Re: NAND review

[Rutger Hofman <rutger@cs.vu.nl>]

Bart Veer wrote:
>>>>>> "Andrew" == Andrew Lunn <andrew@lunn.ch> writes:
> 
>     >> The partition definition is necessarily platform-specific, so
>     >> doesn't fit anywhere else.
> 
>     Andrew> I simply don't get this. 
> 
>     Andrew> Take a case i recently had with a NOR based system. I had
>     Andrew> a linux kernel bug i had to trace down. So that i had
>     Andrew> human readable kernel opps information, i rebuilt the
>     Andrew> kernel to include debug symbols. The resulting kernel was
>     Andrew> too big to fit in the space allocated to it. So i used
>     Andrew> redboot fis to zap both the root filesystem and the space
>     Andrew> holding the kernel. I recreated the kernel partition a bit
>     Andrew> bigger and made the root filesystem a bit smaller. I then
>     Andrew> installed the new kernel and the root filesystem. I then
>     Andrew> had a booting system with opps with symbols, not hex
>     Andrew> addresses.
> 
>     Andrew> At no point did i need to edit the HAL, rebuild and
>     Andrew> install a new redboot. Why should NAND be different? Why
>     Andrew> cannot this partition information be configured by
>     Andrew> redboot? Why must it be platform specific?
> 
> I am not a NAND expert, but I think the answer is that NAND is
> fundamentally different from NOR: it is unreliable.
> 
[snip]
> Or, we could try to implement a robust layer on top of the basic NAND
> layer. If you want to store N pages reliably, reserve (N+f(N)+k)
> pages in the NAND flash. When one of the pages starts failing, start
> using one of the spare pages. Assume that updates will be infrequent
> so that you do not need to worry about wear-levelling, just bad
> blocks. This would give us a way of storing the partition info in the
> NAND flash with a high degree of reliability. Unfortunately it means a
> fairly complicated extra layer which will not be needed by e.g.
> NAND-aware flash filesystems - those expect to handle the bad pages
> etc. themselves.

FWIW, this is the approach taken by MTD with the BBT (Bad Block Table).

Rutger