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Date: Thu, 22 Aug 2013 10:05:33 +0200 From: Hector Palacios <hector.palacios@...i.com> To: Alexandre Belloni <alexandre.belloni@...e-electrons.com> CC: Pawel Moll <pawel.moll@....com>, Jonathan Cameron <jic23@...nel.org>, "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>, "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 Dear Alexandre, On 08/22/2013 12:13 AM, Alexandre Belloni wrote: > Hi Pawel, > > On 14/08/2013 16:44, Pawel Moll wrote: >> 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? >> > > You are not so wrong. There is indeed actually only one reference > voltage (and that is 1.85V). But, before feeding the voltage to the ADC > channels, you sometimes have a divider. Then, after the channel muxing, > you can add a by 2 divider. > > Mandatory ascii art: > > +-----+ > | | > +-ch1--->| | > | | > | | > | | +-----+ > +-ch2--->| | | | > | MUX |++-->| ADC +-----------> > ch3 | | | | | > +----+ | | | +-----+ > | | | | | | > +-> :4 +->| | | +---+--+ > | | | | | | | > +----+ | | +->| :2 | > +-----+ | | > +------+ > > >> 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. >> > > So, from my point of view, the divider that is before the mux (the by 4 > divider on channel 3 on my drawing) is not part of the the ADC, it is > not fixed by that IP. And indeed, that changed between the i.mx23 and > i.mx28 while the IP is the same. The dividers only make sense and affect the ADC, so whether they should be considered part of the ADC IP or not is a philosophical question. In my opinion, the different dividers between the i.mx23 and i.mx28 channels are the kind of hardware differences that fit nicely in the DeviceTree, describing the hardware. > So, the two solutions you suggest are: > 1/ using a fixed-regulator phandle per channel Since the dividers only affect and have meaning on the ADC channels, creating a regulator for each channel that has a different divider looks to me like an overworked solution. These are not real voltage sources. They are just indicators of the maximum reference voltage that an ADC channel can measure. > 2/ hard-coding the dividers in the driver using the compatible string to > know which divider is on which channel. > > I feel that solution 2 is less future proof but at the same time, I > don't believe we will see that IP in another chip in the future. This was what I originally submitted but it then looked like it would better fit in the DeviceTree. The spear-adc seemed to use a similar approach: http://permalink.gmane.org/gmane.linux.kernel.iio/7994 Best regards, -- Hector Palacios -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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