linux-kernel - Re: [RFC net-next 1/3] dt-bindings: net: dsa: yt921x: Add Motorcomm YT921x switch support

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Message-ID: <0c94e763-8963-4a04-8157-87725e80075c@lunn.ch>
Date: Fri, 15 Aug 2025 01:09:37 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Krzysztof Kozlowski <krzk@...nel.org>
Cc: David Yang <mmyangfl@...il.com>, netdev@...r.kernel.org,
	Vladimir Oltean <olteanv@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>, Simon Horman <horms@...nel.org>,
	Russell King <linux@...linux.org.uk>, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC net-next 1/3] dt-bindings: net: dsa: yt921x: Add Motorcomm
 YT921x switch support

> > +  motorcomm,switch-id:
> > +    description: |
> > +      When managed via mdio, hard-configured switch id to distinguish between
> > +      multiple devices.
> 
> IDs are not allowed.

Please describe in more detail what this is used for.

When the switch is hanging off an MDIO bus, there is a reg
property. Maybe that is what you mean here? Take a look at mv88e6xxx,
and other DSA devices which are on MDIO busses.

> > +    enum: [0, 1, 2, 3]
> > +    default: 0
> > +
> > +  mdio:
> > +    $ref: /schemas/net/mdio.yaml#
> > +    unevaluatedProperties: false
> > +    description: MDIO bus for the internal GbE PHYs.
> > +
> > +  mdio-external:
> > +    $ref: /schemas/net/mdio.yaml#
> > +    unevaluatedProperties: false
> > +    description: External MDIO bus.
> > +
> > +    properties:
> > +      compatible:
> > +        const: motorcomm,yt921x-mdio-external
> 
> Incomplete compatible... but also not needed in the first place.

So this device has two MDIO busses. You need to somehow describe each
in DT, so you can PHYs off them. And you need to know which is
which. For mv88e6xxx, which i know is maybe not the best example
because a lot of best practices have changed since then, some variants
also have two MDIO busses, and we give the second one a compatible so
we can tell it part from the other which is common to all mv88e6xxx
devices.

	Andrew