Re: NAND technical review

[Ross Younger <wry at ecoscentric dot com>]

Rutger Hofman wrote:
> R has part-read and part-write support. One thing that has always
> puzzled me is how this interacts with ECC. ECC often works on a complete
> subpage, like 256 bytes on a 2KB page chip; then I understand. But what
> if the read/write is not of such a subpage?

This is a very good question - I revisited it the other day when working on
hardware ECC support for the customer port I'm working on - and I don't have
a particularly good answer for it.

If the read is less than an ECC stride[*], one could perhaps fill in the ECC
calculation by reading the rest of that stride's worth anyway and not
passing it to the caller. Similarly, a write that is less than a stride
could be "filled in" with 0xFF for the purposes of computing its ECC. How
this would be achieved efficiently is an exercise for the reader as a bit of
refactoring is likely to be involved...

[*] I'm using "stride" here to mean the amount of data that an ECC
calculation operates over. The Samsung algorithm which computes 22 bits of
ECC over 256 bytes of data is common, not least of which because that's the
one used by the Linux MTD layer.

I did wonder about not supporting less-than-page reads and writes at all,
but my code currently tries its best on the grounds of being liberal in what
it accepts.

In passing, I note that some large page devices allow the data and spare
areas to be written in subpages (e.g. this Samsung K9 chip to hand - 2048
main + 64 spare per page - allows writes in units of 512 main and 16 spare);
there might be a use to be found here in allowing an application to treat a
large page device as if it were a small-page device.


Ross

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