Re: [Linux-fbdev-devel] [PATCH] video: arch specific page protection support for deferred io

[Arnd B. <ar...@ar...>]

On Thursday 25 June 2009, Jaya Kumar wrote:
> The patch looks good. I was going to suggest that it might be
> attractive to use __attribute__(weak) for each of the dummy functions
> instead of ifdefs in this case, but I can't remember if there was a
> consensus about attribute-weak versus ifdefs.

We rarely use weak functions, the canonical way to express an optional
subsystem is along the lines of

/* include/linux/foo.h */
#ifdef CONFIG_FOO
extern int bar(void);
#else
static inline int bar(void)
{
	return 0;
}
#endif
---
/* foo/foo.c */
int bar(void)
{
	/* the real thing here */
	...
}
---
# foo/Makefile 
obj-$(CONFIG_FOO) += foo.c

Most uses of __weak or __attribute__((weak)) are for working default
implementations that can be overridden by architecture specific code.
However, for these the preferred way to express seems to have shifted
towards variations of:

/* include/linux/foo.h */
#include <asm/foo.h>
#ifndef bar
static inline int bar(void)
{
	/* generic implementation */
	...
}
#endif
/* arch/*/include/asm/foo.h */
#define bar bar
static inline int bar(void)
{
	/* arch specific implementation */
	...
}

	Arnd <><