linux-kernel - Re: [PATCH 1/2] drivers/hwmon/ad7314.c: Do proper sign extension

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Message-ID: <54C184B0.7060606@roeck-us.net>
Date:	Thu, 22 Jan 2015 15:16:00 -0800
From:	Guenter Roeck <linux@...ck-us.net>
To:	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Jean Delvare <jdelvare@...e.de>
CC:	lm-sensors@...sensors.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] drivers/hwmon/ad7314.c: Do proper sign extension

On 01/22/2015 02:44 PM, Rasmus Villemoes wrote:
> The comment above (data << 2) >> 2 explains what the intention is: To
> use bit 13 of the 14-bit value data as the sign bit. However, this
> doesn't work due to C's promotion rules. data has type s16, but data
> << 2 has type int. To get sign extension, that expression would have
> to be cast back to an s16 before being shifted (at which point C's
> promotion rules would then kick in again and promote the left operand
> to int). As it stands, both expressions are no-ops for any value of
> data.
>
> Avoid these subtleties by using the existing API for
> this. sign_extend32 works equally well for 8 and 16 bit types.
>
> Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
> ---

Good catch. Applied.

Thanks,
Guenter

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