Subject: Re: [PATCH net-next 2/5] net: phy: realtek: simplify C22 reg access via MDIO_MMD_VEND2

From: "Russell King (Oracle)" <[email-masked]>
Date: Fri, 9 Jan 2026 21:32:18 +0000

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.

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