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

[Andrew Lunn]

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@xxxxxxxxxxx>

Reviewed-by: Andrew Lunn <andrew@xxxxxxx>

    Andrew