Re: [PATCH 2/6] dt-bindings: iio: ad7949: Add adi,reference-select

[Adam Michaelis <adam.michaelis@rockwellcollins.com>]

The property name "adi,reference-select" was copied from the
adi,ad7124 bindings as a similar hardware register configuration value
field. If the property was separated into three independent fields,
there would be a lot of explanation and checking required since many
of the combinations are invalid (for example, temperature sensor and
buffer are always enabled if internal reference is used). I could
possibly see removing the temperature sensor configuration from the
device tree, but, the current driver (even after these patches) does
not provide any support to read the temperature sensor's value. I
include that information in the configuration options as a summary of
the datasheet.

On Sun, May 5, 2019 at 7:22 AM Jonathan Cameron <jic23@kernel.org> wrote:
>
> On Wed,  1 May 2019 16:16:59 -0500
> Adam Michaelis <adam.michaelis@rockwellcollins.com> wrote:
>
> > Adding optional parameter to AD7949 to specify the source for the
> > reference voltage signal. Default value is maintaned with option '6' to
> > match previous version of driver.
> >
> > Signed-off-by: Adam Michaelis <adam.michaelis@rockwellcollins.com>
> > ---
> >  .../devicetree/bindings/iio/adc/ad7949.txt         | 22 ++++++++++++++++++++--
> >  1 file changed, 20 insertions(+), 2 deletions(-)
> >
> > diff --git a/Documentation/devicetree/bindings/iio/adc/ad7949.txt b/Documentation/devicetree/bindings/iio/adc/ad7949.txt
> > index c7f5057356b1..14ee9a2cb2a5 100644
> > --- a/Documentation/devicetree/bindings/iio/adc/ad7949.txt
> > +++ b/Documentation/devicetree/bindings/iio/adc/ad7949.txt
> > @@ -6,11 +6,29 @@ Required properties:
> >       * "adi,ad7682"
> >       * "adi,ad7689"
> >   - reg: spi chip select number for the device
> > - - vref-supply: The regulator supply for ADC reference voltage
> >
> > -Example:
> > +Optional properties:
> > + - adi,reference-select: Select the reference voltage source to use
> > + when converting the input voltages. Valid values are:
> So my immediate thought here is we are mapping one binding to several
> different things. Some of which are definitely better described in other
> ways.
>
> So let us break it down:
>
> Internal vs external.
> - External should require a regulator.  If the regulator is there, normal
> assumption would be you want to use it.
>
> Which internal reference?  Hmm. This would be incompatible with the external
> regulator and I'd expect the presence of such a regulator to override this.
> That does need a new binding.
> adi,internal-reference-milivolts = 2500 or 4096.   Much nicer to have
> real numbers for someone wondering how it is configured than an enum.
>
> Temperature sensor enabled: Why is this a devicetree question rather than
> a runtime decision?
>
> Buffer enabled: This needs a custom binding
> adi,external-reference-buffer-enable or something like that?
>
> Makes for a more consistent binding where some elements can be common
> across similar devices.  It would be good to see if similar bindings
> already exist.  Potentially tings like the reference-buffer enable
> may be worth making standard ADC properties rather than device
> specific.
>
> Thanks,
>
> Jonathan
>
>
> > +   0: Internal 2.5V reference; temperature sensor enabled
> > +   1: Internal 4.096V reference; temperature sensor enabled
> > +   2: External reference, temperature sensor enabled, no buffer
> > +   3: External reference, temperature sensor enabled, buffer enabled
> > +   6: External reference, temperature sensor disabled, no buffer
> > +   7: External reference, temperature sensor disabled, buffer enabled
> > + - vref-supply: The regulator supply for ADC reference voltage. Required
> > + if external reference selected by 'adi,reference-select'.
> > +
> > +Examples:
> >  adc@0 {
> >       compatible = "adi,ad7949";
> >       reg = <0>;
> > +     adi,reference-select = <2>;
> >       vref-supply = <&vdd_supply>;
> >  };
> > +
> > +adc@0 {
> > +     compatible = "adi,ad7949";
> > +     reg = <0>;
> > +     adi,reference-select = <0>;
> > +};
>


-- 

Adam Michaelis | Sr Software Engineer | Comm Engineering | Mission Systems

COLLINS AEROSPACE

400 Collins Road, Cedar Rapids, IA 52498 U.S.A.

Tel: +1 319 295 4102

adam.michaelis@collins.com | collinsaerospace.com



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