linux-kernel - Re: [PATCH net] net: mdio: mdio-i2c: Hold the i2c bus lock during smbus transactions

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Message-ID: <13a81c5e-3dc8-44fe-aa04-5239c328c0c5@lunn.ch>
Date: Mon, 6 Oct 2025 15:24:39 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Maxime Chevallier <maxime.chevallier@...tlin.com>
Cc: davem@...emloft.net, Jakub Kicinski <kuba@...nel.org>,
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
	Russell King <linux@...linux.org.uk>,
	Heiner Kallweit <hkallweit1@...il.com>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, thomas.petazzoni@...tlin.com,
	Florian Fainelli <f.fainelli@...il.com>,
	Köry Maincent <kory.maincent@...tlin.com>,
	Simon Horman <horms@...nel.org>,
	Romain Gantois <romain.gantois@...tlin.com>,
	Marek Behún <kabel@...nel.org>
Subject: Re: [PATCH net] net: mdio: mdio-i2c: Hold the i2c bus lock during
 smbus transactions

On Fri, Oct 03, 2025 at 09:03:06AM +0200, Maxime Chevallier wrote:
> When accessing an MDIO register using single-byte smbus accesses, we have to
> perform 2 consecutive operations targeting the same address,
> first accessing the MSB then the LSB of the 16 bit register:
> 
>   read_1_byte(addr); <- returns MSB of register at address 'addr'
>   read_1_byte(addr); <- returns LSB
> 
> Some PHY devices present in SFP such as the Broadcom 5461 don't like
> seeing foreign i2c transactions in-between these 2 smbus accesses, and
> will return the MSB a second time when trying to read the LSB :
> 
>   read_1_byte(addr); <- returns MSB
> 
>   	i2c_transaction_for_other_device_on_the_bus();
> 
>   read_1_byte(addr); <- returns MSB again
> 
> Given the already fragile nature of accessing PHYs/SFPs with single-byte
> smbus accesses, it's safe to say that this Broadcom PHY may not be the
> only one acting like this.
> 
> Let's therefore hold the i2c bus lock while performing our smbus
> transactions to avoid interleaved accesses.
> 
> Fixes: d4bd3aca33c2 ("net: mdio: mdio-i2c: Add support for single-byte SMBus operations")
> Signed-off-by: Maxime Chevallier <maxime.chevallier@...tlin.com>

Reviewed-by: Andrew Lunn <andrew@...n.ch>

    Andrew