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SubjectRe: [RFC PATCH v1 0/2] hwmon: (iio_hwmon) optionally force iio channel type
On 5/16/21 11:14 AM, Liam Beguin wrote:
> On Sun May 16, 2021 at 12:26 PM EDT, Jonathan Cameron wrote:
>> On Sun, 16 May 2021 08:54:06 -0700
>> Guenter Roeck <> wrote:
>>> 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 <> 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.
>> Agreed, I'd forgotten it existed :( Had a feeling we'd solved that
>> problem before
>> but couldn't remember the name of the driver.
>> The afe driver already deals with current / voltage scaling and
>> conversion
>> for common analog circuits. Potential dividers, current shunts etc, but
>> they
>> are all the linear cases IIRC.
>> ntc_thermistor deals with the much more complex job of dealing with a
>> thermistor.
> I agree, no need to reinvent this.
> Like Jonathan said, the ntc_thermistor driver seems to handle much more
> complex cases. Where would be the best place to add support for PT100
> and PT1000? iio-rescale?

Those sensors don't seem to be even useful for hardware monitoring, so
if they are linear (and it looks like that that the case) iio would be
a better place.


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