linux-kernel - Re: [PATCH v3 5/5] iio: adc: ad7192: Add AD7194 support

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Date: Mon, 19 Feb 2024 10:10:15 -0600
From: David Lechner <dlechner@...libre.com>
To: Jonathan Cameron <jic23@...nel.org>
Cc: Alisa-Dariana Roman <alisadariana@...il.com>, alexandru.tachici@...log.com, 
	alisa.roman@...log.com, conor+dt@...nel.org, devicetree@...r.kernel.org, 
	krzysztof.kozlowski+dt@...aro.org, krzysztof.kozlowski@...aro.org, 
	lars@...afoo.de, linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org, 
	michael.hennerich@...log.com, robh+dt@...nel.org, 
	Nuno Sa <nuno.sa@...log.com>
Subject: Re: [PATCH v3 5/5] iio: adc: ad7192: Add AD7194 support

On Sat, Feb 17, 2024 at 10:25 AM Jonathan Cameron <jic23@...nel.org> wrote:
>
> On Fri, 16 Feb 2024 10:57:33 -0600
> David Lechner <dlechner@...libre.com> wrote:
>
> > On Fri, Feb 16, 2024 at 8:22 AM Jonathan Cameron <jic23@...nel.org> wrote:
> > >
> > > On Thu, 15 Feb 2024 11:13:19 -0600
> > > David Lechner <dlechner@...libre.com> wrote:
> > >
> >
> > ...
> >
> > > >
> > > > Tables 22, 23 and 24 in the AD7194 datasheet show that this chip is
> > > > much more configurable than AD7192 when it comes to assigning
> > > > channels. There are basically no restrictions on which inputs can be
> > > > used together. So I am still confident that my suggestion is the way
> > > > to go for AD7194. (Although I didn't actually try it on hardware, so
> > > > can't be 100% confident. But at least 90% confident :-p)
> > >
> > > You would have to define a channel number for aincom.  There is an explicit
> > > example in the datasheet of it being at 2.5V using a reference supply.
> > >
> > > I wonder what expectation here is.  Allways a reference regulator on that pin, or
> > > an actually varying input? Maybe in long term we want to support both
> > > options - so if aincom-supply is provided these are single ended with
> > > an offset, but if not they are differential channels between channel X and
> > > channel AINCOM.
> > >
> > > Note though that this mode is described a pseudo differential which normally
> > > means a fixed voltage on the negative.
> > >
> > > So gut feeling from me is treat them as single ended and add an
> > > aincom-supply + the offsets that result if that is provided in DT and
> > > voltage from it is non 0.
> >
> > Calling AINCOM a supply doesn't sound right to me since usually this
> > signal is coming somewhere external, i.e. you have a twisted pair
> > connected to AIN1 and AINCOM going to some signal source that may be
> > hot-pluggable and not known at compile time. As an example, if AINCOM
> > was modeled as a supply, then we would have to change the device tree
> > every time we changed the voltage offset on the signal generator while
> > we are testing using an evaluation board.
>
> We tend to stick away from designing features to support testing with
> devboards where external wiring is involved because anything could be
> wired up there. (Examples are things like shunt resistors - normally
> they are DT only) So sometimes it's a bit painful to work with such boards.
> The main focus has to be production devices or at least stable set ups
> where a fixed DT is sufficient.
>
> So I'm more interested in focusing on production device use cases.
> Do we have an information on how this is this used in those environments?
>

Point taken. I also checked with an apps engineer at ADI and it does
sound like AINCOM should be a supply.