linux-kernel - Re: [PATCH] mtd: nand: s3c2410: fix bug in s3c2410_nand_correct

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Date:	Fri, 8 Apr 2016 04:18:02 +0200
From:	Boris Brezillon <boris.brezillon@...e-electrons.com>
To:	Zeng Zhaoxiu <zhaoxiu.zeng@...il.com>
Cc:	zengzhaoxiu@....com, kgene@...nel.org, k.kozlowski@...sung.com,
	richard@....at, dwmw2@...radead.org, computersforpeace@...il.com,
	linux-arm-kernel@...ts.infradead.org,
	linux-samsung-soc@...r.kernel.org, linux-mtd@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mtd: nand: s3c2410: fix bug in
 s3c2410_nand_correct_data()

On Fri, 8 Apr 2016 09:51:04 +0800
Zeng Zhaoxiu <zhaoxiu.zeng@...il.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@....com wrote:
> >
> >> From: Zeng Zhaoxiu <zhaoxiu.zeng@...il.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