We want io.h primitives (readb(), etc.) to be extern inline.
However, that requires the backing out-of-line implementation
somewhere, preferably kept in sync with the inline ones.

The way it's done is __EXTERN_INLINE macro that defaults to
extern inline, but can be overridden in compilation unit where
the out-of-line instance will be.

That works, but it's brittle - we *must* make sure that asm/io.h
is the very first include in such compilation units.  There'd
been a bunch of bugs of that sort in the past.

Another issue is the choice of overriding definition for
__EXTERN_INLINE; it must be either 'inline' or empty.  Either
will do for compilation purposes - inline void foo(...) {...}
(without extern or static) is going to generate out-of-line
instance.  The difference is that 'definition without a
prototype' heuristics trigger on
void foo(void)
{
	...
}
but not on
inline void foo(void)
{
	...
}

Most of the overrides go for 'inline'; in two cases (sys_jensen
and core_t2) __EXTERN_INLINE is defined as empty.  Without
-Wmissing-prototypes it didn't matter, but now that we have
that thing always on...
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Acked-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>