Re: [PATCH] mtd: nand: s3c2410: fix bug in s3c2410_nand_correct_data()

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在 2016年04月08日 08:18, Boris Brezillon 写道:
Hi Zeng,

On Fri,  8 Apr 2016 00:48:17 +0800
zengzhaoxiu@xxxxxxx wrote:

From: Zeng Zhaoxiu <zhaoxiu.zeng@xxxxxxxxx>

If there is only one bit difference in the ECC, the function should return 1.
The result of "diff0 & ~(1<<fls(diff0))" is equal to diff0, so the function
actually returns -1.

Here, we can use the simple expression "(diff0 & (diff0 - 1)) == 0" to determine
whether the diff0 has only one 1-bit.
Missing Signed-off-by here.

---
  drivers/mtd/nand/s3c2410.c | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/mtd/nand/s3c2410.c b/drivers/mtd/nand/s3c2410.c
index 9c9397b..c9698cf 100644
--- a/drivers/mtd/nand/s3c2410.c
+++ b/drivers/mtd/nand/s3c2410.c
@@ -542,7 +542,7 @@ static int s3c2410_nand_correct_data(struct mtd_info *mtd, u_char *dat,
  	diff0 |= (diff1 << 8);
  	diff0 |= (diff2 << 16);
- if ((diff0 & ~(1<<fls(diff0))) == 0)
+	if ((diff0 & (diff0 - 1)) == 0)
Or just

	if (hweight_long((unsigned long)diff0) == 1)

which is doing exactly what the comment says.

BTW, I don't understand why the current code is wrong? To me, it seems
it's correctly detecting the case where only a single bit is different.
What are you trying to fix exactly?

Best Regards,

Boris


For example, assuming diff0 is 1, then fls(diff0) is equal to 1, then "~(1 << fls(diff0))" is equal to 0xfffffffd,
then the result of "(diff0 & ~(1 << fls(diff0)))" is 1 , not we expected 0.

__fls(diff0) and "(fls(diff0) - 1)" are all right, but fls(diff0) is wrong.

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