linux-kernel - Re: [RFC PATCH] kconfig: introduce KCONFIG

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Date:	Sat, 24 May 2008 21:58:47 +0100
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Sam Ravnborg <sam@...nborg.org>
CC:	linux-kbuild <linux-kbuild@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Roman Zippel <zippel@...ux-m68k.org>,
	Tom Spink <tspink@...il.com>
Subject: Re: [RFC PATCH] kconfig: introduce KCONFIG_* symbols for .c files

Jeremy Fitzhardinge wrote:
> Sam Ravnborg wrote:
>> We have many places in the kernel that looks like
>> the following:
>>
>> #ifdef CONFIG_FOO
>>     ...
>> #endif
>>
>> Which has the disadvantage that the code denoted '...'
>> are not even built if CONFIG_FOO is not selected in
>> the current configuration.
>>
>> We know that gcc do simple code-elimination for
>> conditionals which is always true/false and
>> thus the above code could be turned into:
>>
>>     if (CONFIG_FOO)
>>         ...
>>
>> One line smaller and we follow the normal flow in the program.
>> The code is always build but we do not waste space as gcc will
>> do proper code-elimination for us.
>>
>> Today this is not possible because kconfig will only
>> define CONFIG_FOO if selected and FOO is not a module.
>>
>> The following patch implement a new set of defines in
>> the KCONFIG_* namespace.
>>
>> For a tristate symbol the following are defined:
>>
>> FOO not selected: #define KCONFIG_FOO        0
>> #define KCONFIG_FOO_MODULE 0
>>
>> FOO is built-in ('y')
>> #define KCONFIG_FOO        1
>> #define KCONFIG_FOO_MODULE 0
>>
>> FOO is a module ('m'):
>> #define KCONFIG_FOO        1
>> #define KCONFIG_FOO_MODULE 1
>>
>> In other words KCONFIG_FOO will say if the
>> symbol is selected and KCONFIG_FOO_MODULE
>> will say if it is a module.
>>
>> With the above included we can now do:
>>
>>     if (KCONFIG_FOO)
>>         ...
>>
>> This is not a replacement for the CONFIG_*
>> defines but a pleasant supplement.
>> Using KCONFIG_FOO will also give us a nice
>> error message the day that FOO is no longer part
>> of the configuration.
>>   
>
> How about rather than defining a pile of new constants, we just define:
>
> #define KCONFIG(x)   (x - 0)      /* XXX choose better macro name */

Would

    #define KCONFIG(x)   (CONFIG_##x - 0)

    if (KCONFIG(PREEMPT)) {
        ...
    }

work?

    J
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