linux-kernel - Re: [PATCH 2/2] iio: afe: unit-converter: add support for adi,lt6106

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Message-ID: <6fff0256-c9f6-6302-45ac-afc7a3276b06@metafoo.de>
Date:   Fri, 13 Apr 2018 10:11:27 +0200
From:   Lars-Peter Clausen <lars@...afoo.de>
To:     Peter Rosin <peda@...ntia.se>, "Andrew F. Davis" <afd@...com>,
        linux-kernel@...r.kernel.org
Cc:     Jonathan Cameron <jic23@...nel.org>,
        Hartmut Knaack <knaack.h@....de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        "David S. Miller" <davem@...emloft.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Fabio Estevam <festevam@...il.com>, linux-iio@...r.kernel.org,
        devicetree@...r.kernel.org
Subject: Re: [PATCH 2/2] iio: afe: unit-converter: add support for adi,lt6106

On 04/13/2018 12:31 AM, Peter Rosin wrote:
> On 2018-04-12 17:35, Andrew F. Davis wrote:
>> On 04/12/2018 09:29 AM, Peter Rosin wrote:
>>> On 2018-04-11 18:13, Andrew F. Davis wrote:
>>>> On 04/11/2018 10:51 AM, Lars-Peter Clausen wrote:
>>>>> On 04/11/2018 05:43 PM, Andrew F. Davis wrote:
>>>>>> On 04/11/2018 09:15 AM, Peter Rosin wrote:
>>>>>>> This is a current sense amplifier from Analog Devices.
>>>>>>>
>>>>>>> Signed-off-by: Peter Rosin <peda@...ntia.se>
>>>>>>> ---
>>>>>>>  drivers/iio/afe/Kconfig              |  3 +-
>>>>>>>  drivers/iio/afe/iio-unit-converter.c | 54 ++++++++++++++++++++++++++++++++++++
>>>>>>>  2 files changed, 56 insertions(+), 1 deletion(-)
>>>>>>>
>>>>>>> diff --git a/drivers/iio/afe/Kconfig b/drivers/iio/afe/Kconfig
>>>>>>> index 642ce4eb12a6..0e10fe8f459a 100644
>>>>>>> --- a/drivers/iio/afe/Kconfig
>>>>>>> +++ b/drivers/iio/afe/Kconfig
>>>>>>> @@ -10,7 +10,8 @@ config IIO_UNIT_CONVERTER
>>>>>>>  	depends on OF || COMPILE_TEST
>>>>>>>  	help
>>>>>>>  	  Say yes here to build support for the IIO unit converter
>>>>>>> -	  that handles voltage dividers and current sense shunts.
>>>>>>> +	  that handles voltage dividers, current sense shunts and
>>>>>>> +	  the LT6106 Current Sense Amplifier from Analog Devices.
>>>>>>
>>>>>> Could work better to split these out into separate drivers. Maybe a
>>>>>> iio-shunt-resistor.c that does just voltage->current with the
>>>>>> appropriate scaling. Then make a a separate lt6106.c.
>>>>>
>>>>> I don't think we need a separate driver here. There are tons of circuits
>>>>> that all work the same way and all require the same properties. If we'd add
>>>>> a driver for each of them we'd get buried in boilerplate code.
>>>>>
>>>>
>>>> Fair enough, then it should at least be renamed to something generic
>>>> like current-sense-amplifier, as you said lots of circuits do this, not
>>>> just lt6106s. We will have then have support for:
>>>>
>>>> current-sense-amplifier
>>>> current-sense-shunt
>>>> voltage-divider
>>>
>>> For the compatible "current-sense-amplifier", I would advocate the
>>> properties...
>>>
>>>  sense-resistor-micro-ohms
>>>  sense-gain
>>>
>>> (or something close to that)
>>>
>>> ...and not input-resistor-ohms and output-resistor-ohms which are way
>>> more particular to the LT6106.
>>>
>>> But as I said in the cover letter, I didn't go with sense-gain since I
>>> thought I would end up with requests for non-integer gains. There is
>>> yet to be a comment on the non-integer gain problem, and before there
>>> is a path forward for that case, I'm reluctant.
>>>
>>
>> Why not similar to what you had before with the resistor:
>>
>> sense-gain-multiplier
>> sense-gain-divider
>>
>> if either are missing assume they are 1.
> 
> Hmm, how about sense-gain for the normal integer case, and then divide
> by sense-attenuation if needed? I.e. exactly the same functionality as
> you describe, just different names.

There is some precedence in the clock bindings for using -mult and -div as
the suffix for fractional scales. See fixed-factor-clock.txt.