linux-kernel - Re: [PATCH v1 4/8] iio: dac: ds4424: reject -128 RAW value

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Message-ID: <aXc5agTyH4XmzKUi@pengutronix.de>
Date: Mon, 26 Jan 2026 10:52:42 +0100
From: Oleksij Rempel <o.rempel@...gutronix.de>
To: Jonathan Cameron <jic23@...nel.org>
Cc: Andy Shevchenko <andriy.shevchenko@...el.com>,
	Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>, stable@...r.kernel.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 v1 4/8] iio: dac: ds4424: reject -128 RAW value

On Fri, Jan 23, 2026 at 09:33:57AM +0000, Jonathan Cameron wrote:
> On Mon, 19 Jan 2026 21:03:46 +0200
> Andy Shevchenko <andriy.shevchenko@...el.com> wrote:
> 
> > On Mon, Jan 19, 2026 at 07:24:20PM +0100, Oleksij Rempel wrote:
> > > The DS442x DAC uses sign-magnitude encoding, so -128 cannot be
> > > represented in hardware.
> > > 
> > > With the previous check, userspace could pass -128, which gets converted
> > > to a magnitude of 128 and then truncated by the 7-bit DAC field. This
> > > ends up programming a zero magnitude with the sign bit set, i.e. an
> > > unintended output (effectively 0 mA instead of -128 steps).
> > > 
> > > Reject -128 to avoid silently producing the wrong current.  
> > 
> > ...
> > 
> > > -		if (val < S8_MIN || val > S8_MAX)
> > > +		if (val <= S8_MIN || val > S8_MAX)
> > >  			return -EINVAL;  
> > 
> > Hmm... So the range is [ -127 .. 0 .. 127 ] ?
> > 
> > I think in such case the plain numbers would be more specific than
> > the type related limits.
> > 
> 
> Check the abs(val) <= 127 given that's what we care about I think?
> Or make it explicit and do
> FIELD_FIT() against a mask that you then use to fill the register
> value (another mask for the sign bit).
> 
> Btw use abs(val) to set raw.dx and drop it out of the conditional.
> Even better get rid of the bitfield stuff and just add
> two defines + fill val directly in this function using FIELD_PREP().
> Then both the checking and the field filling use the same defines
> and it should be easy to see what is going on.

FIELD_* macros require compile-time constant masks. Since the next patch
adds support for variants with different data widths (making the mask a
runtime variable), I prefer using an implementation now that remains
consistent with the followup changes.

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