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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>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.