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

David Jander
2026-01-28T08:02:29Z

On Tue, 27 Jan 2026 19:55:26 +0000
Conor Dooley <conor@kernel.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@pengutronix.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:
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.

> > > +
> > > 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
> > >
>
>

--
David Jander