linux-kernel - Re: [PATCH RFC 1/3] Decompressors: Add XZ decompressor module

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Message-Id: <20101124142549.c1b9ad14.akpm@linux-foundation.org>
Date:	Wed, 24 Nov 2010 14:25:49 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Lasse Collin <lasse.collin@...aani.org>
Cc:	linux-kernel@...r.kernel.org, linux-embedded@...r.kernel.org,
	"H. Peter Anvin" <hpa@...or.com>, Alain Knaff <alain@...ff.lu>,
	Albin Tonnerre <albin.tonnerre@...e-electrons.com>,
	Phillip Lougher <phillip@...gher.demon.co.uk>
Subject: Re: [PATCH RFC 1/3] Decompressors: Add XZ decompressor module

On Wed, 24 Nov 2010 22:51:52 +0200
Lasse Collin <lasse.collin@...aani.org> wrote:

> From: Lasse Collin <lasse.collin@...aani.org>
> 
> In userspace, the .lzma format has become mostly a legacy
> file format that got superseded by the .xz format. Similarly,
> LZMA Utils was superseded by XZ Utils.
> 
> These patches add support for XZ decompression into
> the kernel. Most of the code is as is from XZ Embedded
> <http://tukaani.org/xz/embedded.html>. It was written for
> the Linux kernel but is usable in other projects too.
> 
> Advantages of XZ over the current LZMA code in the kernel:
>   - Nice API that can be used by other kernel modules; it's
>     not limited to kernel, initramfs, and initrd decompression.
>   - Integrity check support (CRC32)
>   - BCJ filters improve compression of executable code on
>     certain architectures. These together with LZMA2 can
>     produce a few percent smaller kernel or Squashfs images
>     than plain LZMA without making the decompression slower.
> 
> This patch: Add the main decompression code (xz_dec), testing
> module (xz_dec_test), wrapper script (xz_wrap.sh) for the xz
> command line tool, and documentation. The xz_dec module is
> enough to have a usable XZ decompressor e.g. for Squashfs.

I'm not seeing any documentation which tells me how to create, install
and execute xs-compressed kernels. There are new makefile targets?

>
> ...
>
> +#define bcj_x86_test_msbyte(b) ((b) == 0x00 || (b) == 0xFF)

This should be written in C.  It looks nicer, and so
bcj_x86_test_msbyte(*p++) won't explode.

> +static noinline_for_stack size_t bcj_x86(

hm, but it uses little stack space.

> +static noinline_for_stack size_t bcj_x86(
> +		struct xz_dec_bcj *s, uint8_t *buf, size_t size)

The preferred style is

static noinline_for_stack size_t bcj_x86(struct xz_dec_bcj *s, uint8_t *buf,
					size_t size)

or

static noinline_for_stack size_t
bcj_x86(struct xz_dec_bcj *s, uint8_t *buf, size_t size)

(lots of dittoes)
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