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

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

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