[PATCH v5 05/10] dt-bindings: net: dwmac-sun8i: update documentation about integrated PHY

[Rob Herring]

On Thu, Sep 14, 2017 at 2:19 PM, Andrew Lunn <andrew at lunn.ch> wrote:
>> > Is the MDIO controller "allwinner,sun8i-h3-emac" or "snps,dwmac-mdio"?
>> > If the latter, then I think the node is fine, but then the mux should be
>> > a child node of it. IOW, the child of an MDIO controller should either
>> > be a mux node or slave devices.
>
> Hi Rob
>
> Up until now, children of an MDIO bus have been MDIO devices. Those
> MDIO devices are either Ethernet PHYs, Ethernet Switches, or the
> oddball devices that Broadcom iProc has, like generic PHYs.
>
> We have never had MDIO-muxes as MDIO children. A Mux is not an MDIO
> device, and does not have the properties of an MDIO device. It is not
> addressable on the MDIO bus. The current MUXes are addressed via GPIOs
> or MMIO.

The DT parent/child relationship defines the bus topology. We describe
MDIO buses in that way and if a mux is sitting between the controller
and the devices, then the DT hierarchy should reflect that. Now
sometimes we have 2 options for what interface has the parent/child
relationship (e.g. an I2C controlled USB hub chip), but in this case
we don't.

> There other similar cases. i2c-mux-gpio is not a child of an i2c bus,
> nor i2c-mux-reg or gpio-mux. nxp,pca9548 is however a child of the i2c
> bus, because it is an i2c device itself...

Some are i2c controlled mux devices, but some can be GPIO controlled.

>
> If the MDIO mux was an MDIO device, i would agree with you. Bit it is
> not, so lets not make it a child.
>
>             Andrew
>
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