Re: [PATCH v3 7/8] dt-bindings: iio: adc: adi,ad4030: Add ADAQ4216 and ADAQ4224

Rob Herring
2025-09-29T14:31:33Z

On Sun, Sep 28, 2025 at 11:19:55AM +0100, Jonathan Cameron wrote:
> On Fri, 26 Sep 2025 17:40:47 -0300
> Marcelo Schmitt <marcelo.schmitt@analog.com> wrote:
>
> > ADAQ4216 and ADAQ4224 are similar to AD4030 except that ADAQ devices have a
> > PGA (programmable gain amplifier) that scales the input signal prior to it
> > reaching the ADC inputs. The PGA is controlled through a couple of pins (A0
> > and A1) that set one of four possible signal gain configurations.
> >
> > Signed-off-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
> > ---
> > Change log v2 -> v3
> > - PGA gain now described in decibels.
> >
> > The PGA gain is not going to fit well as a channel property because it may
> > affect more than one channel as in AD7191.
> > https://www.analog.com/media/en/technical-documentation/data-sheets/AD7191.pdf
> >
> > I consulted a very trustworthy source [1, 2] and learned that describing signal
> > gains in decibels is a common practice. I now think it would be ideal to describe
> > these PGA and PGA-like gains with properties in decibel units and this patch
> > is an attempt of doing so. The only problem with this approach is that we end up
> > with negative values when the gain is lower than 1 (the signal is attenuated)
> > and device tree specification doesn't support signed integer types. As the
> > docs being proposed fail dt_binding_check, I guess I have to nack the patch myself.
> > Any chance of dt specification eventually support signed integers?
> > Any suggestions appreciated.
> >
> > [1] https://en.wikipedia.org/wiki/Decibel
> > [2] https://en.wikipedia.org/wiki/Gain_(electronics)
>
> I still wonder if the better way to describe this is to ignore that it
> has anything to do with PGA as such and instead describe the pin strapping.
>
> DT folk, is there an existing way to do that? My grep skills are failing to
> spot one.
>
> We've papered over this for a long time in various IIO drivers by controlling
> directly what the pin strap controls with weird and wonderful device specific
> bindings. I wonder if we can't have a gpio driver + binding that rejects all
> config and just lets us check the current state of an output pin. Kind of a
> fixed mode regulator equivalent for gpios.

If these are connected to GPIOs, isn't it possible that someone will
want to change their value?

Other than some generic 'pinstrap-gpios' property, I don't see what we'd
do here? I don't feel like pin strapping GPIOs is something that we see
all that often.

Rob