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

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Message-ID: <aW6AEszfRQzuHf6j@smile.fi.intel.com>
Date: Mon, 19 Jan 2026 21:03:46 +0200
From: Andy Shevchenko <andriy.shevchenko@...el.com>
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>, 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 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.

-- 
With Best Regards,
Andy Shevchenko