linux-kernel - Re: [PATCH 2/2] Add support for

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Message-ID: <OFF4CB8B0C.41A8912A-ON882574B3.00613BB8-882574B3.0061CE65@selinc.com>
Date:	Thu, 28 Aug 2008 10:48:32 -0700
From:	Bruce_Leonard@...inc.com
To:	Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006@....net>
Cc:	Bruce Leonard <brucle@...thlink.net>,
	Jamie Lokier <jamie@...reable.org>,
	linux-kernel@...r.kernel.org, linux-mtd@...ts.infradead.org,
	linux-mtd-bounces@...ts.infradead.org
Subject: Re: [PATCH 2/2] Add support for > 2GiB MTD devices

linux-mtd-bounces@...ts.infradead.org wrote on 08/27/2008 02:52:24 PM:

> On 27.08.2008 20:51, Jamie Lokier wrote:
> > Bruce_Leonard@...inc.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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