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@nxp.com>
552c88273e