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

[Guenter Roeck]

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@xxxxxxxxx> 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