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

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Message-ID: <d99331c5-b3d6-5e87-3a3d-8cf2817dea11@roeck-us.net>
Date:   Sun, 16 May 2021 16:10:56 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Liam Beguin <liambeguin@...il.com>,
        Jonathan Cameron <jic23@...nel.org>
Cc:     jdelvare@...e.com, lars@...afoo.de, pmeerw@...erw.net,
        linux-hwmon@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-iio@...r.kernel.org, devicetree@...r.kernel.org,
        robh+dt@...nel.org, Peter Rosin <peda@...ntia.se>
Subject: Re: [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 <linux@...ck-us.net> 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 <liambeguin@...il.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.
> 
>> 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.

Guenter