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

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Message-Id: <CBEREZMZ2Z8U.13BH8G7RKPPL7@shaak>
Date:   Sun, 16 May 2021 11:02:52 -0400
From:   "Liam Beguin" <liambeguin@...il.com>
To:     "Jonathan Cameron" <jic23@...nel.org>
Cc:     <jdelvare@...e.com>, <linux@...ck-us.net>, <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

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!

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