Re: [PATCH 19/39] mtd: nand: denali: perform erased check against raw transferred page
From: Masahiro Yamada
Date: Wed Nov 30 2016 - 00:36:42 EST
Hi Boris
2016-11-28 1:12 GMT+09:00 Boris Brezillon <boris.brezillon@xxxxxxxxxxxxxxxxxx>:
> On Sun, 27 Nov 2016 03:06:05 +0900
> Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx> 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