Messages in this thread | | | Subject | Re: [PATCH] mtd: nand: s3c2410: fix bug in s3c2410_nand_correct_data() | From | Zeng Zhaoxiu <> | Date | Fri, 8 Apr 2016 09:51:04 +0800 |
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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@163.com wrote: > >> From: Zeng Zhaoxiu <zhaoxiu.zeng@gmail.com> >> >> 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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