[PATCH v3] block: simplify blksize_bits() implementation

From: Dawei Li
Date: Sun Oct 30 2022 - 01:26:19 EST


Convert current looping-based implementation into bit operation,
which can bring improvement for:

1) bitops is more efficient for its arch-level optimization.

2) Given that blksize_bits() is inline, _if_ @size is compile-time
constant, it's possible that order_base_2() _may_ make output
compile-time evaluated, depending on code context and compiler behavior.

v1: https://lore.kernel.org/all/TYCP286MB2323169D81A806A7C1F7FDF1CA309@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

v2: Remove the ternary operator, based on Bart's suggestion
But this may lead to break for corner cases below:
BUILD_BUG_ON(blksize_bits(1025) != 11);
So make a minor modification by adding (SECTOR_SIZE - 1) before
shifting.

v3: Remove the rounding stuff.

base-commit: 30209debe98b6f66b13591e59e5272cb65b3945e

Signed-off-by: Dawei Li <set_pte_at@xxxxxxxxxxx>
---
include/linux/blkdev.h | 7 +------
1 file changed, 1 insertion(+), 6 deletions(-)

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 57ed49f20d2e..32137d85c9ad 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -1349,12 +1349,7 @@ static inline int blk_rq_aligned(struct request_queue *q, unsigned long addr,
/* assumes size > 256 */
static inline unsigned int blksize_bits(unsigned int size)
{
- unsigned int bits = 8;
- do {
- bits++;
- size >>= 1;
- } while (size > 256);
- return bits;
+ return order_base_2(size >> SECTOR_SHIFT) + SECTOR_SHIFT;
}

static inline unsigned int block_size(struct block_device *bdev)
--
2.25.1