JFFS2 on dataflash problem

[Ulf Samuelsson]

> I have an embedded device based on Atmel's AT91RM9200DK board, which is
> using serial dataflash (AT45DB642).  I've allocated a JFFS2 partition to
> store non-volatile data.  In testing I stumbled across a particular
> problem that only occurs after heavy hammering on our device, but is
> fairly consistent in how and when it occurs.  The pattern has been
> narrowed down so that a script doing something like this:

> while [ 1 ]; do
>   cp /mnt/jffs2/$RANDOM_FILE /mnt/jffs2/$BLAH
>   # File size has been tested between 8K and 64K
> done

If I wrote this script I would call it:

wear_out_dataflash_quickly.sh

There are some limitations to the number of erase cycles in the dataflash.
(IN any flash to be correct)
You can expect to reprogram it 50,000-100.000 times before the first errors
occur.
The second thing is that you need to do a block erase after the sum
of erases inside an 8 page block exceeds 10,000.

I am not at all sure that the MTD drivers/JFFS2 handle this (did not look at
the code).
I assume that JFFS may be able to detect a bad write and map
the block out from time to time, so this could explain why you can do the
recover.

If you really want to test the dataflash, write a CRC in the extra bytes
available.
(The page is 1024 + 32 bytes) and read back, checking CRC.

Best Regards,
Ulf Samuelsson
ulf at a-t-m-e-l.com