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

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Message-ID: <20250929143132.GA4099970-robh@kernel.org>
Date: Mon, 29 Sep 2025 09:31:32 -0500
From: Rob Herring <robh@...nel.org>
To: Jonathan Cameron <jic23@...nel.org>
Cc: Marcelo Schmitt <marcelo.schmitt@...log.com>, linux-iio@...r.kernel.org,
	devicetree@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-spi@...r.kernel.org, linux-kernel@...r.kernel.org,
	michael.hennerich@...log.com, nuno.sa@...log.com,
	eblanc@...libre.com, dlechner@...libre.com, andy@...nel.org,
	krzk+dt@...nel.org, conor+dt@...nel.org, corbet@....net,
	marcelo.schmitt1@...il.com,
	Linus Walleij <linus.walleij@...aro.org>,
	Bartosz Golaszewski <brgl@...ev.pl>, linux-gpio@...r.kernel.org
Subject: Re: [PATCH v3 7/8] dt-bindings: iio: adc: adi,ad4030: Add ADAQ4216
 and ADAQ4224

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@...log.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@...log.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