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SubjectRe: [PATCH] mtd: nand: s3c2410: fix bug in s3c2410_nand_correct_data()
On Fri, 8 Apr 2016 09:51:04 +0800
Zeng Zhaoxiu <zhaoxiu.zeng@gmail.com> wrote:

>
>
> 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.
>

Indeed, I forgot that fls() was returning (position + 1). Anyway, I
still think using hweight clarifies what you really want to test.

--
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com

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