linux-kernel - Re: [PATCH net-next 2/5] net: phy: realtek: simplify C22 reg access via MDIO_MMD

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Message-ID: <aWF_cmmcT03Q03Nv@makrotopia.org>
Date: Fri, 9 Jan 2026 22:21:38 +0000
From: Daniel Golle <daniel@...rotopia.org>
To: "Russell King (Oracle)" <linux@...linux.org.uk>
Cc: Andrew Lunn <andrew@...n.ch>, Heiner Kallweit <hkallweit1@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Vladimir Oltean <vladimir.oltean@....com>,
	Michael Klein <michael@...sekall.de>,
	Aleksander Jan Bajkowski <olek2@...pl>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 2/5] net: phy: realtek: simplify C22 reg access
 via MDIO_MMD_VEND2

On Fri, Jan 09, 2026 at 09:32:18PM +0000, Russell King (Oracle) wrote:
> On Fri, Jan 09, 2026 at 03:03:22AM +0000, Daniel Golle wrote:
> > RealTek 2.5GE PHYs have all standard Clause-22 registers mapped also
> > inside MDIO_MMD_VEND2 at offset 0xa400. This is used mainly in case the
> > PHY is inside a copper SFP module which uses the RollBall MDIO-over-I2C
> > method which *only* supports Clause-45.
> 
> It isn't just Rollball. There are SoCs out there which have separate
> MDIO buses, one bus signals at 3.3V and can generate only clause 22
> frames. The other operates at 1.2V and can only generate clause 45
> frames.
> 
> While hardware may elect to generate and recognise either frame types
> at either voltage, this goes some way to explain why there are
> implementations that only support one or the other on a particular
> pair of MDC/MDIO wires.
> 
> Armada 8040 has this setup - there is one MDIO bus that only supports
> clause 22 frames, and there is a separate MDIO bus that only supports
> clause 45 frames.

Interesting. And a bit annoying. I wasn't aware of the electrical
difference (signal voltage).

Never the less, even with this change applied you now get a driver which
uses *only* Clause-45 access in case phydev->is_45 is true, and only
Clause-22 in case phydev->is_45 is false.

>From what I understood this was the intended outcome of having two
dedicated drivers, and you can have the very same results now with a
single driver. If you would like me to broaden the commit message and
clarify this, please let me know.