Re: [RFC PATCH] UBI fixable bit-flip issue

[Boris Brezillon <boris.brezillon@bootlin.com>]

On Fri, 17 Aug 2018 16:53:22 +0200
Boris Brezillon <boris.brezillon@bootlin.com> wrote:

> On Sat, 18 Aug 2018 00:33:25 +1000
> Mark Spieth <mspieth@digivation.com.au> wrote:
> 
> > >> I hope this description is clear enough.    
> > > Well, I think selecting the bitflip threshold properly is really
> > > important, simply because some NANDs (including SLC NANDs) are showing
> > > bitflips even on blocks that have a low EC. Check the NAND ECC
> > > requirements, and if it's something like 8bit/512bytes, I guess that's
> > > more or less expected (it all depends on how many bitflips you have in
> > > the faulty block). It's less likely on NANDs requiring 1bit/512bytes
> > > ECC, and if that happens on such NANDs, you may have a problem in the
> > > controller driver.    
> > 4 bits ECC per 512 bytes, from memory 28 bytes in OOB, using software 
> > ECC in the MTD driver.
> > As I said, I believe the better threshold is hiding the root cause. It 
> > is only a band-aid.  
> 
> What you describe will anyway happen sooner or later: if you're using
> almost al LEBs, and the remaining free ones are all impacted by the
> correctable bit-flip issue you'll have to use them anyway. So, yes,
> this is a band-aid, just like your solution is just improving things
> but not really solving the issue. This being said, if the blocks
> really show too many bitflips, they should be marked bad at some point,
> because during the scrubbing process we do write a pattern and check
> that we can read it back. I'll have to double check, but I think we're
> also checking for EUCLEAN and mark the block bad when that happens.

Hm, actually we're not torturing the source PEB when moving a LEB
because of bitflips (probably because it's expensive and tends to wear
the block even faster) :-/. The destination PEB is tortured if we fail
to read the VID header back, which is definitely not a guarantee that
other data are readable or do not contain too much bitflips.

There's definitely something to improve there.