Re: [PATCH v3 net-next 12/14] dt-bindings: net: dsa: ocelot: add ocelot-ext documentation

[Vladimir Oltean <olteanv@gmail.com>]

On Wed, Oct 05, 2022 at 10:09:06AM +0200, Krzysztof Kozlowski wrote:
> > The /spi/soc@0 node actually has a compatible of "mscc,vsc7512" which
> > Colin did not show in the example (it is not "simple-bus"). It is covered
> > by Documentation/devicetree/bindings/mfd/mscc,ocelot.yaml. Still waiting
> > for a better suggestion for how to name the mfd container node.
> 
> Then still the /spi node does not seem related. If I understand
> correctly, your device described in this bindings is a child of soc@0.
> Sounds fine. How that soc@0 is connected to the parent - via SPI or
> whatever - is not related to this binding, is it? It is related to the
> soc binding, but not here.

It's an example, it's meant to be informative. It is the first DSA
driver of its kind. When everybody else ATM puts the ethernet-switch node
under the &spi controller node, this puts it under &spi/soc@<chip-select>/,
for reasons that have to do with scalability. If the examples aren't a
good place to make this more obvious, I don't know why we don't just
tell people to RTFD.

> > Unrelated to your "existing soc example" (the VSC9953), but relevant and
> > you may want to share your opinion on this:
> > 
> > The same hardware present in the VSC7514 SoC can also be driven by an
> > integrated MIPS processor, and in that case, it is indeed expected that
> > the same dt-bindings cover both the /soc and the /spi/soc@0/ relative
> > positioning of their OF node. This is true for simpler peripherals like
> > "mscc,ocelot-miim", "mscc,ocelot-pinctrl", "mscc,ocelot-sgpio". However
> > it is not true for the main switching IP of the SoC itself.
> > 
> > When driven by a switchdev driver, by the internal MIPS processor (the
> > DMA engine is what is used for packet I/O), the switching IP follows the
> > Documentation/devicetree/bindings/net/mscc,vsc7514-switch.yaml binding
> > document.
> > 
> > When driven by a DSA driver (external processor, host frames are
> > redirected through an Ethernet port instead of DMA controller),
> > the switching IP follows the Documentation/devicetree/bindings/net/dsa/mscc,ocelot.yaml
> > document.
> > 
> > The switching IP is special in this regard because the hardware is not
> > used in the same way. The DSA dt-binding also needs the 'ethernet'
> > phandle to be present in a port node. The different placement of the
> > bindings according to the use case of the hardware is a bit awkward, but
> > is a direct consequence of the separation between DSA and pure switchdev
> > drivers that has existed thus far (and the fact that DSA has its own
> > folder in the dt-bindings, with common properties in dsa.yaml and
> > dsa-port.yaml etc). It is relatively uncommon for a switching IP to have
> > provisioning to be used in both modes, and for Linux to support both
> > modes (using different drivers), yet this is what we have here.
> 
> Is there a question here to me? What shall I do with this paragraph? You
> know, I do not have a problem of lack of material to read...

For mscc,vsc7514-switch we have a switchdev driver. For mscc,vsc7512-switch,
Colin is working on a DSA driver. Their dt-bindings currently live in
different folders. The mscc,vsc7514-switch can also be used together
with a DSA driver, and support for that will inevitably be added. When
it will, how and where do you recommend the dt-bindings should be added?
In net/dsa/mscc,ocelot.yaml, together with the other switches used in
DSA mode, or in net/mscc,vsc7514-switch.yaml, because its compatible
string already exists there? We can't have a compatible string present
in multiple schemas, right?

This matters because it has implications upon what Colin should do with
the mscc,vsc7512-switch. If your answer to my question is "add $ref: dsa.yaml#
to net/mscc,vsc7514-switch.yaml", then I don't see why we wouldn't do
that now, and wait until the vsc7514 to make that move anyway.