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

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Message-Id: <CBEVHCPHGJJD.2IDUUG1RPDF64@shaak>
Date:   Sun, 16 May 2021 14:14:02 -0400
From:   "Liam Beguin" <liambeguin@...il.com>
To:     "Jonathan Cameron" <jic23@...nel.org>,
        "Guenter Roeck" <linux@...ck-us.net>
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 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?

Thanks,
Liam

>
> Thanks,
>
> Jonathan
>
> > 
> > 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  
> > >   
> >