linux-kernel - Re: [PATCH 1/2 ] Add support LZO in cramfs

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Date:	Sat, 27 Oct 2007 13:09:45 +0400
From:	Michael Tokarev <mjt@....msk.ru>
To:	vince.kim@...ess-company.com
CC:	linux-kernel@...r.kernel.org,
	"Richard Purdie [rpurdie"@openedhand.com
Subject: Re: [PATCH 1/2 ] Add support LZO in cramfs

vince kim wrote:
> This is a kernel patch to add support LZO compression in cramfs. 
[]
> --- linux-2.6.23/fs/cramfs/inode.c      2007-10-09 13:31:38.000000000 -0700
> +++ linux-2.6.23_cramfs_lzo/fs/cramfs/inode.c    2007-10-26 14:35:59.000000000 -0700
> @@ -31,6 +31,8 @@
>  static const struct inode_operations cramfs_dir_inode_operations;
>  static const struct file_operations cramfs_directory_operations;
>  static const struct address_space_operations cramfs_aops; 
> +/* function pointer to uncompress block */
> +static int (* cramfs_uncompress_block) (void *dst, int dstlen, void *src, int srclen);

Shouldn't this pointer be mountpoint-specific?  I mean,
if I've two cramfs images, one using zlib and another
using lzo, the two will not work at the same time.

[]
> --- linux-2.6.23/fs/Kconfig     2007-10-09 13:31:38.000000000 -0700
> +++ linux-2.6.23_cramfs_lzo/fs/Kconfig  2007-10-26 14:19:01.000000000 -0700
> @@ -1348,6 +1348,7 @@
>         tristate "Compressed ROM file system support (cramfs)" 
>         depends on BLOCK
>         select ZLIB_INFLATE
> +       select LZO_DECOMPRESS
>         help
>           Saying Y here includes support for CramFs (Compressed ROM File
>           System).  CramFs is designed to be a simple, small, and compressed 

Hmm.  How about using modular decompressor? I mean,
it isn't really necessary for a given config to handle
both lzo- and zlib-compressed cramfs images, only one
may be needed, so the other compression library becomes
a useless dependency.  This is especially important for
embedded environments where memory/disk space is limited.
Since you're using a pointer-to-function anyway, it can
be done fully dynamically, by requesting a module to de-
compress the thing at runtime.  Pretty much like it's
done f.e. in crypto/ipsec layer currently.

By the way, your patch is word-wrapped.

/mjt
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