[PATCH 2/5] dt-bindings: connector: Add fsl,io-connector binding

[Chancel Liu (OSS)]

> >>>>>>>>> +description:
> >>>>>>>>> +  The NXP I/O connector represents a physically present I/O
> >>>>>>>>> +connector on the
> >>>>>>>>> +  base board. It acts as a nexus that exposes a constrained
> >>>>>>>>> +set
> >>>> of
> >>>>>>>>> +I/O
> >>>>>>>>> +  resources, such as GPIOs, clocks, PWMs and interrupts,
> >>>>>>>>> +through fixed
> >>>>>>>>> +  electrical wiring. All actual hardware providers reside on
> >>>>>>>>> +the
> >>>> base
> >>>>>> board.
> >>>>>>>>> +  The connector node only defines index-based mappings to
> >>>>>>>>> + those
> >>>>>>>> providers.
> >>>>>>>>> +
> >>>>>>>>> +properties:
> >>>>>>>>> +  compatible:
> >>>>>>>>> +    const: fsl,io-connector
> >>>>>>>>
> >>>>>>>> Everything is IO. Everything is connector, so your compatible
> >>>>>>>> does not match requirements from writing bindings.
> >>>>>>>>
> >>>>>>>
> >>>>>>> Yes, this compatible is too generic. I will rename the
> >>>>>>> compatible to fsl,aud-io-connector.
> >>>>>>
> >>>>>> aud is not much better. Which boards have it? What's the pinout?
> >>>> What's
> >>>>>> standard? Is it described anywhere? If so, provide reference to
> >>>> spec/docs.
> >>>>>>
> >>>>>
> >>>>> This is not an industry standard electrical interface. This
> >>>>> connector
> >>>>
> >>>> Then if you do not have standard, then you have board specific
> >>>> layouts thus you need board-specific compatibles. You can use
> >>>> fallbacks. Generic fallback could work, but both io-connector and
> >>>> aud-io-connector are just too generic. Every connector is
> >>>> "connector" and "io", thus absolutely anything can be
> >>>> "io-connector". "aud" improves it only a bit, thus honestly I would
> go with board specific fallback as well.
> >>>>
> >>>
> >>> How about board specific + common fallback compatible like this:
> >>>   compatible:
> >>>     items:
> >>>       - enum:
> >>>           - fsl,imx95-19x19-evk-aud-io-connector
> >>>           - fsl,imx952-evk-aud-io-connector
> >>>       - const: fsl,imx-aud-io-connector Since the daughter board is
> >>> named “IMX-AUD-IO” in publicly available
> >>
> >> I don't think it is named like that.
> >>
> >> git grep -i imx-aud-io
> >>
> >>> documentation, common compatible clearly indicates that this
> >>> connector is intended for that.
> >>>
> >>> Also, I want to talk about the topic of generic connector. It's a
> >>> common design that daughter board is connected to base board through
> >>> a connector. This connector more often acts as a nexus that exposes
> >>> a constrained subset of GPIO, clock, PWM and interrupt resources to
> >>> the daughter board. Can we document this kind of connector as a
> >>> generic binding?
> >>
> >> So this binding is the connector between carrier and some addon? Then
> >> you don't get a compatible for that at all, because it is not
> >> necessary, not useful and NEVER used. Do you see socket LGA "connector"
> bindings? No.
> >
> > Not exactly. Any connector connects a carrier board with an add-on
> board.
> > The key point here is that this connector type is reused across
> > different boards, even though it is not an industry-standard
> > connector. Both the signal definitions and the mechanical layout are
> defined.
> >
> > The same add-on boards can therefore be reused across different base
> > boards that use this type of connector.
> >
> > There are also GPIO mappings involved. For example, pin 1 on the
> > connector may represent reset-gpios, but it could be connected to
> > GPIO0 on board A and GPIO1 on board B.
> >
> > Without a connector definition layer, this would create an N × M
> > combination problem. The Nexus node discussion already covered this
> topic:
> > https://osseu2025.sched.com/event/25Vrw
> >
> > An LGA socket is a CPU socket, where the signals are completely
> > transparent to software, so it is not a good comparison. A PCIe M.2
> > Key-M/E connector would be a more appropriate comparison.
> >
> 
> So the terminology of daughter and carrier boards was confusing. If this
> is a hat, mezzanine or other addon, it's fine.
> 

The IMX-AUD-IO is an add-on board that attaches to the base board. To
make it clearer, I will replace "daughter board" with "add-on board"
throughout descriptions.

> I still insist on board specific compatibles - fallback and specific.
> 

The base board has a slot component that is mechanically compatible
with a PCIe x8 connector. However, it carries no PCIe signals and the
pins are repurposed to carry fixed board-level audio I/O related
signals.

I think we can name a compatible reflects a standard mechanical form
factor.
For the compatibles (specific + fallback) I propose:
- enum:
    - fsl,imx95-19x19-evk-aud-io-pcie-x8-slot 
    - fsl,imx952-evk-aud-io-pcie-x8-slot
- const: fsl,aud-io-pcie-x8-slot

I understand the concern about implying PCIe functionality. In the
binding I will explicitly state that this is a repurposed mechanical
form factor only, with no PCIe protocol present.

> Best regards,
> Krzysztof

Regards, 
Chancel Liu