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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>
81 lines
2.7 KiB
C
81 lines
2.7 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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#ifndef __LINUX_KCONFIG_H
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#define __LINUX_KCONFIG_H
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/* CONFIG_CC_VERSION_TEXT (Do not delete this comment. See help in Kconfig) */
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#include <generated/autoconf.h>
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#ifdef CONFIG_CPU_BIG_ENDIAN
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#define __BIG_ENDIAN 4321
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#else
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#define __LITTLE_ENDIAN 1234
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#endif
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#define __ARG_PLACEHOLDER_1 0,
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#define __take_second_arg(__ignored, val, ...) val
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/*
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* The use of "&&" / "||" is limited in certain expressions.
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* The following enable to calculate "and" / "or" with macro expansion only.
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*/
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#define __and(x, y) ___and(x, y)
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#define ___and(x, y) ____and(__ARG_PLACEHOLDER_##x, y)
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#define ____and(arg1_or_junk, y) __take_second_arg(arg1_or_junk y, 0)
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#define __or(x, y) ___or(x, y)
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#define ___or(x, y) ____or(__ARG_PLACEHOLDER_##x, y)
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#define ____or(arg1_or_junk, y) __take_second_arg(arg1_or_junk 1, y)
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/*
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* Helper macros to use CONFIG_ options in C/CPP expressions. Note that
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* these only work with boolean and tristate options.
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*/
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/*
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* Getting something that works in C and CPP for an arg that may or may
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* not be defined is tricky. Here, if we have "#define CONFIG_BOOGER 1"
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* we match on the placeholder define, insert the "0," for arg1 and generate
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* the triplet (0, 1, 0). Then the last step cherry picks the 2nd arg (a one).
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* When CONFIG_BOOGER is not defined, we generate a (... 1, 0) pair, and when
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* the last step cherry picks the 2nd arg, we get a zero.
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*/
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#define __is_defined(x) ___is_defined(x)
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#define ___is_defined(val) ____is_defined(__ARG_PLACEHOLDER_##val)
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#define ____is_defined(arg1_or_junk) __take_second_arg(arg1_or_junk 1, 0)
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/*
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* IS_BUILTIN(CONFIG_FOO) evaluates to 1 if CONFIG_FOO is set to 'y', 0
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* otherwise. For boolean options, this is equivalent to
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* IS_ENABLED(CONFIG_FOO).
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*/
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#define IS_BUILTIN(option) __is_defined(option)
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/*
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* IS_MODULE(CONFIG_FOO) evaluates to 1 if CONFIG_FOO is set to 'm', 0
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* otherwise.
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*/
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#define IS_MODULE(option) __is_defined(option##_MODULE)
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/*
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* IS_REACHABLE(CONFIG_FOO) evaluates to 1 if the currently compiled
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* code can call a function defined in code compiled based on CONFIG_FOO.
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* This is similar to IS_ENABLED(), but returns false when invoked from
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* built-in code when CONFIG_FOO is set to 'm'.
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*/
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#define IS_REACHABLE(option) __or(IS_BUILTIN(option), \
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__and(IS_MODULE(option), __is_defined(MODULE)))
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/*
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* IS_ENABLED(CONFIG_FOO) evaluates to 1 if CONFIG_FOO is set to 'y' or 'm',
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* 0 otherwise.
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*/
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#define IS_ENABLED(option) __or(IS_BUILTIN(option), IS_MODULE(option))
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/*
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* IF_ENABLED(CONFIG_FOO, ptr) evaluates to (ptr) if CONFIG_FOO is set to 'y'
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* or 'm', NULL otherwise.
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*/
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#define IF_ENABLED(option, ptr) (IS_ENABLED(option) ? (ptr) : NULL)
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#endif /* __LINUX_KCONFIG_H */
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