[PATCH] LOG2: Implement a general integer log2 facility in the kernel
authorDavid Howells <dhowells@redhat.com>
Fri, 8 Dec 2006 10:37:49 +0000 (02:37 -0800)
committerLinus Torvalds <torvalds@woody.osdl.org>
Fri, 8 Dec 2006 16:28:51 +0000 (08:28 -0800)
commitf0d1b0b30d250a07627ad8b9fbbb5c7cc08422e8
tree0aa5379150574374351fb92af7881a48dbfcf2ce
parentb3d7ae5f47a58a9f7b152deeaf7daa1fc558a8f1
[PATCH] LOG2: Implement a general integer log2 facility in the kernel

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>
36 files changed:
arch/alpha/Kconfig
arch/arm/Kconfig
arch/arm26/Kconfig
arch/avr32/Kconfig
arch/cris/Kconfig
arch/frv/Kconfig
arch/h8300/Kconfig
arch/i386/Kconfig.cpu
arch/ia64/Kconfig
arch/m32r/Kconfig
arch/m68k/Kconfig
arch/m68knommu/Kconfig
arch/mips/Kconfig
arch/parisc/Kconfig
arch/powerpc/Kconfig
arch/ppc/Kconfig
arch/s390/Kconfig
arch/sh/Kconfig
arch/sh64/Kconfig
arch/sparc/Kconfig
arch/sparc64/Kconfig
arch/v850/Kconfig
arch/x86_64/Kconfig
arch/xtensa/Kconfig
drivers/infiniband/hw/mthca/mthca_provider.c
drivers/infiniband/hw/mthca/mthca_qp.c
drivers/infiniband/hw/mthca/mthca_srq.c
drivers/infiniband/ulp/iser/iser_memory.c
drivers/md/dm-crypt.c
drivers/mmc/tifm_sd.c
fs/ext2/super.c
fs/ext3/super.c
include/asm-frv/bitops.h
include/linux/kernel.h
include/linux/log2.h [new file with mode: 0644]
mm/page_alloc.c