Date:	Fri, 18 Jul 2008 08:14:29 +0200 (CEST)
From:	Geert Uytterhoeven <Geert.Uytterhoeven@...ycom.com>
To:	Robert Lougher <rob.lougher@...il.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?

	Hi Robert,

On Thu, 17 Jul 2008, Robert Lougher wrote:
> On Thu, Jul 17, 2008 at 4:35 PM, Geert Uytterhoeven
> <Geert.Uytterhoeven@...ycom.com> wrote:
> > 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

Sorry, I don't want to offend anyone. Nor do I intent to become the maintainer.
I just answered Peter's question, because I think people should know the
answer.

> 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.

If there are good reasons for breaking compatibility, we're more than happy to
accept that. Don't you think it would be a good idea to explain them to the
readers of the squashfs-devel mailing list?

Phillip, can you please elaborate? Many thanks in advance!

With kind regards,

Geert Uytterhoeven
Software Architect

Sony Techsoft Centre Europe
The Corporate Village · Da Vincilaan 7-D1 · B-1935 Zaventem · Belgium

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