linux-kernel - Re: [PATCH v3 3/3] Documentation: ABI: adl8113: add documentation

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Message-ID: <62109f602caefd06a23bbd96e05c327647d054e9.camel@gmail.com>
Date: Tue, 18 Nov 2025 08:08:44 +0000
From: Nuno Sá <noname.nuno@...il.com>
To: David Lechner <dlechner@...libre.com>, Jonathan Cameron
 <jic23@...nel.org>,  Antoniu Miclaus <antoniu.miclaus@...log.com>
Cc: robh@...nel.org, conor+dt@...nel.org, linux-iio@...r.kernel.org, 
	linux-kernel@...r.kernel.org, devicetree@...r.kernel.org, Nuno
 Sá
	 <nuno.sa@...log.com>, Andy Shevchenko <andy@...nel.org>
Subject: Re: [PATCH v3 3/3] Documentation: ABI: adl8113: add documentation

On Mon, 2025-11-17 at 17:59 -0600, David Lechner wrote:
> On 11/16/25 9:50 AM, Jonathan Cameron wrote:
> > On Fri, 14 Nov 2025 11:57:25 +0000
> > Antoniu Miclaus <antoniu.miclaus@...log.com> wrote:
> > 
> > > Add ABI documentation for the ADL8113 Low Noise Amplifier,
> > > covering the 4 pin-selectable operating modes.
> > > 
> > > Signed-off-by: Antoniu Miclaus <antoniu.miclaus@...log.com>
> > +CC all the IIO subsystem reviewers as I'm not sure how to handle
> > what this device does, and would like some more opinions.
> > 
> > Anyone who has expertise or design experience with this sort
> > of amplifier with external bypass path: how are these actually used?
> > 
> > The development board seems to just be suited to measurement of
> > device characteristics, not representative of a design using it.
> > 
> > > ---
> > > 
> > > Changes in v3:
> > > - Replace "mode" attribute documentation with "signal_path" attribute
> > > - Update descriptions to reflect separation of gain control (hardwaregain) and
> > > path control (signal_path)
> > > - Add explanation of automatic bypass mode switching when selecting external
> > > paths
> > > - Update attribute names from mode/mode_available to
> > > signal_path/signal_path_available
> > > - Clarify that external paths enable connection of external components for
> > > filtering or amplification
> > > ---
> > >  .../testing/sysfs-bus-iio-amplifiers-adl8113  | 30 +++++++++++++++++++
> > >  1 file changed, 30 insertions(+)
> > >  create mode 100644 Documentation/ABI/testing/sysfs-bus-iio-amplifiers-adl8113
> > > 
> > > diff --git a/Documentation/ABI/testing/sysfs-bus-iio-amplifiers-adl8113
> > > b/Documentation/ABI/testing/sysfs-bus-iio-amplifiers-adl8113
> > > new file mode 100644
> > > index 000000000000..31fb1e6e80fc
> > > --- /dev/null
> > > +++ b/Documentation/ABI/testing/sysfs-bus-iio-amplifiers-adl8113
> > > @@ -0,0 +1,30 @@
> > > +What:		/sys/bus/iio/devices/iio:deviceX/signal_path
> > > +Date:		January 2025
> > > +KernelVersion:	6.14
> > > +Contact:	linux-iio@...r.kernel.org
> > > +Description:
> > > +		This attribute controls the signal routing path for the
> > > ADL8113
> > > +		Low Noise Amplifier. The supported signal paths are:
> > > +
> > > +		* internal - Signal passes through internal path. Can be used
> > > with
> > > +		  either amplifier mode (14dB gain) or bypass mode (0dB gain).
> > > +
> > > +		* external_a - Signal routes from RFIN to OUT_A and from IN_A
> > > to RFOUT
> > > +		  (VA=0, VB=1). External components can be connected for
> > > filtering or
> > > +		  additional amplification. Automatically switches to bypass
> > > mode (0dB).
> > > +
> > > +		* external_b - Signal routes from RFIN to OUT_B and from IN_B
> > > to RFOUT
> > > +		  (VA=1, VB=0). External components can be connected for
> > > filtering or
> > > +		  additional amplification. Automatically switches to bypass
> > > mode (0dB).
> > 
> > I'm still not sure how to handle these external paths.  The one thing they won't
> > be
> > is truly zero gain (because that would be pointless given the internal bypass
> > path does that).
> > 
> > So how are these used in practice?
> 
> Afraid I don't have any experience with this. But I would assume that the two
> external
> bypasses each have another amplifier (or attenuator) connected to them.
> 

Unfortunately me neither. Antoniu, it would make sense to figure out who the APPs
engineer for this part is. He is the one who can answer how the bypass feature is
supposed to be used in practise.

> > 
> > Do we need to figure out how to describe the real characteristics of those paths?
> 
> It sounds like this is just a multiplexer. So a few ideas:
> 
> 1. We could handle this like iio-mux.c and have 4 channels and just restrict use
>    to one channel at a time. Not exactly sure how we would do that. Perhaps an
>    enable attribute that is interlocked so that it will fail to enable if another
>    channel is already enabled? In this case, the devicetree would need to have an
>    io-channels property to get the two bypass amplifier channels and basically pass
>    them through.
> 
> 2. Have the devicetree specify the gain of the two external bypass amplifiers
>    so that we can use the hardwaregain attribute to select between the 4 mux
>    positions without needing a custom attribute. This works as long as none
>    of the amplifiers have the same gain.
> 
> I'm envisioning this would be e.g. part of a range selection on some sort of
> test equipment. In that case, option 2 seems the right way to go about it.

This makes sense but I would still try to have some confirmation about this.

- Nuno Sá
>