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Date: Sat, 24 May 2008 12:53:16 -0700
From: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [RFC PATCH] kconfig: introduce KCONFIG_* symbols for .c files
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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>, Roman Zippel <zippel@linux-m68k.org>, Jeremy Fitzhardinge <jeremy@goop.org>

On Sat, 24 May 2008 21:25:40 +0200 Sam Ravnborg <sam@ravnborg.org> 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.

It could help to get us out of the occasional sticky situation, but it
does seem a bit risky.  What happens with Kconfig variables which are
just not known about at all with some .configs?

Silly example, one could add

	if (KCONFIG_DVB_VES1820)

to kernel/sched.c and that would work happily until someone sets DVB=n,
in which case I assume KCONFIG_DVB_VES1820 doesn't get defined
anywhere?

A more realistic example might be using an x86-only KCONFIG_* in non-x86
code.