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

Jeremy Fitzhardinge
2008-05-24T20:59:25Z

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