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[PATCH] LOG2: Implement a general integer log2 facility in the kernel

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This facility provides three entry points:

	ilog2()		Log base 2 of unsigned long
	ilog2_u32()	Log base 2 of u32
	ilog2_u64()	Log base 2 of u64

These facilities can either be used inside functions on dynamic data:

	int do_something(long q)
	{
		...;
		y = ilog2(x)
		...;
	}

Or can be used to statically initialise global variables with constant values:

	unsigned n = ilog2(27);

When performing static initialisation, the compiler will report "error:
initializer element is not constant" if asked to take a log of zero or of
something not reducible to a constant.  They treat negative numbers as
unsigned.

When not dealing with a constant, they fall back to using fls() which permits
them to use arch-specific log calculation instructions - such as BSR on
x86/x86_64 or SCAN on FRV - if available.

[akpm@osdl.org: MMC fix]
Signed-off-by: David Howells <dhowells@redhat.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: David Howells <dhowells@redhat.com>
Cc: Wojtek Kaniewski <wojtekka@toxygen.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
This commit is contained in:
David Howells 2006-12-08 02:37:49 -08:00 • committed by Linus Torvalds
parent b3d7ae5f47
commit f0d1b0b30d
36 changed files with 384 additions and 28 deletions
Download patch file Download diff file Expand all files Collapse all files

9
include/linux/kernel.h
View file

@ -13,6 +13,7 @@
#include <linux/types.h>
#include <linux/compiler.h>
#include <linux/bitops.h>
#include <linux/log2.h>
#include <asm/byteorder.h>
#include <asm/bug.h>
@ -157,14 +158,6 @@ static inline int printk(const char *s, ...) { return 0; }
unsigned long int_sqrt(unsigned long);
static inline int __attribute_pure__ long_log2(unsigned long x)
{
int r = 0;
for (x >>= 1; x > 0; x >>= 1)
r++;
return r;
}
static inline unsigned long
__attribute_const__ roundup_pow_of_two(unsigned long x)
{