[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>

2 files changed, 4 insertions, 8 deletions

diff --git a/fs/ext2/super.c b/fs/ext2/super.c
index 255cef5..6347c2d 100644
--- a/fs/ext2/super.c
+++ b/fs/ext2/super.c
@@ -597,8 +597,6 @@ static int ext2_check_descriptors (struct super_block * sb)
 	return 1;
 }
 
-#define log2(n) ffz(~(n))
- 
 /*
  * Maximal file size.  There is a direct, and {,double-,triple-}indirect
  * block limit, and also a limit of (2^32 - 1) 512-byte sectors in i_blocks.
@@ -834,9 +832,9 @@ static int ext2_fill_super(struct super_block *sb, void *data, int silent)
 	sbi->s_sbh = bh;
 	sbi->s_mount_state = le16_to_cpu(es->s_state);
 	sbi->s_addr_per_block_bits =
-		log2 (EXT2_ADDR_PER_BLOCK(sb));
+		ilog2 (EXT2_ADDR_PER_BLOCK(sb));
 	sbi->s_desc_per_block_bits =
-		log2 (EXT2_DESC_PER_BLOCK(sb));
+		ilog2 (EXT2_DESC_PER_BLOCK(sb));
 
 	if (sb->s_magic != EXT2_SUPER_MAGIC)
 		goto cantfind_ext2;
diff --git a/fs/ext3/super.c b/fs/ext3/super.c
index 580b8a6..b348867 100644
--- a/fs/ext3/super.c
+++ b/fs/ext3/super.c
@@ -1347,8 +1347,6 @@ static void ext3_orphan_cleanup (struct super_block * sb,
 	sb->s_flags = s_flags; /* Restore MS_RDONLY status */
 }
 
-#define log2(n) ffz(~(n))
-
 /*
  * Maximal file size.  There is a direct, and {,double-,triple-}indirect
  * block limit, and also a limit of (2^32 - 1) 512-byte sectors in i_blocks.
@@ -1597,8 +1595,8 @@ static int ext3_fill_super (struct super_block *sb, void *data, int silent)
 	sbi->s_desc_per_block = blocksize / sizeof(struct ext3_group_desc);
 	sbi->s_sbh = bh;
 	sbi->s_mount_state = le16_to_cpu(es->s_state);
-	sbi->s_addr_per_block_bits = log2(EXT3_ADDR_PER_BLOCK(sb));
-	sbi->s_desc_per_block_bits = log2(EXT3_DESC_PER_BLOCK(sb));
+	sbi->s_addr_per_block_bits = ilog2(EXT3_ADDR_PER_BLOCK(sb));
+	sbi->s_desc_per_block_bits = ilog2(EXT3_DESC_PER_BLOCK(sb));
 	for (i=0; i < 4; i++)
 		sbi->s_hash_seed[i] = le32_to_cpu(es->s_hash_seed[i]);
 	sbi->s_def_hash_version = es->s_def_hash_version;