Re: [PATCH v10 2/5] mtd: nand: vf610_nfc: add hardware BCH-ECC support

[Brian Norris <computersforpeace@gmail.com>]

On Wed, Aug 26, 2015 at 10:57:38AM -0700, Stefan Agner wrote:
> On 2015-08-25 12:54, Brian Norris wrote:
> > On Mon, Aug 03, 2015 at 11:28:43AM +0200, Stefan Agner wrote:
> >> Btw, if the ECC check fails, the controller seems kind of count the
> >> amount of bitflips. It works for most devices reliable, but we had
> >> devices for which that number was not accurate, see:
> >> http://thread.gmane.org/gmane.linux.ports.arm.kernel/357439
> > 
> > I'm a little confused there. Why would you be expecting to get a count
> > of bitflips, when the ECC engine can't correct all errors? How is it
> > supposed to know what the "right" data is if the bit errors are beyond
> > the correction strength?
> 
> When printing the ECC error count on ECC fail when reading an erased
> NAND flash, the numbers of bit flips (stuck at zero) seem to widely
> correlate with the number returned by the controller. While it seems to
> correlate widely, there are exceptions, as discussed in the thread:
> http://thread.gmane.org/gmane.linux.ports.arm.kernel/295424
> 
> Maybe this is an artifact of the ECC algorithm we just can't/shouldn't
> rely on? I am not sure where this originated, I did not found any
> indication in the reference manual about what that value contains in the
> error case.

Doesn't sound too reliable to me. And I'm not sure even if it was
reliable, that it would provide much value. We have to a lot of
re-counting anyway, so we might as well just be using our own threshold.
Or maybe I'm missing the point.

> Bill, do you have an idea why we used that value as threshold in early
> implementations?
> 
> Otherwise I also think we should just drop the use of this value.

Brian