Re: [PATCH 2/4] mtd: nand: implement two pairing scheme

Boris Brezillon
2016-06-14T20:29:33Z

On 14 Jun 2016 05:07:26 -0400
"George Spelvin" <linux@sciencehorizons.net> wrote:

> Boris Brezillon wrote:
> > On 12 Jun 2016 16:24:53 George Spelvin wrote:
> >> Boris Brezillon wrote:
> >> My problem is that I don't really understand MLC programming.
>
> > I came to the same conclusion: we really have these 2 cases in the
> > wild, which makes it even more complicated to define a standard
> > behavior.
>
> I did find a useful stuy of the issue: "Program Interference in MLC NAND
> Flash Memory: Characterization, Modeling, and Mitigation"
>
> https://users.ece.cmu.edu/~omutlu/pub/flash-programming-interference_iccd13.pdf
>
> It describes the write-disturb-precompensation technique, and also
> shows how the two-stage programming works. (Although the fact that the
> "least significant bit" is the *largest* voltage difference and is shown
> on the *left* makes no sense at all.)

I think I read this document back when I started to look at how MLC
NANDs were working, but I didn't have the background to understand what
all this meant.

Reading it again makes a lot more sense, and actually I now understand
why those NANDs require data scrambling/randomization (their program
disturb modeling was done with random data, which makes it irrelevant
when repeated pattern are programmed).

>
> Looking at the demonstrated programming sequence, it looks like
> it should be possible to probe for the bit assignment. If you have
> a half-programmed page, then any bits programmed to "0" are actually
> sitting close to the threshold between the two middle voltage levels.
>
> So you'll get a lot of errors reading them as "1", but the interesting
> part is the read-back of the unprogrammed bit.
>
> If the chip is using the binary sequence, you'll read either 10 or 01.
> If the chip us ising the Gray-code sequence, you'll read 10 or 00.
>
> Basically, you read both pages and see which bit combination never
> appears. That is the combination that corresponds to the highest voltage
> level.
>
> Another interesting paper is "Read Disturb Errors in MLC NAND Flash
> Memory: Characterization, Mitigation, and Recovery"
> https://users.ece.cmu.edu/~omutlu/pub/flash-read-disturb-errors_dsn15.pdf
>
> That talks about tricks that do as you observe: increase read error to start.
> (In order to decreaease read disturb, and thus read errors later.)
>
> >> It's more considering it to have 16K pages that can be accessed in half-pages.
>
> > Yes, I know, but it's not really easy to fake that at the NAND level,
> > because programming 2 pages still requires 2 page program operation.
> > The MTD user could detect that the pairing scheme always exposes 2
> > consecutive non-paired pages, but as you've seen, this condition does
> > not necessarily imply the 'pair coupling' constraint, and we don't want
> > to increase the min_io_size value if it's not really necessary.
>
> Ideally, it would be nice to separate the "SLC hack" from the "later
> write failures can corrupt earlier data" workaround.
>
> First, you get the latter working on SLC flash. Then you add MLC, and
> make MLC another reason why it can happen.
>
> But I'm not certain this is actually necessary. Could listing 4 pages
> rather than 2 as in other data sheets just be an editing or translation
> error? Maybe someoe got confused about "in the same row" when they
> wrote that clarifying example.
>
> > I'm just realizing this is actually a non-issue for the solution we
> > developed with Ricard. As I said, it's unsafe to partially write a
> > block in MLC mode, so the only sane way is either to write a block in
> > SLC mode, or atomically write a block in MLC mode, and that's what
> > we're doing with our 'UBI LEB consolidation' approach. I'm pretty sure
> > the problem described in the Hynix datasheet does not happen when only
> > writing in SLC mode. So, even if the pairing scheme does not account
> > for this extra 'coupling' constraint, we should be safe.
>
> I can't see any reason why it would affect MLC and not SLC.