Re: NAND ECC capabilities

[Josh Wu <josh.wu@atmel.com>]

Hi, Brian

On 1/9/2015 3:09 AM, Brian Norris wrote:
> On Thu, Jan 08, 2015 at 01:42:53PM -0300, Ezequiel Garcia wrote:
>> On 01/08/2015 05:32 AM, Ricard Wanderlof wrote:
>>> On Thu, 8 Jan 2015, Steve deRosier wrote:
>>>> Is my general understanding correct?
>>> I'd say yes, but the ECC threshold should be per 512 byte ECC block (which
>>> seems to be the correct term rather than 'sector'), rather than per page.
>>> Are you sure that the threshold is set to 4 (see
>>> /sys/devices/virtual/mtd/mtd<n>/bitflip_threshold )?
>>>
>>> Normally the threshold is set below the ECC correction capability, so that
>>> bit scrubbing has a chance to occur before the bits rot too far. Say you
>>> have the threshold set at 4 bits, and you have 3 bits that have flipped.
>>> If another bit flips, the block would be scrubbed, but say that two bits
>>> flipped before you read the data the next time. You would have lost your
>>> chance of recovery, so it makes sense to have the threshold lower than the
>>> ECC capability. I would say 3/4 of the ECC capability would be a
>>> reasonable value.
>>>
>> This makes a lot of sense. However, do we have any way of telling if the
>> bitflips where produced on the same ECC sector?
>>
>>  From a cursory look to the code, I'd say there's no such feature with
>> the current MTD/NAND design. So, if an mtd_read reports 3 bitflips you
>> have no way of telling they happened on the same sector or not, so you
>> can't implement your idea.
> I'm not sure if I'm misunderstanding you or if you are misunderstanding
> the code. Please review Documentation/ABI/testing/sysfs-class-mtd for
> the 'bitflip_threshold' description. We only deal with the max # of
> bitflips per sector (or block, or whatever you want to call it). No
> ECC-related concept is handled on a per-page basis.
>
> So I believe Ricard is accurately describing the current reality, not
> "his idea."
>
> Side note: I wonder if we want to change the nand_base defaults so
>
> 	bitflip_threshold == 3/4 * ecc_strength
>
> rather than
>
> 	bitflip_threshold == ecc_strength
>
> See in nand_scan_tail():
>
> 	if (!mtd->bitflip_threshold)
> 		mtd->bitflip_threshold = mtd->ecc_strength;
According to the discussion, this change really make sense.
Will you generate a patch for this? Thanks.

Best Regards,
Josh Wu

>
> Brian
>
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