Message-ID: <CAK7LNATA_iy=ntNixpxiJKpTTYy2tXnFaPd4UQ3uiS34tNNT7A@mail.gmail.com>
Date:   Wed, 30 Nov 2016 14:36:13 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     Boris Brezillon <boris.brezillon@...e-electrons.com>
Cc:     linux-mtd@...ts.infradead.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Marek Vasut <marek.vasut@...il.com>,
        Brian Norris <computersforpeace@...il.com>,
        Richard Weinberger <richard@....at>,
        David Woodhouse <dwmw2@...radead.org>,
        Cyrille Pitchen <cyrille.pitchen@...el.com>
Subject: Re: [PATCH 19/39] mtd: nand: denali: perform erased check against raw
 transferred page

Hi Boris


2016-11-28 1:12 GMT+09:00 Boris Brezillon <boris.brezillon@...e-electrons.com>:
> On Sun, 27 Nov 2016 03:06:05 +0900
> Masahiro Yamada <yamada.masahiro@...ionext.com> wrote:
>
>> The erased page check must be done against the raw transferred data.
>> The current first call of is_erase() is against the data after ECC
>> correction.  I saw cases where not all of the data in the page are
>> 0xFF after they are manipulated by the ECC correction engine.
>
> Hm, that's surprising. Usually ECC engines leaves data unchanged when
> uncorrectable errors are detected. Do you have examples where this
> happens? How did you trigger this case?


It always happens on some of my boards.
At the bottom of this mail, I attached a dump of an erased page
after it was manipulated by the ECC engine.

See offset 0x2c1, 0x6c1
The data is 0xfd, not 0xff.


I think basically two ways for erased page checking.

[1] Read the whole of page + oob in raw mode transfer,
    then check if all the data are 0xFF.

[2] Read the ECC-corrected page + oob,
    then check if *most* of the data are 0xFF.
    bit-flips less than ecc.strength are allowed.


I think [2] is equivalent to calling nand_check_erased_ecc_chunk()
with ecc.strength for threshold,
as you suggested in 09/39


But, the current Denali driver does:

[3] Read the ECC-corrected page + oob,
    then check if all the data are 0xFF.
    (no bit-flips are allowed)

I think this is wrong
because this erased page checking is false negative.



This patches adopts [1] to solve my problem, but [2] will work as well.

In my case, only one bit-flip (0xff -> 0xfd) in each ECC section.



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-- 
Best Regards
Masahiro Yamada

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