Re: [PATCH v2 8/8] dt-bindings: iio: afe: add binding for temperature-sense-amplifier

["Liam Beguin" <liambeguin@gmail.com>]

On Wed Jun 9, 2021 at 4:52 PM EDT, Jonathan Cameron wrote:
> On Mon, 7 Jun 2021 10:47:18 -0400
> Liam Beguin <liambeguin@gmail.com> wrote:
>
> > From: Liam Beguin <lvb@xiphos.com>
> > 
> > An ADC is often used to measure other quantities indirectly. This
> > binding describe such a use case, the measurement of a temperature
> > through an analog front end connected to a voltage channel.
> > 
> > Signed-off-by: Liam Beguin <lvb@xiphos.com>
> > ---
> >  .../iio/afe/temperature-sense-amplifier.yaml  | 57 +++++++++++++++++++
> >  MAINTAINERS                                   |  1 +
> >  2 files changed, 58 insertions(+)
> >  create mode 100644 Documentation/devicetree/bindings/iio/afe/temperature-sense-amplifier.yaml
> > 
> > diff --git a/Documentation/devicetree/bindings/iio/afe/temperature-sense-amplifier.yaml b/Documentation/devicetree/bindings/iio/afe/temperature-sense-amplifier.yaml
> > new file mode 100644
> > index 000000000000..08f97f052a91
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/iio/afe/temperature-sense-amplifier.yaml
> > @@ -0,0 +1,57 @@
> > +# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
> > +%YAML 1.2
> > +---
> > +$id: http://devicetree.org/schemas/iio/afe/temperature-sense-amplifier.yaml#
> > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > +
> > +title: Temperature Sense Amplifier
> > +
> > +maintainers:
> > +  - Liam Beguin <lvb@xiphos.com>
> > +
> > +description: |
> > +  When an io-channel measures the output voltage of a temperature analog front
> > +  end such as an RTD (resistance thermometer) or a temperature to current
> > +  sensor, the interesting measurement is almost always the corresponding
> > +  temperature, not the voltage output. This binding describes such a circuit.
>
> Perhaps add something about this only covering the linear cases...
>

Okay, will do.

> > +
> > +properties:
> > +  compatible:
> > +    const: temperature-sense-amplifier
> > +
> > +  io-channels:
> > +    maxItems: 1
> > +    description: |
> > +      Channel node of a voltage io-channel.
> > +
> > +  '#io-channel-cells':
> > +    const: 1
> > +
> > +  sense-gain-mult:
> > +    $ref: /schemas/types.yaml#/definitions/uint32
> > +    description: Amplifier gain multiplier. The default is <1>.
> > +
> > +  sense-gain-div:
> > +    $ref: /schemas/types.yaml#/definitions/uint32
> > +    description: Amplifier gain divider. The default is <1>.
> > +
> > +  sense-offset-millicelsius:
> > +    description: Amplifier offset. The default is <0>.
>
> Whilst it may seem obvious I'd like to see a statement of
> how these are used somewhere in here.
>
> temp_celcius = voltage * gain-mult / gain-div + offset
>
> Mainly because those familiar with the IIO usage of offset
> would expect
> (voltage + offset) * gain-mult/gain-div
>
> which doesn't make sense for this device but might leave
> people confused!
>

You're right, I'll add an explicit statement of how these parameters are
used.

Thanks,
Liam

> > +
> > +additionalProperties: false
> > +required:
> > +  - compatible
> > +  - io-channels
> > +
> > +examples:
> > +  - |
> > +    pt1000_1: temperature-sensor {
> > +        compatible = "temperature-sense-amplifier";
> > +        #io-channel-cells = <1>;
> > +        io-channels = <&temp_adc 3>;
> > +
> > +        sense-gain-mult = <1000000>;
> > +        sense-gain-div = <3908>;
> > +        sense-offset-millicelsius = <(-255885)>;
> > +    };
> > +...
> > diff --git a/MAINTAINERS b/MAINTAINERS
> > index e679d422b472..4f7b4ee9f19b 100644
> > --- a/MAINTAINERS
> > +++ b/MAINTAINERS
> > @@ -8887,6 +8887,7 @@ L:	linux-iio@vger.kernel.org
> >  S:	Maintained
> >  F:	Documentation/devicetree/bindings/iio/afe/current-sense-amplifier.yaml
> >  F:	Documentation/devicetree/bindings/iio/afe/current-sense-shunt.yaml
> > +F:	Documentation/devicetree/bindings/iio/afe/temperature-sense-amplifier.yaml
> >  F:	Documentation/devicetree/bindings/iio/afe/voltage-divider.yaml
> >  F:	drivers/iio/afe/iio-rescale.c
> >