linux-kernel - Re: [Squashfs-devel] Squashfs mainlining status?

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Date:	Thu, 17 Jul 2008 17:35:30 +0200 (CEST)
From:	Geert Uytterhoeven <Geert.Uytterhoeven@...ycom.com>
To:	Peter Korsgaard <jacmet@...site.dk>
cc:	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 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?

With kind regards,

Geert Uytterhoeven
Software Architect

Sony Techsoft Centre Europe
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