Date:	Thu, 17 Jul 2008 16:47:13 +0100
From:	"Robert Lougher" <rob.lougher@...il.com>
To:	"Geert Uytterhoeven" <Geert.Uytterhoeven@...ycom.com>
Cc:	"Peter Korsgaard" <jacmet@...site.dk>,
	"Phillip Lougher" <phillip@...gher.demon.co.uk>,
	squashfs-devel@...ts.sourceforge.net,
	"Linux Kernel Development" <linux-kernel@...r.kernel.org>
Subject: Re: [Squashfs-devel] Squashfs mainlining status?

On Thu, Jul 17, 2008 at 4:35 PM, Geert Uytterhoeven
<Geert.Uytterhoeven@...ycom.com> wrote:
>        Hi Peter,
>
> On Wed, 4 Jun 2008, Peter Korsgaard wrote:
>> >>>>> "Phillip" == Phillip Lougher <phillip@...gher.demon.co.uk> writes:
>>  Phillip> The major work of course is the move to a little endian
>>  Phillip> layout without any bit fields in the structures to make it
>>  Phillip> easier to swap member fields on big-endian architectures.
>>  Phillip> I'm currently working on this on a non-CVS branch.
>>  Phillip> Most of the work till now has been without any attempt to
>>  Phillip> retain backwards compatibility with the current 3.3
>>  Phillip> little-endian filesystem layout.  After your private email
>>  Phillip> regarding retaining backwards compatibility I will look
>>  Phillip> again at this issue, it may well be that I will abandon my
>>  Phillip> current layout changes that I have so far made.
>>
>> What private mail? Anything of interest to the rest of the group? As I
>
> Oops, I've just returned from holidays, and realize I never actually responded
> to this question. So here is the gist of it:
>
> | I had a closer look at the structures and bitfields, and I still think it's
> | possible to stay compatible with the current little endian layout:
> |   1. About half of the bitfields are actually bytes or 16-bit words. Hence
> |      these can easily be converted to __u8 or __le16.
> |   2. Most of the remaining bitfields sit in an 8, 16, or 32 bit integer.
> |      Hence these can be converted to such an integer (or an array of bytes,
> |      cfr. 3 below), and accessed using get_unaligned_le{16,32}() and a
> |      shift/mask operation.
> |   3. There are a few bitfields that sit in an `odd' number of bytes. These
> |      can be converted to an array of bytes, and accessed using
> |      get_unaligned_le{16,32}() and a shift/mask operation.
> |   4. The integral fields in the structs should be accessed using
> |      get_unaligned_le{8,16,32}(), too.
>
> I noticed the squashfs CVS repository is getting a new (incompatible) 4.0
> layout now?
>
> What do people think?
>

Last time I looked you were not the maintainer of squashfs.  Phillip
spent a long time examining the possibility of maintaining
compatibility but decided asthat there were sufficient advantages in a
new layout to warrant the change.

Rob.

> With kind regards,
>
> Geert Uytterhoeven
> Software Architect
>
> Sony Techsoft Centre Europe
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