linux-kernel - Re: [PATCH net] net: mdio: mdio-bitbang: Fix C45 read/write protocol

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Date:   Wed, 16 Aug 2023 21:39:37 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     Serge Semin <fancer.lancer@...il.com>
Cc:     Heiner Kallweit <hkallweit1@...il.com>,
        Russell King <linux@...linux.org.uk>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Michael Walle <michael@...le.cc>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] net: mdio: mdio-bitbang: Fix C45 read/write protocol

On Wed, Aug 16, 2023 at 09:06:52PM +0300, Serge Semin wrote:
> Based on the original code semantic in case of Clause 45 MDIO, the address
> command is supposed to be followed by the command sending the MMD address,
> not the CSR address. The commit 002dd3de097c ("net: mdio: mdio-bitbang:
> Separate C22 and C45 transactions") has erroneously broken that. So most
> likely due to an unfortunate variable name it switched the code to sending
> the CSR address. In our case it caused the protocol malfunction so the
> read operation always failed with the turnaround bit always been driven to
> one by PHY instead of zero. Fix that by getting back the correct
> behaviour: sending MMD address command right after the regular address
> command.

I'm actually surprised we have not got more reports of me breaking
MDIO with this patchset. I did test bit-banging, but only for C22.

> Fixes: 002dd3de097c ("net: mdio: mdio-bitbang: Separate C22 and C45 transactions")
> Signed-off-by: Serge Semin <fancer.lancer@...il.com>

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

    Andrew