Re: [RFC PATCH v1 0/2] hwmon: (iio_hwmon) optionally force iio channel type

[Guenter Roeck <linux@roeck-us.net>]

On 5/16/21 8:02 AM, Liam Beguin wrote:
> Hi Jonathan,
> 
> On Sun May 16, 2021 at 5:06 AM EDT, Jonathan Cameron wrote:
>> On Sun, 16 May 2021 00:43:13 -0400
>> Liam Beguin <liambeguin@gmail.com> wrote:
>>
>>> Add a devicetree binding to optionally force a different IIO channel
>>> type.
>>>
>>> This is useful in cases where ADC channels are connected to a circuit
>>> that represent another unit such as a temperature or a current.
>>>
>>> `channel-types` was chosen instead of `io-channel-types` as this is not
>>> part of the iio consumer bindings.
>>>
>>> In the current form, this patch does what it's intended to do:
>>> change the unit displayed by `sensors`, but feels like the wrong way to
>>> address the problem.
>>>
>>> Would it be possible to force the type of different IIO channels for
>>> this kind of use case with a devicetree binding from the IIO subsystem?
>>>
>>> It would be convenient to do it within the IIO subsystem to have the
>>> right unit there too.
>>>
>>> Thanks for your time,
>>> Liam
>>
>> Hi Liam,
>>
>> +CC Peter for AFE part.
>>
>> It's an interesting approach, but I would suggest we think about this
>> a different way.
>>
>> Whenever a channel is being used to measure something 'different' from
>> what it actually measures (e.g. a voltage ADC measuring a current) that
>> reflects their being some analog component involved.
>> If you look at drivers/iio/afe/iio-rescale.c you can see the approach
>> we currently use to handle this.
> 
> Many thanks for pointing out the AFE code. That look like what I was
> hoping to accomplish, but in a much better way.
> 
>>
>> Effectively what you add to devicetree is a consumer of the ADC channel
>> which in turn provides services to other devices. For this current case
>> it would be either a current-sense-amplifier or a current-sense-shunt
>> depending on what the analog front end looks like. We have to describe
>> the characteristics of that front end which isn't something that can
>> be done via a simple channel type.
>>
> 
> Understood. My original intention was to use sensors.conf to do the
> conversions and take into accounts those parameters.
> 
>> That afe consumer device can then provide services to another consumer
>> (e.g. iio-hwmon) which work for your usecase.
>>
>> The main limitation of this approach currently is you end up with
>> one device per channel. That could be improved upon if you have a
>> usecase
>> where it matters.
>>
>> I don't think we currently have an equivalent for temperature sensing
>> but it would be easy enough to do something similar.
> 
> Wonderful, thanks again for pointing out the AFE!
> 

Please don't reinvent the ntc_thermistor driver.

Thanks,
Guenter

> Liam
> 
>>
>> Jonathan
>>
>>
>>>
>>> Liam Beguin (2):
>>>    hwmon: (iio_hwmon) optionally force iio channel type
>>>    dt-bindings: hwmon: add iio-hwmon bindings
>>>
>>>   .../devicetree/bindings/hwmon/iio-hwmon.yaml  | 41 +++++++++++++++++++
>>>   drivers/hwmon/iio_hwmon.c                     |  2 +
>>>   2 files changed, 43 insertions(+)
>>>   create mode 100644 Documentation/devicetree/bindings/hwmon/iio-hwmon.yaml
>>>
>>>
>>> base-commit: 9f4ad9e425a1d3b6a34617b8ea226d56a119a717
>