linux-kernel - Re: [PATCH v2 2/8] dt-bindings: iio: dac: maxim,ds4424: add maxim,rfs-ohms property

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Message-ID: <20260128-omit-bubbling-9224777f4071@spud>
Date: Wed, 28 Jan 2026 17:00:10 +0000
From: Conor Dooley <conor@...nel.org>
To: David Jander <david@...tonic.nl>
Cc: Oleksij Rempel <o.rempel@...gutronix.de>,
	Jonathan Cameron <jic23@...nel.org>, Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>, kernel@...gutronix.de,
	linux-kernel@...r.kernel.org, linux-iio@...r.kernel.org,
	devicetree@...r.kernel.org, Andy Shevchenko <andy@...nel.org>,
	David Lechner <dlechner@...libre.com>,
	Nuno Sá <nuno.sa@...log.com>
Subject: Re: [PATCH v2 2/8] dt-bindings: iio: dac: maxim,ds4424: add
 maxim,rfs-ohms property

On Wed, Jan 28, 2026 at 09:01:18AM +0100, David Jander wrote:
> On Tue, 27 Jan 2026 19:55:26 +0000
> Conor Dooley <conor@...nel.org> wrote:
> 
> > On Tue, Jan 27, 2026 at 07:49:20PM +0000, Conor Dooley wrote:
> > > On Tue, Jan 27, 2026 at 07:09:33AM +0100, Oleksij Rempel wrote:  
> > > > The Maxim DS4422/DS4424 and DS4402/DS4404 current DACs determine their
> > > > full-scale output current via external resistors (Rfs) connected to the
> > > > FSx pins. Without knowing these values, the full-scale range of the
> > > > hardware is undefined.
> > > > 
> > > > Add the 'maxim,rfs-ohms' property to describe these physical components.
> > > > This property is required to provide a complete description of the
> > > > hardware configuration.
> > > > 
> > > > Signed-off-by: Oleksij Rempel <o.rempel@...gutronix.de>
> > > > ---
> > > > changes v2:
> > > > - make maxim,rfs-ohms a required property as the hardware range is undefined
> > > >   without external resistors.
> > > > - add allOf constraints to enforce 2 vs 4 items in maxim,rfs-ohms based on
> > > >   compatible string.
> > > > - drop explicit $ref for maxim,rfs-ohms to fix dt_binding_check warning.
> > > > - update example in binding to include the new required property.
> > > > ---
> > > >  .../bindings/iio/dac/maxim,ds4424.yaml        | 36 +++++++++++++++++++
> > > >  1 file changed, 36 insertions(+)
> > > > 
> > > > diff --git a/Documentation/devicetree/bindings/iio/dac/maxim,ds4424.yaml b/Documentation/devicetree/bindings/iio/dac/maxim,ds4424.yaml
> > > > index efe63e6cb55d..400afd8771aa 100644
> > > > --- a/Documentation/devicetree/bindings/iio/dac/maxim,ds4424.yaml
> > > > +++ b/Documentation/devicetree/bindings/iio/dac/maxim,ds4424.yaml
> > > > @@ -27,9 +27,44 @@ properties:
> > > >  
> > > >    vcc-supply: true
> > > >  
> > > > +  maxim,rfs-ohms:
> > > > +    description: |
> > > > +      Array of resistance values in Ohms for the external Rfs resistors
> > > > +      connected to the FS pins.
> > > > +      - For DS44x2 (2 channels): 2 values required.
> > > > +      - For DS44x4 (4 channels): 4 values required.
> > > > +      Typical values range from 40000 (40 kOhm) to 160000 (160 kOhm).  
> > > 
> > > Add here
> > > maxItems: 4
> > > minItems: 2
> > > 
> > > Are 40kOhm and 160kOhm actual max/min values, or just a suggested range?  
> > 
> > Datasheet for the ds4424 seems to imply they're actual max/min values:
> > "Input resistors (RFS) must be between the speciifed values to ensure the
> > device meets its accuracy and linearity specifications."
> > In that case, consider also adding something like:
> >     items:
> >       maximum: 40000
> >       minimum: 160000
> > 
> > Although, that would complain about the 5100 Ohms you're using. How come
> > the example lies outside of the "typical" range?
> 
> Sorry to chime in here out of nowhere with this, but 2 things:

No no, don't apologise - that's good info and makes sense. Thanks.

> 1. Rfs for DS4402/4 has a different "typical" range than DS4424 (different
> Vref and different output current range).
> 2. "Typical" or "recommended" ranges should not translate to a hard limit in
> the driver. IMHO, no max or min value should be enforced here.

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