Re: [PATCH v3 2/5] ARM: dts: add reference voltage property for MXS LRADC

[Pawel Moll <pawel.moll@arm.com>]

On Tue, 2013-08-13 at 22:23 +0100, Jonathan Cameron wrote:
> On 07/22/13 15:04, Hector Palacios wrote:
> > Some LRADC channels have fixed pre-dividers so they can measure
> > different voltages at full scale. The reference voltage allows to
> > expose a scaling attribute through the IIO sysfs so that a user can
> > compute the real voltage out of a measured sample value.
> >=20
> > diff --git a/Documentation/devicetree/bindings/staging/iio/adc/mxs-lrad=
c.txt b/Documentation/devicetree/bindings/staging/iio/adc/mxs-lradc.txt
> > index 4688205..6ec485c 100644
> > --- a/Documentation/devicetree/bindings/staging/iio/adc/mxs-lradc.txt
> > +++ b/Documentation/devicetree/bindings/staging/iio/adc/mxs-lradc.txt
> > @@ -1,9 +1,12 @@
> >  * Freescale i.MX28 LRADC device driver
> > =20
> >  Required properties:
> > -- compatible: Should be "fsl,imx28-lradc"
> > +- compatible: "fsl,imx28-lradc", "fsl,imx23-lradc"
> >  - reg: Address and length of the register set for the device
> >  - interrupts: Should contain the LRADC interrupts
> > +- fsl,vref: Reference voltage (in mV) for each LRADC channel. This is =
the
> > +=09    maximum voltage that can be measured at full scale in each chan=
nel
> > +=09    considering fixed pre-dividers.

So, let me try to rephrase what I read above.

There's an ADC with X channels. And there's a reference voltage source
(one?). Now, each of the ADC channels have a (different?) voltage
divider, taking the voltage from the reference source and feeding it to
the ADC comparator. How much am I wrong?

If I'm not wrong at all, I'd say that the reference source could be
described as a standard fixed regulator
(Documentation/devicetree/bindings/regulator/fixed-regulator.txt) and
the ADC node should have some king of "reference-supply" phandle to the
regulator node. Now, if the dividers factors are *really* fixed, the
driver could know about them and calculate the effective reference
voltage on its own, couldn't it?

Let me repeat the "DT standard disclaimer": the tree, in general, should
describe the way components are *wired up*, not much more.

Pawel