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SubjectRe: [PATCH 2/2] Add support for > 2GiB MTD devices
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linux-mtd-bounces@lists.infradead.org wrote on 08/27/2008 02:52:24 PM:

> On 27.08.2008 20:51, Jamie Lokier wrote:
> > Bruce_Leonard@selinc.com wrote:
> >
> >> I'm still reluctant to change size to a 64-bit value. There's a
vague
> >> recolection of early conversations on the list that there would be
little
> >> acceptance for that. And that probably has to do with the ongoing
> >> conversation about ABI changes. What I could do to eliminate the
> >> multiplication is introduce the same concept that the NAND layer
uses,
> >> shift values. After all, erasesize should always be a power of 2,
making
> >> that a power of 2 multiplication which can be done via shifts. By
> >> changing erasesize to erasesize_shift, I'd get something like this:
> >>
> >> return a->num_eraseblocks == 0 ? a->size : a->num_eraseblocks <<
> >> a->erasesize_shift
> >>
> >> How would that suit you?
> >>
> >
> > Are you sure it's always going to be a power of 2?
> >
> > What if someone targets a board with 3 chips wired to shared address
> > and parallel data buses?
> >
> > Or if someone makes a weird chip? Or if you can format it in
> > different ways according to desired ECC level (like you can with CDs)?
> >
>
> IIRC I saw a datasheet for such a chip (selectable erasesize with
> non-power-of-2 default) some weeks ago and it had entered production a
> few months ago. The erasesize was alwas a multiple of 16, though. Sorry
> for not remembering more details.
>
> Regards,
> Carl-Daniel
>

Well in that case I don't see that there's much option other than a
multiplication. If there were an odd number of eraseblocks (i.e., 3
chips) you could still do the shifting operations. But if someone has
come up with a flash part that has a non-power of two sector/erasesize,
then there's no way to do it by shift. I supose I could just change
erasesize to size64, make it a 64-bit type and do this:

return a->num_eraseblocks == 0 ? a->size : a->size64

Doesn't seem quite as elegant, but it is simpler. What ever I do, I can't
change the meaning or type of size. That's an kernel <=> userspace ABI
change, and we know what happens when I try to do that ;).

Bruce

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