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Date: Tue, 12 Jun 2012 17:42:00 -0700
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Subject: Re: [PATCH] mtd: nand: Use the mirror BBT descriptor when reading its
 version
From: Brian Norris <computersforpeace@gmail.com>
To: dedekind1@gmail.com
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Cc: David Woodhouse <dwmw2@infradead.org>,
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On Tue, Jun 12, 2012 at 3:42 AM, Artem Bityutskiy <dedekind1@gmail.com> wrote:
> On Sun, 2012-06-10 at 22:59 -0700, Brian Norris wrote:
>> On Sun, Jun 10, 2012 at 3:58 AM, Shmulik Ladkani
>> <shmulik.ladkani@gmail.com> wrote:
>> > - Not runtime tested.
>>
>> Patch looks fine, but this makes me curious: does anyone use
>> NAND_BBT_ABSPAGE? It looked broken to me when I tried it not too long
>> ago, but I didn't look too deeply. (I doubt that this typo was its
>> only problem, but I may try again...)
>
> Looks like this is used in diskonchip.c.

Hmm, OK. This driver seems to be very stale (very few real
fixes/improvements in git history). Anyone know if diskonchip.c even
works with the current BBT code?

I retried NAND_BBT_ABSPAGE on my own driver, and it seems that this
code-path has no ability to *create* a bad block table where one
didn't exist previously. It simply reads whatever data is present at
the given page(s) (according to td->pages[]), regardless of ECC
errors, junk data, lack of BBT markers (i.e., "Bbt0" or "1tbB"), and
versioning.

So when I use NAND_BBT_ABSPAGE with an erased block at page 1024, I
get the following:

   Bad block table at page 1024, version 0xFF

And no blocks are detected bad (simply because there was 0xff "table
data"). But then, I get even worse results if the uninitialized BBT
block has arbitrary (non-0xff) data! nand_bbt would just mark random
blocks as bad...

So, this "feature" seems severely limited - designed for a somewhat
static, pre-initialized BBT. I can probably survive by continuing to
ignore this eyesore, but I'd rather just fix it or kill it.

Brian