typesafe_cb: wrappers for typesafe callbacks.

cast_if_type is clever but annoying to use, so we make some slightly
less annoying wrappers for callback registration.

These allow const pointer-taking functions as well as the normal
non-const ones.  We also have a typesafe_cb_preargs() for callbacks
which don't just take one argument.

Here's an example of use:

BEFORE:
	void foo_set_callback(int (*callback)(void *), void *arg);

AFTER:
	#define foo_set_callback(cb, arg) \
		__foo_set_callback(typesafe_cb(int, (cb), (arg)), (arg))
	void __foo_set_callback(int (*callback)(void *), void *arg);

o If cb is of form "int cb(typeof(arg))" or "int cb(const typeof(arg))" it
  will be cast to form "int cb(void *)".
o Otherwise, if cb is not already of form "int cb(void *)" it will cause a
  warning when passed to __foo_set_callback().
o If arg is not a pointer, then handing it to __foo_set_callback() will cause
  a warning.

Note: there are at least two possible additions we don't yet need.  We
don't cover callbacks which take a volatile pointer, even though that
would be fine, and we don't have a typesafe_cb_postargs().

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
---
 include/linux/kernel.h |   35 +++++++++++++++++++++++++++++++++++
 1 file changed, 35 insertions(+)

diff -r 907c55be656f include/linux/kernel.h
--- a/include/linux/kernel.h	Mon Apr 21 06:58:00 2008 +1000
+++ b/include/linux/kernel.h	Mon Apr 21 07:04:02 2008 +1000
@@ -436,4 +436,39 @@ struct sysinfo {
 #define NUMA_BUILD 0
 #endif
 
+/* If fn is of type ok1 or ok2, cast to desttype */
+#define __typesafe_cb(desttype, fn, ok1, ok2) \
+	cast_if_type(cast_if_type((fn), ok1, desttype), ok2, desttype)
+
+/**
+ * typesafe_cb - cast a callback function if it matches the arg
+ * @rettype: the return type of the callback function
+ * @fn: the callback function to cast
+ * @arg: the (pointer) argument to hand to the callback function.
+ *
+ * If a callback function takes a single argument, this macro does
+ * appropriate casts to a function which takes a single void * argument if the
+ * callback provided matches the @arg (or a const version).
+ *
+ * It is assumed that @arg is of pointer type: usually @arg is passed
+ * or assigned to a void * elsewhere anyway.
+ */
+#define typesafe_cb(rettype, fn, arg)					\
+	__typesafe_cb(rettype (*)(void *), (fn),			\
+		      rettype (*)(const typeof(arg)),			\
+		      rettype (*)(typeof(arg)))
+
+/**
+ * typesafe_cb_preargs - cast a callback function if it matches the arg
+ * @rettype: the return type of the callback function
+ * @fn: the callback function to cast
+ * @arg: the (pointer) argument to hand to the callback function.
+ *
+ * This is a version of typesafe_cb() for callbacks that take other arguments
+ * before the @arg.
+ */
+#define typesafe_cb_preargs(rettype, fn, arg, ...)			\
+	__typesafe_cb(rettype (*)(__VA_ARGS__, void *), (fn),		\
+		      rettype (*)(__VA_ARGS__, const typeof(arg)),	\
+		      rettype (*)(__VA_ARGS__, typeof(arg)))
 #endif