static_call: Provide DEFINE_STATIC_CALL_RET0()

DECLARE_STATIC_CALL() must pass the original function targeted for a
given static call. But DEFINE_STATIC_CALL() may want to initialize it as
off. In this case we can't pass NULL (for functions without return value)
or __static_call_return0 (for functions returning a value) directly
to DEFINE_STATIC_CALL() as that may trigger a static call redeclaration
with a different function prototype. Type casts neither can work around
that as they don't get along with typeof().

The proper way to do that for functions that don't return a value is
to use DEFINE_STATIC_CALL_NULL(). But functions returning a actual value
don't have an equivalent yet.

Provide DEFINE_STATIC_CALL_RET0() to solve this situation.
Signed-off-by: NFrederic Weisbecker <frederic@kernel.org>
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: NIngo Molnar <mingo@kernel.org>
Link: https://lkml.kernel.org/r/20210118141223.123667-3-frederic@kernel.org

include/linux/static_call.h

@@ -144,13 +144,13 @@ extern int static_call_text_reserved(void *start, void *end);
 
 extern long __static_call_return0(void);
 
-#define DEFINE_STATIC_CALL(name, _func)					\
+#define __DEFINE_STATIC_CALL(name, _func, _func_init)			\
 	DECLARE_STATIC_CALL(name, _func);				\
 	struct static_call_key STATIC_CALL_KEY(name) = {		\
-		.func = _func,						\
+		.func = _func_init,					\
 		.type = 1,						\
 	};								\
-	ARCH_DEFINE_STATIC_CALL_TRAMP(name, _func)
+	ARCH_DEFINE_STATIC_CALL_TRAMP(name, _func_init)
 
 #define DEFINE_STATIC_CALL_NULL(name, _func)				\
 	DECLARE_STATIC_CALL(name, _func);				\
@@ -178,12 +178,12 @@ struct static_call_key {
 	void *func;
 };
 
-#define DEFINE_STATIC_CALL(name, _func)					\
+#define __DEFINE_STATIC_CALL(name, _func, _func_init)			\
 	DECLARE_STATIC_CALL(name, _func);				\
 	struct static_call_key STATIC_CALL_KEY(name) = {		\
-		.func = _func,						\
+		.func = _func_init,					\
 	};								\
-	ARCH_DEFINE_STATIC_CALL_TRAMP(name, _func)
+	ARCH_DEFINE_STATIC_CALL_TRAMP(name, _func_init)
 
 #define DEFINE_STATIC_CALL_NULL(name, _func)				\
 	DECLARE_STATIC_CALL(name, _func);				\
@@ -234,10 +234,10 @@ static inline long __static_call_return0(void)
 	return 0;
 }
 
-#define DEFINE_STATIC_CALL(name, _func)					\
+#define __DEFINE_STATIC_CALL(name, _func, _func_init)			\
 	DECLARE_STATIC_CALL(name, _func);				\
 	struct static_call_key STATIC_CALL_KEY(name) = {		\
-		.func = _func,						\
+		.func = _func_init,					\
 	}
 
 #define DEFINE_STATIC_CALL_NULL(name, _func)				\
@@ -286,4 +286,10 @@ static inline int static_call_text_reserved(void *start, void *end)
 
 #endif /* CONFIG_HAVE_STATIC_CALL */
 
+#define DEFINE_STATIC_CALL(name, _func)					\
+	__DEFINE_STATIC_CALL(name, _func, _func)
+
+#define DEFINE_STATIC_CALL_RET0(name, _func)				\
+	__DEFINE_STATIC_CALL(name, _func, __static_call_return0)
+
 #endif /* _LINUX_STATIC_CALL_H */