kconfig.h: Add IF_ENABLED() macro

kconfig.h: Add IF_ENABLED() macro

IF_ENABLED(CONFIG_FOO, ptr) evaluates to (ptr) if CONFIG_FOO is set to 'y'
or 'm', NULL otherwise. The (ptr) argument must be a pointer.

The IF_ENABLED() macro can be very useful to help GCC drop dead code.

For instance, consider the following:

    #ifdef CONFIG_FOO_SUSPEND
    static int foo_suspend(struct device *dev)
    {
       ...
    }
    #endif

    static struct pm_ops foo_ops = {
	#ifdef CONFIG_FOO_SUSPEND
        .suspend = foo_suspend,
	#endif
    };

While this works, the foo_suspend() macro is compiled conditionally,
only when CONFIG_FOO_SUSPEND is set. This is problematic, as there could
be a build bug in this function, we wouldn't have a way to know unless
the config option is set.

An alternative is to declare foo_suspend() always, but mark it as maybe
unused:

    static int __maybe_unused foo_suspend(struct device *dev)
    {
       ...
    }

    static struct pm_ops foo_ops = {
	#ifdef CONFIG_FOO_SUSPEND
        .suspend = foo_suspend,
	#endif
    };

Again, this works, but the __maybe_unused attribute is required to
instruct the compiler that the function may not be referenced anywhere,
and is safe to remove without making a fuss about it. This makes the
programmer responsible for tagging the functions that can be
garbage-collected.

With this patch, it is now possible to write the following:

    static int foo_suspend(struct device *dev)
    {
       ...
    }

    static struct pm_ops foo_ops = {
        .suspend = IF_ENABLED(CONFIG_FOO_SUSPEND, foo_suspend),
    };

The foo_suspend() function will now be automatically dropped by the
compiler, and it does not require any specific attribute.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20201213235447.138271-1-paul@crapouillou.net
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>