[U-Boot] Why data corrected by ecc are not rewrite to the flash?

[Ivan Nardi]

Thank you for your replay and for your interest.

You are right: rewriting the corrected data may be a critical
operation that may fail and having an untouch flash content is useful
to prevent damages from buggy algorithms.

Yet I was thinking about a particular scenario. If flash memory is
wearing a little at a time (ie, the first time I read a page I find 1
bit wrong, then I found 2 errors in the same page and so on until I
am not able to correct data) rewrite corrected data may be useful.

Is this scenario so unlikely?

On 15 June 2010 13:01, Albert ARIBAUD <albert.aribaud at free.fr> wrote:
> Le 15/06/2010 12:42, Ivan Nardi a écrit :
>> Hello!
>>
>> I'am having a look at the ecc algorithms in nand code and I observe (both in
>> kernel and in u-boot) that when ecc corrects some data, corrected data are
>> given to the user, but the data storage in the nand do not change. In other
>> words, I expected the flash would be written with the new (corrected) data.
>>
>> Why data corrected by ecc are not rewrite to the flash?
>>
>> I think that it should be easy achieve this goal. In the read function, if
>> ecc corrects some data on a page, I read the entire block, erase it and
>> write back (correct) data to the flash.
>>
>> Nobody is interested in this feature? Or is it more difficult than I think?
>>
>> Thanks a lot!
>
> Let me play the devil's advocate here: what would be the interest of
> writing back corrected data to the flash? Here's my take at the pros and
> cons of writing ECC-corrected data back to flash:
>
> Cons:
>
> - risk of the write failing more or less critically;
>
> - performance hit (albeit marginal hopefully);
>
> - risk of spurious data corruption: if the ECC detection/correction or
> write decision code is buggy, you may end up with a write-back where
> unnecessary, and possibly write the wrong data back to flash. The
> current situation, at least, guarantees that the Flash content is
> preserved even if ECC detection/correction gets it wrong.
>
> Pros:
>
> - marginal performance gain for subsequent reads of ECC, assuming the
> data is read frequently enough that the correction time becomes
> significant wrt all the rest of the bootloader execution.
>
> But the bottom line is: there is no benefit as far as the data itself is
> concerned since it is correct without writing it back to Flash.
>
> Amicalement,
> --
> Albert.
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