Re: [PATCH v2] mtd: nand: write bad block marker even with BBT

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 12/19/2011 11:03 PM, Brian Norris wrote:
> Currently, the flash-based BBT implementation writes bad block data only
> to its flash-based table and not to the OOB marker area. Then, as new
> bad blocks are marked over time, the OOB markers become out of date and
> the flash-based table becomes the only source of current bad block
> information. This can be a problem when:
>
>   * bootloader cannot read the flash-based BBT format
>   * BBT is corrupted and the flash must be rescanned for bad
>     blocks; we want to remember bad blocks that were marked from Linux
>
> In an attempt to keep the bad block markers in sync with the flash-based
> BBT, this patch changes the default so that we write bad block markers
> to the proper OOB area on each block in addition to flash-based BBT.
>
> Theoretically, the bad block table and the OOB markers can still get out
> of sync if the system experiences a power cut between writing the BBT to
> flash and writing the OOB marker to a newly-marked bad block. However,
> this is a relatively unlikely event, as new bad blocks shouldn't appear
> frequently.
>

The marker and BBT may get out of sync. You should use either the one 
_or_ the other but not both. Why use both anyway? I use BBT because I
have no room left in OOB or updating a single area in OOB is too
complicated / hardly possible.
In the former case, reading that area again and interpreting it as
something as it was once is a bad thing.

Why is the BBT corrupted? You have two tables. If one is broken you
have still the other. If you lose one block which was about to be
marked bad you find soon enough.

Sebastian