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Date: Sat, 24 May 2008 21:48:11 +0100
From: Jeremy Fitzhardinge <jeremy@goop.org>
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Subject: Re: [RFC PATCH] kconfig: introduce KCONFIG_* symbols for .c files
References: <20080524192540.GA28067@uranus.ravnborg.org>
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To: Sam Ravnborg <sam@ravnborg.org>
Cc: linux-kbuild <linux-kbuild@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>, Linus Torvalds <torvalds@linux-foundation.org>, Andrew Morton <akpm@linux-foundation.org>, Roman Zippel <zippel@linux-m68k.org>, Tom Spink <tspink@gmail.com>

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 */

That would allow CONFIG_X variables to be used in C expressions, while 
still coping with non-existent/unknown CONFIG vars.  Also saves on a lot 
of #defines...

    J