linux-kernel - Re: [PATCH 6/7] dt-bindings: iio: adc: add adi,ad7606c-{16,18} compatible strings

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Message-ID: <CA+GgBR-ZPfJuOJJuXqUu6YEBmzHf6SLYWFprQUo8X-UhyRTyEg@mail.gmail.com>
Date: Wed, 28 Aug 2024 13:23:10 +0300
From: Alexandru Ardelean <aardelean@...libre.com>
To: Krzysztof Kozlowski <krzk@...nel.org>
Cc: Jonathan Cameron <jic23@...nel.org>, linux-iio@...r.kernel.org, 
	linux-kernel@...r.kernel.org, devicetree@...r.kernel.org, krzk+dt@...nel.org, 
	robh@...nel.org, lars@...afoo.de, michael.hennerich@...log.com, 
	gstols@...libre.com
Subject: Re: [PATCH 6/7] dt-bindings: iio: adc: add adi,ad7606c-{16,18}
 compatible strings

On Fri, Aug 23, 2024 at 12:09 PM Krzysztof Kozlowski <krzk@...nel.org> wrote:
>
> On 21/08/2024 22:26, Jonathan Cameron wrote:
> >
> >>>> +    type: object
> >>>> +    $ref: adc.yaml
> >>>> +    unevaluatedProperties: false
> >>>> +
> >>>> +    properties:
> >>>> +      reg:
> >>>> +        description: The channel number.
> >>>> +        minimum: 0
> >>>> +        maximum: 7
> >>>> +
> >>>> +      diff-channel:
> >>>> +        description: Channel is bipolar differential.
> >>>
> >>> There is diff-channels property, why do we need one more?
> >>
> >> Yeah, I wanted to use that.
> >> Maybe I will try another spin at that.
> >> The thing with "diff-channels" is that it requires 2 ints.
> >> So,  diff-channels = <0 0>.
> >> To use it here, a rule would need to be put in place where  "reg ==
> >> diff-channels[0] && reg == diff-channels[1]".
> >> That also works from my side.
> >> Part of the reason for this patchset, was to also get some feedback
> >> (if this is the correct direction).
> >>
> > So I 'think' this is a datasheet matching thing.
> > In many cases, even for strictly differential devices, the pin
> > naming allows for a clear A - B channel description. Here
> > in the non differential modes, the negative pins are effectively
> > not used (from a really quick look at the datasheet)
> >
> > So we 'could' introduce magic channels (give them high numbers) for
> > the negative ends. I think we may want to do that for the
> > userspace ABI (0-0 on the few times it has come up has been a
> > calibration / self check mode not what you have here - it
> > wires the actual inputs together).  Alternative is just present
> > them as a simple voltage and don't worry about the differential aspect
> > as it's not hugely different to bipolar (where the zero level is
> > effectively the negative input of a differential ADC.
> >
> > For the binding I'm fine with the binding using A, A as you suggest
> > with an update to adc.yaml to cover this corner.

The main difference the "diff-channels" property brings is a change to
the available scales.
They differ a bit between differential, and single-ended (unipolar and bipolar).

I'll update the adc.yaml file then.

>
> Yep, let's add it to adc.yaml.
>
> >
> > We never (I think) have bindings for the self check case where the input
> > is wired to both sides. It's just a mode that is applied to
> > any inputs that are wired.
> >
> Best regards,
> Krzysztof
>