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

[Krzysztof Kozlowski <krzysztof.kozlowski@xxxxxxxxxx>]

On 04/10/2022 18:01, Vladimir Oltean wrote:
> On Tue, Oct 04, 2022 at 04:59:02PM +0200, Krzysztof Kozlowski wrote:
>> On 04/10/2022 14:15, Vladimir Oltean wrote:
>>> On Tue, Oct 04, 2022 at 01:19:33PM +0200, Krzysztof Kozlowski wrote:
>>>>> +  # Ocelot-ext VSC7512
>>>>> +  - |
>>>>> +    spi {
>>>>> +        soc@0 {
>>>>
>>>> soc in spi is a bit confusing.
>>>
>>> Do you have a better suggestion for a node name? This is effectively a
>>> container for peripherals which would otherwise live under a /soc node,
>>
>> /soc node implies it does not live under /spi node. Otherwise it would
>> be /spi/soc, right?
> 
> Did you read what's written right below? I can explain if you want, but
> there's no point if you're not going to read or ask other clarification
> questions.
> 
>>> if they were accessed over MMIO by the internal microprocessor of the
>>> SoC, rather than by an external processor over SPI.
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> 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.

> 
>>>> How is this example different than previous one (existing soc example)?
>>>> If by compatible and number of ports, then there is no much value here.
>>>
>>> The positioning relative to the other nodes is what's different.
>>
>> Positioning of nodes is not worth another example, if everything else is
>> the same. What is here exactly tested or shown by example? Using a
>> device in SPI controller?
> 
> Everything is not the same, it is not the same hardware as what is currenly
> covered by Documentation/devicetree/bindings/net/dsa/mscc,ocelot.yaml.
> The "existing soc example" (mscc,vsc9953-switch) has a different port
> count, integration with a different SERDES, interrupt controller, pin
> controller, things like that. The examples already differ in port count
> and phy-mode values, I expect they will start diverging more in the
> future. If you still believe it's not worth having an example of how to
> instantiate a SPI-controlled VSC7512 because there also exists an
> example of an MMIO-controlled VSC9953, then what can I say.
> 
> ------ cut here ------
> 
> 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...

Best regards,
Krzysztof