[PATCH 1/3] kconfig.h: abstract empty functions with STUB_UNLESS macro

[Andrew Murray <andrew.murray@arm.com>]

A common pattern found in header files is a function declaration dependent
on a CONFIG_ option being enabled, followed by an empty function for when
that option isn't enabled. This can often take up a lot of space and impact
code readability.

Let's introduce the following STUB_UNLESS macro:

STUB_UNLESS(CONFIG_FOO, [body], prototype)

This evaluates to 'prototype' prepended with 'extern' if CONFIG_FOO is set
to 'y'. Otherwise it will evaluate to 'prototype' prepended with 'static
inline' followed by an empty function body. Where optional argument 'body'
then 'body' will be used as the function body, intended to allow for simple
return statements.

Signed-off-by: Andrew Murray <andrew.murray@arm.com>
---
 include/linux/kconfig.h | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/include/linux/kconfig.h b/include/linux/kconfig.h
index cc8fa10..216a27f 100644
--- a/include/linux/kconfig.h
+++ b/include/linux/kconfig.h
@@ -12,6 +12,7 @@
 
 #define __ARG_PLACEHOLDER_1 0,
 #define __take_second_arg(__ignored, val, ...) val
+#define __take_fourth_arg(__ignored, __ignored2, __ignored3, val, ...) val
 
 /*
  * The use of "&&" / "||" is limited in certain expressions.
@@ -70,4 +71,34 @@
  */
 #define IS_ENABLED(option) __or(IS_BUILTIN(option), IS_MODULE(option))
 
+
+
+/*
+ * Allow for __stub to be overloaded by making the second argument optional
+ */
+#define __stub_overload(...)		__take_fourth_arg(__VA_ARGS__, \
+					__stub_body, __stub_void)(__VA_ARGS__)
+#define __stub_body(option, body, ptype) __stub(option, body, ptype)
+#define __stub_void(option, ptype)	__stub(option, , ptype)
+
+/*
+ * Using the same trick as __defined we use the config option to conditionally
+ * expand to an extern function declaration or a static stub function.
+ */
+#define __stub(option, body, ptype)	___stub( \
+					__ARG_PLACEHOLDER_##option, body, ptype)
+#define ___stub(arg1_or_junk, body, ptype) __take_second_arg( \
+		arg1_or_junk __extern_ptype(ptype), __static_ptype(body, ptype))
+#define __static_ptype(body, ptype)	static inline ptype { body; }
+#define __extern_ptype(ptype)		extern ptype
+
+/*
+ * STUB_UNLESS(CONFIG_FOO, [body], prototype) evaluates to 'prototype' prepended
+ * with 'extern' if CONFIG_FOO is set to 'y'. Otherwise it will evaluate to
+ * 'prototype' prepended with 'static inline' followed by an empty function
+ * body. Where optional argument 'body' is present then 'body' will be used
+ * as the function body, intended to allow for simple return statements.
+ */
+#define STUB_UNLESS(...) __stub_overload(__VA_ARGS__)
+
 #endif /* __LINUX_KCONFIG_H */
-- 
2.7.4