Re: NAND ECC capabilities

[Brian Norris <computersforpeace@gmail.com>]

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;

Brian