Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753931Ab3HVIFr (ORCPT ); Thu, 22 Aug 2013 04:05:47 -0400 Received: from mail1.bemta7.messagelabs.com ([216.82.254.109]:52516 "EHLO mail1.bemta7.messagelabs.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751930Ab3HVIFn (ORCPT ); Thu, 22 Aug 2013 04:05:43 -0400 X-Env-Sender: Hector.Palacios@digi.com X-Msg-Ref: server-3.tower-201.messagelabs.com!1377158738!10979855!2 X-Originating-IP: [66.77.174.14] X-StarScan-Received: X-StarScan-Version: 6.9.11; banners=-,-,- X-VirusChecked: Checked Message-ID: <5215C64D.3040004@digi.com> Date: Thu, 22 Aug 2013 10:05:33 +0200 From: Hector Palacios Organization: Digi International User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130804 Thunderbird/17.0.8 MIME-Version: 1.0 To: Alexandre Belloni CC: Pawel Moll , Jonathan Cameron , "linux-iio@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "devicetree-discuss@lists.ozlabs.org" , "lars@metafoo.de" , "fabio.estevam@freescale.com" , "marex@denx.de" , "rob.herring@calxeda.com" , Mark Rutland , Stephen Warren , Ian Campbell Subject: Re: [PATCH v3 2/5] ARM: dts: add reference voltage property for MXS LRADC References: <1374501843-19651-1-git-send-email-hector.palacios@digi.com> <1374501843-19651-3-git-send-email-hector.palacios@digi.com> <520AA3CD.1040008@kernel.org> <1376491467.18617.41.camel@hornet> <52153B8E.7050309@free-electrons.com> In-Reply-To: <52153B8E.7050309@free-electrons.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4780 Lines: 111 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@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/