Re: [PATCH] Newly erased page read workaround

[Ivan Djelic <ivan.djelic@parrot.com>]

On Fri, Apr 01, 2011 at 10:06:51AM +0100, Vipin Kumar wrote:
> That's the problem. Ideally the ecc should have been programmed in OOB and then 
> the driver would be able to correct the flipped bits. The problem happens only 
> if we try to read the erased pages.
> 
> >> Ideally, any filesystem would mark it as a bad block 
> > 
> > That's the point - no. This is normal on modern flashes.
> > 
> > I think one solution could be that you make your check more
> > sophisticated. You check for 0xFFs, if this is not true, you see is this
> > "almost all 0xFFs" and count amount of non-0xFF bits. If the count is,
> > say, 2, you assume this page contains all 0xFFs plus 2 bit-flips. But
> > I'm not sure it would work.
> > 
> > Anyway, If you do not care about such bit-flips for your SoC - fine. I
> > just wanted you to understand and accept the issue and write about it in
> > the comment. And I also wanted you to _not_ do expensive 0xFF comparison
> > every time - but it seems you accepted this :-)
> > 
> 
> Yes, I had to accept this :-)
> The flip side is that the hardware itself should not report errors when it 
> reads all ff data and ff ecc..It should assume it as an erased page and not 
> report any errors
 
Hello Vipin,

Did you consider this idea: if you have an unused byte available in oob,
program it to 0x00 when a page is programmed.

That way, you just need to check a single byte when you read a page in order
to distinguish erased pages from programmed pages. And by counting the number
of 1s in the byte, you can be robust to bitflips.

As a special refinement, you could also "cleanup" pages detected as erased, in
order to iron out possible bitflips.

I think that this method is used by Micron for their internal on-die ecc engine:
they add a parity byte (0x00 or 0x01) to their BCH code, which can be used to:

1) detect failures (using parity) when the max error count is reached
2) distinguish between erased and programmed pages

Best Regards,

Ivan