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

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Message-ID: <20260127-rockiness-finalize-e0b50b9d823f@spud>
Date: Tue, 27 Jan 2026 19:55:26 +0000
From: Conor Dooley <conor@...nel.org>
To: Oleksij Rempel <o.rempel@...gutronix.de>
Cc: 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>,
	David Jander <david@...tonic.nl>
Subject: Re: [PATCH v2 2/8] dt-bindings: iio: dac: maxim,ds4424: add
 maxim,rfs-ohms property

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?

> > +
> >  required:
> >    - compatible
> >    - reg
> > +  - maxim,rfs-ohms
> > +
> > +allOf:
> > +  - if:
> > +      properties:
> > +        compatible:
> > +          contains:
> > +            enum:
> > +              - maxim,ds4402
> > +              - maxim,ds4422
> > +    then:
> > +      properties:
> > +        maxim,rfs-ohms:
> 
> > +          minItems: 2
> 
> Remove this...
> 
> > +          maxItems: 2
> > +  - if:
> > +      properties:
> > +        compatible:
> > +          contains:
> > +            enum:
> > +              - maxim,ds4404
> > +              - maxim,ds4424
> > +    then:
> > +      properties:
> > +        maxim,rfs-ohms:
> > +          minItems: 4
> 
> 
> > +          maxItems: 4
> 
> ...and this. The outer-most constraints should be in the definition and
> the if/else bit just adjusts whichever bounds it needs to.
> 
> Cheers,
> Conor.
> 
> pw-bot: changes-requested
> 
> >  
> >  additionalProperties: false
> >  
> > @@ -43,6 +78,7 @@ examples:
> >              compatible = "maxim,ds4424";
> >              reg = <0x10>; /* When A0, A1 pins are ground */
> >              vcc-supply = <&vcc_3v3>;
> > +            maxim,rfs-ohms = <5100>, <5100>, <5100>, <5100>;
> >          };
> >      };
> >  ...
> > -- 
> > 2.47.3
> > 



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