linux-kernel - [PATCH] target/iblock: use ilog2 to compute block size

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Date:	Wed, 22 Jun 2016 14:55:12 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
Cc:	Arnd Bergmann <arnd@...db.de>, Jens Axboe <axboe@...com>,
	Mike Christie <mchristi@...hat.com>,
	Andy Grover <agrover@...hat.com>,
	Hannes Reinecke <hare@...e.com>, linux-scsi@...r.kernel.org,
	target-devel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] target/iblock: use ilog2 to compute block size

Enabling CONFIG_UBSAN_SANITIZE_ALL on ARM caused a link error:

drivers/target/built-in.o: In function `iblock_emulate_read_cap_with_block_size.constprop.1':
target_core_iblock.c:(.text+0xc2774): undefined reference to `____ilog2_NaN'
target_core_iblock.c:(.text+0xc27f8): undefined reference to `__aeabi_uldivmod'
target_core_iblock.c:(.text+0xc299c): undefined reference to `__aeabi_uldivmod'

This is caused by gcc not behaving in the expected ways with __builtin_constant_p(),
but it also points to somewhat inefficient code: As we know that the
block size is a power-of-two value, we can turn the expensive 64-bit
division into a simpler variable bit shift.

Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
 drivers/target/target_core_iblock.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/target/target_core_iblock.c b/drivers/target/target_core_iblock.c
index 22af12f8b8eb..3ab4e2d1202c 100644
--- a/drivers/target/target_core_iblock.c
+++ b/drivers/target/target_core_iblock.c
@@ -201,9 +201,9 @@ static unsigned long long iblock_emulate_read_cap_with_block_size(
 	struct block_device *bd,
 	struct request_queue *q)
 {
-	unsigned long long blocks_long = (div_u64(i_size_read(bd->bd_inode),
-					bdev_logical_block_size(bd)) - 1);
 	u32 block_size = bdev_logical_block_size(bd);
+	unsigned int block_shift = ilog2(block_size);
+	unsigned long long blocks_long = (i_size_read(bd->bd_inode) >> block_shift) - 1;
 
 	if (block_size == dev->dev_attrib.block_size)
 		return blocks_long;
-- 
2.9.0