linux-kernel - Re: [PATCH net-next 06/16] net: pcs: xpcs: Avoid creating dummy XPCS MDIO device

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Date:   Tue, 5 Dec 2023 13:52:34 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     Serge Semin <fancer.lancer@...il.com>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Russell King <linux@...linux.org.uk>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        Jose Abreu <joabreu@...opsys.com>,
        Jose Abreu <Jose.Abreu@...opsys.com>,
        Maxime Chevallier <maxime.chevallier@...tlin.com>,
        Tomer Maimon <tmaimon77@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, openbmc@...ts.ozlabs.org,
        netdev@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 06/16] net: pcs: xpcs: Avoid creating dummy XPCS
 MDIO device

On Tue, Dec 05, 2023 at 01:35:27PM +0300, Serge Semin wrote:
> If the DW XPCS MDIO devices are either left unmasked for being auto-probed
> or explicitly registered in the MDIO subsystem by means of the
> mdiobus_register_board_info() method

mdiobus_register_board_info() has exactly one caller, and that is
dsa_loop. I don't understand the relevance of it w.r.t. Synopsys XPCS.
I'm reading the patches in order from the beginning.

> there is no point in creating the dummy MDIO device instance in order

Why dummy? There's nothing dummy about the mdio_device. It's how the PCS
code accesses the hardware.

> to get the DW XPCS handler since the MDIO core subsystem will create
> the device during the MDIO bus registration procedure.

It won't, though? Unless someone is using mdiobus_register_board_info()
possibly, but who does that?

> All what needs to be done is to just reuse the MDIO-device instance
> available in the mii_bus.mdio_map array (using some getter for it
> would look better though). It shall prevent the XPCS devices been
> accessed over several MDIO-device instances.
> 
> Note since the MDIO-device instance might be retrieved from the MDIO-bus
> map array its reference counter shall be increased. If the MDIO-device
> instance is created in the xpcs_create_mdiodev() method its reference
> counter will be already increased. So there is no point in toggling the
> reference counter in the xpcs_create() function. Just drop it from there.
> 
> Signed-off-by: Serge Semin <fancer.lancer@...il.com>
> ---

Sorry, because the commit log lost me at the "context presentation" stage,
I failed to understand the "what"s and the "why"s.

Are you basically trying to add xpcs support on top of an mdio_device
where the mdio_device_create() call was made externally to the xpcs code,
through mdiobus_register_board_info() and mdiobus_setup_mdiodev_from_board_info()?