Re: [PATCH] mtd: nand: default bitflip-reporting threshold to 75% of correction strength

[Richard Weinberger <richard@nod.at>]

Am 17.01.2015 um 20:42 schrieb Boris Brezillon:
> On Sat, 17 Jan 2015 20:26:44 +0100
> Richard Weinberger <richard@nod.at> wrote:
> 
>> Am 17.01.2015 um 20:01 schrieb Boris Brezillon:
>>> Just sharing my experience with MLC NANDs requiring read-retry: the
>>> number of reported bitflips often raise ecc_strength value (at least
>>> with the current read-retry approach).
>>> This patch will definitely make UBI move NAND blocks over and over
>>> again considering the threshold has been raised and the block is not
>>> reliable anymore.
>>
>> Within the last 6 months I had to face a lot of strange UBI/MTD issues.
>> All showed one "flaw" in UBI, namely that it was designed with good SLC
>> NANDs in mind.
>> Even some modern SLC NANDs show bad behavior like read disturb after
>> less than 100000 reads.
>> I think it is time to bite the bullet and improve UBI wrt. MLC NAND.
>> This is not an easy task as it needs some hardware to play with and
>> time/budget. But I think it is worth the effort.
> 
> I do all my MLC tests on a cubietruck (embedding an Allwinner A20 SoC
> and a Micron MLC NAND).

Maybe I should get me one of these boards.
Despite I'm not really a fan of sunxi.

> I already started to work on randomizer/scrambler support (which are
> needed on some MLC chips), and added support for read-retry on a Micron
> non-ONFI NAND (you can find my work here [1], but it's not ready to be
> mainlined yet).
> But these are all things we can handle in the NAND layer.

Yep.

> Then comes trickier parts, like improved bitflips robustness (as
> you stated), paired pages handling (you cannot reliably write on one
> page without risking to corrupt the page it is paired with, which
> implies specific handling for such cases in upper layers: UBI/UBIFS ?),
> and surely other things I don't remember :-).

Unstable bits for example need also handling.
I really would like to get some input from NAND vendors what they want
us to solve in software.

I'm currently working on a solution for UBI to deal better with
read disturb. Within the next few week I hopefully have something sane
to share. :)

> Anyway, I'd be happy to help with any of these tasks.

Good to know!

Thanks,
//richard