linux-kernel - [PATCH v3] block: simplify blksize

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Date:   Sun, 30 Oct 2022 13:20:08 +0800
From:   Dawei Li <set_pte_at@...look.com>
To:     axboe@...nel.dk
Cc:     bvanassche@....org, hch@....de, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, Dawei Li <set_pte_at@...look.com>
Subject: [PATCH v3] block: simplify blksize_bits() implementation

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@TYCP286MB2323.JPNP286.PROD.OUTLOOK.COM

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@...look.com>
---
 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