Date: Fri, 22 Apr 2005 09:52:09 +0200
From: =?iso-8859-1?Q?J=F6rn?= Engel <joern@wohnheim.fh-wedel.de>
To: Phillip Lougher <phillip@lougher.demon.co.uk>
Cc: linux-kernel@vger.kernel.org
Subject: Re: [PATCH 8/10] squashfs: add swabbing infrastructure
Message-ID: <20050422075209.GI10459@wohnheim.fh-wedel.de>
References: <20050420065500.GA24213@wohnheim.fh-wedel.de> <20050421010817.GB29755@wohnheim.fh-wedel.de> <20050421011045.GC29755@wohnheim.fh-wedel.de> <20050421011126.GD29755@wohnheim.fh-wedel.de> <20050422072037.GB10459@wohnheim.fh-wedel.de> <20050422072200.GC10459@wohnheim.fh-wedel.de> <20050422072251.GD10459@wohnheim.fh-wedel.de> <20050422072355.GE10459@wohnheim.fh-wedel.de>
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On Fri, 22 April 2005 09:23:56 +0200, J?rn Engel wrote:
> +
> +#define SQUASHFS_SWAB(XX)					\
> +u##XX squashfs_swab##XX(u##XX x)	{ return swab##XX(x); }	\
> +u##XX squashfs_ident##XX(u##XX x)	{ return x; }
> +
> +SQUASHFS_SWAB(16)
> +SQUASHFS_SWAB(32)
> +SQUASHFS_SWAB(64)
> +
> +

While it made one function somewhat nicer, I'm not entirely sure this
approach is a good idea in general.  With all the bitfields used,
simple byte-swabbing just doesn't cut it.  Really, the best strategy I
can think of would still be considered A Mess(tm).

It may be time to create a new version of the on-medium layout of the
filesystem.  The new layout is explicitly made big-endian (or
little-endian, whatever) and all structs consist exclusively of u8,
u16, u32 and u64, or their leXX or beXX counterparts.

Advantages:
Swapping is trivially done by beXX_to_cpu and friends.
Code size should decrease significantly.

Disadvantages:
New, incompatible layout.  FS compatibility and code shrinkage are
mutually exclusive.  The thing should be called squashfs2 and have a
completely different code base.
'Bitfield'-access is done by explicit masking and shifting.

Again, this option looks just as rotten as all the previous
alternatives.  The decision is up to you, Phillip.

J?rn

-- 
They laughed at Galileo.  They laughed at Copernicus.  They laughed at
Columbus. But remember, they also laughed at Bozo the Clown.
-- unknown
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