Re: [PATCH v2 4/4] dt-bindings: iio: adc: ad7949: add adi,reference-source

[Liam Beguin]

Hi Jonathan,

On Sat Jul 10, 2021 at 1:00 PM EDT, Jonathan Cameron wrote:
> On Fri, 9 Jul 2021 11:58:56 -0400
> Liam Beguin <liambeguin@xxxxxxxxx> wrote:
>
> > From: Liam Beguin <lvb@xxxxxxxxxx>
> > 
> > Add bindings documentation for the adi,reference-source property.
> > This property is required to properly configure the ADC sample request
> > based on which reference source should be used for the calculation.
>
> Should this be per channel? That will effect some of what I say below...
>

We could make it per channel. Ideally, I'd also like to add support for
differential channels, so might as well add per channel configurations
now.

> > 
> > Signed-off-by: Liam Beguin <lvb@xxxxxxxxxx>
> > ---
> >  .../bindings/iio/adc/adi,ad7949.yaml          | 21 +++++++++++++++++++
> >  1 file changed, 21 insertions(+)
> > 
> > diff --git a/Documentation/devicetree/bindings/iio/adc/adi,ad7949.yaml b/Documentation/devicetree/bindings/iio/adc/adi,ad7949.yaml
> > index 9b56bd4d5510..eae3121cad01 100644
> > --- a/Documentation/devicetree/bindings/iio/adc/adi,ad7949.yaml
> > +++ b/Documentation/devicetree/bindings/iio/adc/adi,ad7949.yaml
> > @@ -35,6 +35,27 @@ properties:
> >    "#io-channel-cells":
> >      const: 1
> >  
> > +  adi,reference-select:
>
> This is one field in the register, but it's not one thing, so lets break
> it up
> in DT. We should do this both to make for more readable dts files and to
> enforce the requirements on regulators...
>
> > +    description: |
> > +      Select the reference voltage source to use when converting samples.
> > +      Acceptable values are:
> > +      - 0: Internal reference and temperature sensor enabled.
> > +           Vref=2.5V, buffered output
> > +      - 1: Internal reference and temperature sensor enabled.
> > +           Vref=4.096V, buffered output
> > +      - 2: Use external reference, temperature sensor enabled.
> > +           Internal buffer disabled
> > +      - 3: Use external reference, internal buffer and temperature sensor
> > +           enabled.
> > +      - 6: Use external reference, internal buffer and temperature sensor
> > +           disabled.
> > +      - 7: Use external reference, internal buffer enabled.
> > +           Internal reference and temperature sensor disabled.
>
> So question 1 is whether to use an external or internal reference.
> Normally we'd make the coarse decision of whether to use an external
> reference
> by whether there is a regulator provided. That won't work so well if we
> make
> this per channel.
>
> Question 2, assuming internal reference, what voltage? Those should take
> an actual voltage (probably in mV and match against an enum of the two
> possible values).
> Binding should check to make sure this isn't specified as well as saying
> we
> are using an external refernce.
>
> Question 3, assuming external reference, is temperature sensor enabled?
> - actually dumb question, but why would anyone not want this enabled?
> Maybe turn it
> off in runtime pm, but in general if you've fitted a chip with a
> temperature sensor
> you at least sometimes want to measure temperature! So my gut feeling is
> don't
> allow this to be controlled (effectively drop cases 6 and 7 above as
> being
> unlikely to be of interest to anyone)
>

I like your suggestion of breaking this down so far, it would look
something like this:

	ad7949: adc@0 {
		compatible = "adi,ad7949";
		reg = <0>;

		vref-supply = <&vdd_supply>;

		channel@0 {
			adi,internal-ref-mv = <2500>;
			reg = <0>;
		};

		channel@1 {
			reg = <1>;
			/*
			 * defaults to vref-supply if defined or error
			 * out
			 */
		};
	};

> Question 4, Is the internal buffer enabled when using and external
> reference.
> This one is interesting. We could just expose it in general, but I
> wonder
> if we can do something that reflects how it is used. From the various
> figures in
> the datasheet this seems to be coupled to whether the external reference
> is on
> pin REF_IN or pin REF. If that's the case can we have two optional regs
> only
> one of which should be supplied? However, this gets more fiddly because
> the default right now is vref-supply actually being connected to the
> vrefin connection.
> That's annoying as it stops us using the obvious naming...
> Hence I think we can have
> vref-supply (actually connected to vrefin) and vref-unbuffered-supply
>

I really like the idea of using the same names as the datasheet
(vref-supply and vrefin-supply), to infer the buffered state,
but it's annoying (and confusing) that it's setup the other way
right now.

I wonder what happens if the reference is connected to refin and we're
configured as unbuffered (and the other way around).
I looked around and I might be able to test it on one setup I have where
the external reference is connected to REF.

If it's not a breaking change, would it be okay with you to follow the
datasheet naming?

Liam

>
>
> > +
> > +    $ref: /schemas/types.yaml#/definitions/uint32
> > +    enum: [0, 1, 2, 3, 6, 7]
> > +    default: 7
> > +
> >  required:
> >    - compatible
> >    - reg