Re: Increased frequency of fastmap failure due to CRC mismatch

[Richard Weinberger <richard.weinberger@gmail.com>]

On Thu, May 17, 2018 at 5:47 PM, Ronak Desai
<ronak.desai@rockwellcollins.com> wrote:
> On one of our units we noticed increase in fastmap failure due to
> fastmap CRC mismatch.  On this unit, on every power-up UBI observed
> fixable bit flips on a specific PEB. We are using SW ECC for ECC
> correction as the processor's NAND controller does not support the
> required ECC strength. We have also implemented read retry in the NAND
> controller driver.
>
> When UBI reads the fastmap data using NAND-MTD framework,  NAND-MTD
> subsystem returns EUCLEAN meaning there were corrections greater or
> equals to ECC strength. But the data should be corrected as the read
> call does not return any other error.
>
> In this failure scenarios, even though NAND-MTD subsystem has fixed
> the corruption with SW ECC, UBI still finds CRC mis-match on fastmap
> data. Successful data read with read retries has already been tested
> at temperature as well so there is no doubt about the reliability of
> read-retries. So, UBI should never receive corrupted data with fixable
> bit flip return code.
>
> So, would like to understand what is causing the fastamp data
> corruption which leads to CRC mis-match.  Interesting thing is we see
> fixable bit flip error message for that specific PEB on every power up
> but we don't see fastmap CRC failure on every power up. All the
> reboots are graceful (UBI partition is  detached and unmounted) and
> there are no abrupt power-cut.

Hmm, did you manually check the fastmap? I wonder what in the fastmap is wrong.
Is it just a bitlfip or are many bytes bad?

Is your mtd driver sane?

-- 
Thanks,
//richard