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

[Jonathan Cameron <jic23@kernel.org>]

On Sun, 16 May 2021 08:54:06 -0700
Guenter Roeck <linux@roeck-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@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.
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.

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  
> >   
>