Re: [PATCH] nand_read_subpage vs. S3C244x NAND: non-word reads

[Werner Almesberger <werner@openmoko.org>]

Ben Dooks wrote:
> As noted on the openmoko list,

Sorry for starting two threads on the same topic. The joy of trying
to do the right thing with lists that don't let you cross-post unless
you're subscribed ...

> I think we can do 256byte subpage reads
> as long as they are aligned to 256bytes. We could make the ECC code
> deal with non-256 byte power-of-two aligned blocks without huge
> changes but my belief is that we cannot support anything that isn't
> a power of two.

In this case, the problem is a bit more subtle: the data blocks
retrieved are perfectly normal, i.e., 256 bytes in size and
properly aligned.

However, nand_read_subpage optimizes retrieval of the OOB data.
So instead of retrieving, say, 64 bytes, it only retrieves 24
(for a 2048 bytes page). Sometimes, not the entire page is
retrieved, and then we get those accesses with an odd size.

> I think the best thing to do is to either force the caller to read
> a power of two (pref. >4 bytes), so either we need some form of flag
> to say this, or change the behaviour of the callers to never try this.

If it's considered generally objectionable to call read_buf for
an amount of data that isn't a whole number of words, those two
approaches would work as well. Making nand_read_subpage align to
4 bytes instead of 1/2 would be fairly simple. A flag would be a
bit messier.

- Werner