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

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Date:	Wed, 27 Aug 2008 15:32:11 -0700 (PDT)
From:	Trent Piepho <tpiepho@...escale.com>
To:	Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006@....net>
cc:	Jamie Lokier <jamie@...reable.org>,
	MTD mailing list <linux-mtd@...ts.infradead.org>,
	linux-kernel@...r.kernel.org, Bruce_Leonard@...inc.com,
	Bruce Leonard <brucle@...thlink.net>
Subject: Re: [PATCH 2/2] Add support for > 2GiB MTD devices

On Wed, 27 Aug 2008, Carl-Daniel Hailfinger wrote:
> 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.

It seems like the device size is always going to have some zeros in the least
significant bits.  Maybe a 32-bit size plus a shift is enough?  That could be
a lot more efficient than a 64-bit size and avoid penalizing most users who
don't need >32-bit size chips.
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