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

[Jonathan Cameron]

On Mon, 12 Jul 2021 13:05:23 -0400
"Liam Beguin" <liambeguin@xxxxxxxxx> wrote:

> 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?

Absolutely. If we can get away with fixing that it would be great.

Jonathan

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