[PATCH 03/14] nand: mxs: fix the bitflips for erased page when uncorrectable error

[sbabic at denx.de]

> This patch is porting from linux:
> http://git.freescale.com/git/cgit.cgi/imx/linux-2.6-imx.git/commit/
> ?h=imx_4.1.15_1.0.0_ga&id=3d42fcece496224fde59f9343763fb2dfc5b0768
> "
> We may meet the bitflips in reading an erased page(contains all 0xFF),
> this may causes the UBIFS corrupt, please see the log from Elie:
> -----------------------------------------------------------------
> [    3.831323] UBI warning: ubi_io_read: error -74 (ECC error) while reading 16384 bytes from PEB 443:245760, read only 16384 bytes, retry
> [    3.845026] UBI warning: ubi_io_read: error -74 (ECC error) while reading 16384 bytes from PEB 443:245760, read only 16384 bytes, retry
> [    3.858710] UBI warning: ubi_io_read: error -74 (ECC error) while reading 16384 bytes from PEB 443:245760, read only 16384 bytes, retry
> [    3.872408] UBI error: ubi_io_read: error -74 (ECC error) while reading 16384 bytes from PEB 443:245760, read 16384 bytes
> ...
> [    4.011529] UBIFS error (pid 36): ubifs_recover_leb: corrupt empty space LEB 27:237568, corruption starts at 9815
> [    4.021897] UBIFS error (pid 36): ubifs_scanned_corruption: corruption at LEB 27:247383
> [    4.030000] UBIFS error (pid 36): ubifs_scanned_corruption: first 6569 bytes from LEB 27:247383
> -----------------------------------------------------------------
> This patch does a check for the uncorrectable failure in the following steps:
>    [0] set the threshold.
>        The threshold is set based on the truth:
>        "A single 0 bit will lead to gf_len(13 or 14) bits 0 after the BCH
>         do the ECC."
>         For the sake of safe, we will set the threshold with half the gf_len, and
>         do not make it bigger the ECC strength.
>    [1] count the bitflips of the current ECC chunk, assume it is N.
>    [2] if the (N <= threshold) is true, we continue to read out the page with
>        ECC disabled. and we count the bitflips again, assume it is N2.
>        (We read out the whole page, not just a chunk, this makes the check
>         more strictly, and make the code more simple.)
>    [3] if the (N2 <= threshold) is true again, we can regard this is a erased
>        page. This is because a real erased page is full of 0xFF(maybe also has
>        several bitflips), while a page contains the 0xFF data will definitely
>        has many bitflips in the ECC parity areas.
>    [4] if the [3] fails, we can regard this is a page filled with the '0xFF'
>        data.
> "
> Signed-off-by: Peng Fan <peng.fan at nxp.com>
Applied to u-boot-imx, master, thanks !

Best regards,
Stefano Babic

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