Re: [PATCH v4 1/5] mtd: nand: Create a BBT flag to access bad block markers in raw mode

From: Archit Taneja
Date: Tue Nov 10 2015 - 00:13:21 EST


On 10/12/2015 01:33 AM, Brian Norris wrote:
Hi Boris,

On Fri, Oct 02, 2015 at 08:27:38AM +0200, Boris Brezillon wrote:
Brian, Archit,

On Thu, 1 Oct 2015 19:44:34 -0700
Brian Norris <computersforpeace@xxxxxxxxx> wrote:

On Wed, Aug 19, 2015 at 10:19:02AM +0530, Archit Taneja wrote:
Some controllers can access the factory bad block marker from OOB only
when they read it in raw mode. When ECC is enabled, these controllers
discard reading/writing bad block markers, preventing access to them

The bbt driver assumes MTD_OPS_PLACE_OOB when scanning for bad blocks.
This results in the nand driver's ecc->read_oob() op to be called, which
works with ECC enabled.

Create a new BBT option flag that tells nand_bbt to force the mode to
MTD_OPS_RAW. This would result in the correct op being called for the
underlying nand controller driver.

Actually I have the same kind of patch in my local tree (for a
different reason though: the HW randomizer can mess up with the BBM
byte if it's not disabled, and the only way to disable it in my current
implementation is to switch to raw mode).

MTD_OPS_RAW is probably the best way to do this, and we should switch
back to it for all users (rather than a new flag).

I'm fine with this solution, but will that be acceptable for everybody?
I mean, some NAND controllers are able to protect some OOB bytes, and
the BBM might fall in those OOB bytes. In this case, shouldn't we rely
on the ECC protection instead of reading the OOB in raw mode?

I think ECC is kind of misused a bit here. It's not really meant for
protecting BBMs, and it's also really not sufficient, esp. given
bitflips in erased areas.

But to do this, we
need to fix up some things. Particularly, we need to extend
'badblockbits' support so that it is applied consistently in all places
(I recall there is one code path in which bad block scanning does take
this into account, and one that doesn't.)

Yes, IIRC Andrea has posted a patch addressing that problem [1].
Another problem I see is that badblockbits is currently assigned a
fixed value by the NAND controller driver (or a default value of 8).
There's no specific logic to correlate it to the required ECC strength.
IMO, we should not let each NAND controller driver decide what is the
appropriate value for each chip but rather implement the logic in
nand_base.c based on ecc->strength and ecc->size, and IIRC this was
the question Andrea asked when he posted his proposal.

About badblockbits: it allows us to do a relaxed heuristic on matching
bad block markers, where we say the BBM is "bad" if more than fewer than
N bits are '1'. Right now, we just say that if there are any 0 bits in
the Bad Block Marker (BBM) region, then the block is bad. But this is
problematic for pages that have been worn down and might have bitflips.
So right now, part of a (bad) solution is to read with ECC, so worn
blocks that have data won't be later interpreted as bad blocks if we
rescan the BBMs (ECC will correct the bitflips, if the OOB is

But that solution is not really good, since ECC is not really a panacea
for misinterpreted BBMs. And HW like yours apparently won't work like

Okay, I see you gave pretty much the same explanation, which makes mine
useless :-).

So in summary: if we can consistently make BBM checks look for 6 or 7
"one" bits (rather than a full 8 bits, i.e. BBM == 0xff), then we can
just unconditionally switch to RAW rather than PLACE_OOB. And we don't
need a flag like this pach introduces.

I guess it all depends whether we want to let NAND controllers that can
protect their BBM keep doing it (which IMO is not such a bad idea).

I think I was the only one consciously trying to do this. (Though I
guess it's possible some people discreetly hacked it in by not
supporting raw mode properly.) And for my cases, I'm pretty sure a
properly-improved raw mode BBM scan would be just as good, or actually
better. So I'm not sure anyone would really notice if we switched back
and properly accounted for flips.

Was there any progress on the badblockbits work? I'd seen a thread on
linux-mtd but that had sort of died too.


Could we get this driver merged for now without BBT support? In my next
revision, I could populate chip->block_bad and chip->block_markbad
and add NAND_SKIP_BBTSCAN to chip->options. I can remove this once
we have badblockbits support.



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