Re: [RFC PATCH] kconfig: introduce KCONFIG_* symbols for .c files

[Jeremy Fitzhardinge]

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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