linux-kernel - Re: [PATCH] thermal: Fix KELVIN_TO

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Message-ID: <532FC307.7050202@roeck-us.net>
Date:	Sun, 23 Mar 2014 22:30:47 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Joe Perches <joe@...ches.com>
CC:	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Zhang Rui <rui.zhang@...el.com>,
	Eduardo Valentin <eduardo.valentin@...com>
Subject: Re: [PATCH] thermal: Fix KELVIN_TO_CELSIUS macro

On 03/23/2014 09:25 PM, Joe Perches wrote:
> On Sun, 2014-03-23 at 21:14 -0700, Guenter Roeck wrote:
>> It is always a good idea to use paranthesis around macro parameters
>> to avoid undesired side effects.
>>
>> In this specific case, KELVIN_TO_CELSIUS() is used in
>> drivers/platform/x86/asus-wmi.c with parameter "value & 0xFFFF",
>> which due to operator evaluation order causes more or less random
>> results.
>
> Maybe it's better to use a statement expression to avoid
> multiple calculations of t
>
> Maybe;
>
> #define KELVIN_TO_CELSIUS(t)				\
> ({							\
> 	long _t = (long)(t) - 2732;			\
> 	_t >= 0 ? (_t + 5) / 10 : (_t - 5) / 10;	\
> })
>
>

Possibly, but I only want to have a bug fixed which should possibly be applied
to -stable at some point. As such, I prefer the minimalistic approach.
Anything more fancy on top of that can be submitted as separate patch.

Guenter

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