linux-kernel - Re: [PATCH v1 2/8] dt-bindings: iio: dac: maxim,ds4424: add maxim,rfs-ohms property

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Message-ID: <aXc50xgca4-BDYVq@pengutronix.de>
Date: Mon, 26 Jan 2026 10:54:27 +0100
From: Oleksij Rempel <o.rempel@...gutronix.de>
To: Jonathan Cameron <jic23@...nel.org>
Cc: Andy Shevchenko <andy@...nel.org>, Rob Herring <robh@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>, devicetree@...r.kernel.org,
	linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org,
	Conor Dooley <conor@...nel.org>,
	Nuno Sá <nuno.sa@...log.com>, kernel@...gutronix.de,
	David Jander <david@...tonic.nl>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	David Lechner <dlechner@...libre.com>
Subject: Re: [PATCH v1 2/8] dt-bindings: iio: dac: maxim,ds4424: add
 maxim,rfs-ohms property

On Sun, Jan 25, 2026 at 10:22:07AM +0000, Jonathan Cameron wrote:
> On Fri, 23 Jan 2026 11:03:38 +0100
> Oleksij Rempel <o.rempel@...gutronix.de> wrote:
> 
> > On Fri, Jan 23, 2026 at 09:28:19AM +0000, Jonathan Cameron wrote:
> > > On Mon, 19 Jan 2026 18:27:52 +0000
> > > Conor Dooley <conor@...nel.org> wrote:
> > >   
> > > > On Mon, Jan 19, 2026 at 07:24:18PM +0100, Oleksij Rempel wrote:  
> > > > > Introduce the 'maxim,rfs-ohms' property. The full-scale output current
> > > > > of these DACs is determined by external resistors (Rfs) connected to
> > > > > the FS pins. The driver requires the physical resistance values to
> > > > > calculate the correct current scale (Amps per step) for the IIO
> > > > > subsystem.
> > > > > 
> > > > > Keep it optional to avoid forcing updates of existing DTs; without it
> > > > > the driver cannot derive a correct IIO scale.    
> > > > 
> > > > I don't really follow the logic here, if the driver doesn't work
> > > > properly without it, shouldn't it be a required property even if that
> > > > means existing devicetrees get new warnings? Warnings are preferable to
> > > > the drivers malfunctioning on those devices, after all!  
> > > 
> > > Agreed. The driver can paper over holes or I guess we could provide a default
> > > if the datasheet has some reference value or similar?    
> > 
> > No, this DAC controls the current. The current range is application specific and
> > defined by the load resistors. Is it better to not provide scale if this
> > information not available, guessing it will make things worse and
> > potentially damage some HW.
> > 
> Then does this need to be a fix on the driver side?
> Or are you just suggesting not proving the scale if we don't have enough info?
> That's fine if so.

I suggest to not provide scale if we don't have enough info. It will be
disabled in the driver if maxim,rfs-ohms is not provided in the DT.

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