Question on how NAND flash BBT is stored in the chip

[Rutger Hofman]

Du Zhongdong wrote:
> 
> On Thu, Oct 16, 2008 at 10:35 PM, Rutger Hofman <rutger at cs.vu.nl 
> <mailto:rutger at cs.vu.nl>> wrote:
> 
>     Good afternoon list,
> 
>     I have a question how the BBT (bad block table) is stored in MTD NAND
>     devices. I googled for a while, and browsed the code, but I am still not
>     sure.
> 
>     The question:
>     Is the BBT stored/retrieved using a spare layout? Is ECC
>     generation/correction done for it?
> 
> I'm not quite sure about what you mean by "using a spare layout",  but I 
> think bbt is stored in the main area of the specific page(the member 
> "int pages[NAND_MAX_CHIPS" in struct nand_bbt_descr). 

With 'spare layout' I mean the thingy that does auto-placement etc, and 
wraps up ECC, bad block mark(s), and user data into the spare area.

> the function scan_write_bbt calls mtd->write_oob with struct 
> mtd_oob_ops.mode = MTD_OOB_PLACE, tracking down this procedure I think 
> the oob data does not change its layout when written to NAND flash.
> 
> hope this information might be of some help to you :)
> 
> 
> 
>     This question derives from the question above: is the BBT marker pattern
>     stored within a spare layout (with slots for ECC, bad block markers
>     etc)? Or is it written verbatim (raw and unmangled) into the spare area?

Thanks. When browsing the code, I thought as much, but I couldn't 
absolutely make sure whether the spare is mangled or not.

So this leaves the question: the BBT itself doesn't seem to use ECC. Is 
there a reason why not? Wouldn't it be better if it does?

Thanks,

Rutger Hofman
VU Amsterdam