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

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Message-ID: <df6d000d-c918-48ec-958c-c8529ae1b75f@email.android.com>
Date:	Thu, 22 Aug 2013 07:17:46 +0100
From:	Jonathan Cameron <jic23@...nel.org>
To:	Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
	Pawel Moll <pawel.moll@....com>
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>,
	"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

Sorry accidental HTML message hence resend

Alexandre Belloni <alexandre.belloni@...e-electrons.com> 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.
>
>So, the two solutions you suggest are:
>1/ using a fixed-regulator phandle per channel
>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.
>
>Are my explanations clear enough to take a decision ?
>
>-- 
>Alexandre Belloni, Free Electrons
>Embedded Linux, Kernel and Android engineering
>http://free-electrons.com
>--
>To unsubscribe from this list: send the line "unsubscribe linux-iio" in
>the body of a message to majordomo@...r.kernel.org
>More majordomo info at  http://vger.kernel.org/majordomo-info.html

I would favour option 2 though some of the discussions going on at the moment about
bindings might result in a generic description of this and any other bits of
 analog front end.
-- 
Sent from my Android phone with K-9 Mail. Please excuse my brevity.
-- 
Sent from my Android phone with K-9 Mail. Please excuse my brevity.
--
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