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

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Date:	Wed, 14 Aug 2013 15:44:27 +0100
From:	Pawel Moll <pawel.moll@....com>
To:	Jonathan Cameron <jic23@...nel.org>
Cc:	Hector Palacios <hector.palacios@...i.com>,
	"linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devicetree-discuss@...ts.ozlabs.org" 
	<devicetree-discuss@...ts.ozlabs.org>,
	"alexandre.belloni@...e-electrons.com" 
	<alexandre.belloni@...e-electrons.com>,
	"lars@...afoo.de" <lars@...afoo.de>,
	"fabio.estevam@...escale.com" <fabio.estevam@...escale.com>,
	"marex@...x.de" <marex@...x.de>,
	"rob.herring@...xeda.com" <rob.herring@...xeda.com>,
	Mark Rutland <Mark.Rutland@....com>,
	Stephen Warren <swarren@...dotorg.org>,
	Ian Campbell <ian.campbell@...rix.com>
Subject: Re: [PATCH v3 2/5] ARM: dts: add reference voltage property for MXS
 LRADC

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.
> > 
> > diff --git a/Documentation/devicetree/bindings/staging/iio/adc/mxs-lradc.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
> >  
> >  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
> > +	    maximum voltage that can be measured at full scale in each channel
> > +	    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


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