   3B 1881  501
>   0010:  250  80C 8620   20 2314  3C8 4716 724F 8024
> 
> BMCR: fixed speed 100 Mb/s
>       The datasheet says "autoneg is fixed to 1 and speed to 100", but
>       it's apparently the case only with 100BASE-TX, not FX.
> 
>       It seems autoneg, speed and duplex bits are read only
>       (bits 13, 12, 9, 8). So, basically, you can't enable autoneg at
>       all and this is maybe the source of the "bug".
> 
> BMSR: 10 and 100 Mb/s FD/HD autoneg support indicated. Hmm. R/O.
> 
> ANAR: quite standard, but doesn't matter since autoneg is disabled.
> 
> I will think about it a bit more.

The PHY in your case should be handled by the ax88796b driver, based on PHY ID
in your register dump being PHY_ID_ASIX_AX88772C. Maybe the PHY driver needs
an extension for handling fiber mode properly. That's something you should check
based on the datasheet. There are examples like the Marvell PHY driver which
has quite some fiber-specific handling.