Re: [PATCH 2/3] MTD: bitflip_threshold added to mtd_info and sysfs

Ivan Djelic
2012-03-16T16:33:07Z

On Thu, Mar 15, 2012 at 05:25:52PM +0000, Mike Dunn wrote:
> +
> +What: /sys/class/mtd/mtdX/bitflip_threshold
> +Date: March 2012
> +KernelVersion: 3.3.1
> +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() and mtd_read_oob(). If the
> + maximum number of bit errors that were corrected on any single
> + writesize region (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. Users who wish
> + to be more paranoid about data integrity can lower the value.
> + If the value exceeds ecc_strength, -EUCLEAN is never returned by
> + the read functions.

Hmmm. I don't think it's a good idea to say "Users who wish to be more paranoid
about data integrity can lower the value"; because this is not exactly true.

Lowering the value is very dangerous, and can have devastating effects: on NAND
devices where sticky bitflips appear (we have plenty of those devices), a low
threshold (say 1) triggers block torture by UBI, then bad block retirement,
quickly reducing the number of valid blocks; the other "sane" blocks
with intermittent bitflips keep being scrubbed, thrashing the whole device.
Even worse: if enough bad blocks appear, UBI runs out of replacement blocks
and stops working.

IMHO the value of 'bitflip_threshold' should be carefully chosen:

- low enough to ensure ecc correction has a safety margin and manufacturer
requirements are met
- high enough to avoid the effects described above

In some cases, controlling bitflip_threshold can be interesting for other
reasons; for instance, on a specific board, I have used a NAND device
requiring 4-bit ecc, but I implemented 8-bit protection through hardware BCH for
extra safety (and future 8-bit NAND upgrades).
In that particular setup, I would set bitflip_threshold to 3 or 4 instead of the
value derived from the ecc strength (8).

So in practice, setting bitflip_threshold is tricky and requires a good
knowledge of the NAND (or Doc/whatever) device your are using, and of how mtd/UBI
will use the threshold.
I suggest we warn about the dangers and discourage people from messing with this
knob unless they know what they are doing.

BR,
--
Ivan