How to correctly nandwrite a nanddump'ed dump with oob?

Question

I am struggling on flashing a previous ROM dump of an embedded device in Linux. My previous dump contains oob data. I wrote it with nandwrite -n -N -o /dev/mtd0 backup.bin, and then take a ROM dump again.

By comparing the old and new ROM dump, I see some un-explainable situation: the last 24 bytes of the oob (ecc bytes) of any empty blocks (filled with 0xFF) is ought to be 0xFF also, but those in the new ROM dump is filled with 0x00, causing later write failures.

oob ought to be:

FFFFFFFF FFFFFFFF FFFFFFFF FFFFFFFF
FFFFFFFF FFFFFFFF FFFFFFFF FFFFFFFF
FFFFFFFF FFFFFFFF FFFFFFFF FFFFFFFF
FFFFFFFF FFFFFFFF FFFFFFFF FFFFFFFF

but for nandwrite:

FFFFFFFF FFFFFFFF FFFFFFFF FFFFFFFF
FFFFFFFF FFFFFFFF FFFFFFFF FFFFFFFF
FFFFFFFF FFFFFFFF 00000000 00000000
00000000 00000000 00000000 00000000

Anyone has any idea why?

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I added a hack in the nandwrite code, to skip writing to NAND if the content to be written is 0xFF, and it worked. So the problem exists when trying to write an empty page to the NAND?

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ADDED:

Now I am having this problem also when writing a bootloader image. The image isn't page-aligned so nandwrite padded it with 0xFF. But for pages with only 0xFF the ecc bytes are still polluted by 0x00 just like above. Seems that my hack doesn't totally solve my problem. Anyone can help? Perhaps it could be a bug in kernel 2.6.35?

This is my hack:

int i;
int needwrite=0;
for (i = 0 ; i < len ; ++i){
    if(((uint8_t*)data)[i]!=0xff){
        needwrite=1;
        break;
    }
}
if(!needwrite)
    return 0;

Answer 1

Sorry, Alvin, but the backup really will not "only work on that particular flash", because you cannot know when a particular bit will go from good to marginal or marginal to bad. You may read it in one state, attempt to write it in the exact same state and fail, on any given day, with any given backup.

The ONLY way to safely backup the data in a NAND device is WITH ECC TURNED ON. You read from the device with ECC corrections to get good data. You then write the known-good data back to NAND with ECC turned on so that any bits which are now marginal or bad from when you read it before can be corrected using the NEW ECC values.