Single bit error correction on NAND.

[DataCom - Virgílio]

On Thursday 28 July 2005 18:06, Thomas Gleixner wrote:
> On Thu, 2005-07-28 at 16:40 -0300, DataCom - Virgílio wrote:
> > I have noticed that NAND support on MTD can detect/correct single bit
> > errors in 256 bytes, using software ECC. However, the correct data is not
> > written back to flash. In other words, the bit stays "flipped" in flash.
> > Isn't that dangerous? I mean, if another bitflip occurs in the same 256
> > bytes, we won't be able to correct by software ECC...
> >
> > Am I missing something here?
>
> Yes.
>
> It is not and can not be the responsibility of the NAND driver to handle
> this. Where to should it write the data ? The existing data _cannot_ be
> overwritten, without erasing the whole eraseblock and the implied risk
> of loosing _ALL_ the data on the block due to a powerfail.
>
> The NAND driver returns the corrected data and an appropriate error code

Maybe I'm missing something, but the function nand_do_read_ecc returns 
-EBADMSG only if an uncorrectable error is detected by nand_correct_data. If 
just one bit is flipped, nand_correct_data corrects it, and nand_do_read_ecc 
returns 0.

> and the calling (e.g. filesystem) driver has to deal with the data
> copying or whatever the developer has decided to be the correct
> solution.

So it would be JFFS2 responsability to write back the correct data to flash, 
after a bit-flip is detected and corrected by the NAND driver. However, 
apparently it doesn't.

My question is: do you think it's safe to just leave the bit flipped in flash? 
I mean, everytime I try to read it, ECC will handle it. But what are the odds 
of another bit-flip in the same 256 bytes?

-- 
VIRGÍLIO Silva