mtd: bitflip_threshold added to mtd_info and sysfs
An element 'bitflip_threshold' is added to struct mtd_info, and also exposed as
a read/write variable in sysfs. This will be used to determine whether or not
mtd_read() returns -EUCLEAN or 0 (absent a hard error). If the driver leaves it
as zero, mtd will set it to a default value of ecc_strength.
This v2 adds the line that propagates bitflip_threshold from the master to the
partitions - thanks Ivan¹.
¹ http://lists.infradead.org/pipermail/linux-mtd/2012-April/040900.html
Signed-off-by: Mike Dunn <mikedunn@newsguy.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
diff --git a/Documentation/ABI/testing/sysfs-class-mtd b/Documentation/ABI/testing/sysfs-class-mtd
index 43d1818..7883508 100644
--- a/Documentation/ABI/testing/sysfs-class-mtd
+++ b/Documentation/ABI/testing/sysfs-class-mtd
@@ -135,3 +135,39 @@
have multiple ecc steps within each writesize region.
In the case of devices lacking any ECC capability, it is 0.
+
+What: /sys/class/mtd/mtdX/bitflip_threshold
+Date: April 2012
+KernelVersion: 3.4
+Contact: linux-mtd@lists.infradead.org
+Description:
+ This allows the user to examine and adjust the criteria by which
+ mtd returns -EUCLEAN from mtd_read(). If the maximum number of
+ bit errors that were corrected on any single region comprising
+ an ecc step (as reported by the driver) equals or exceeds this
+ value, -EUCLEAN is returned. Otherwise, absent an error, 0 is
+ returned. Higher layers (e.g., UBI) use this return code as an
+ indication that an erase block may be degrading and should be
+ scrutinized as a candidate for being marked as bad.
+
+ The initial value may be specified by the flash device driver.
+ If not, then the default value is ecc_strength.
+
+ The introduction of this feature brings a subtle change to the
+ meaning of the -EUCLEAN return code. Previously, it was
+ interpreted to mean simply "one or more bit errors were
+ corrected". Its new interpretation can be phrased as "a
+ dangerously high number of bit errors were corrected on one or
+ more regions comprising an ecc step". The precise definition of
+ "dangerously high" can be adjusted by the user with
+ bitflip_threshold. Users are discouraged from doing this,
+ however, unless they know what they are doing and have intimate
+ knowledge of the properties of their device. Broadly speaking,
+ bitflip_threshold should be low enough to detect genuine erase
+ block degradation, but high enough to avoid the consequences of
+ a persistent return value of -EUCLEAN on devices where sticky
+ bitflips occur. Note that if bitflip_threshold exceeds
+ ecc_strength, -EUCLEAN is never returned by the read functions.
+
+ This is generally applicable only to NAND flash devices with ECC
+ capability. It is ignored on devices lacking ECC capability.