Re: JFFS2 using 'private' zlib header (was [RFC] LZO de/compression support - take 6)

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[ Trimmed Cc list; added original zlib authors. ]

On Tuesday 29 May 2007 09:33:51 Michael-Luke Jones wrote:
> On 29 May 2007, at 12:27, Satyam Sharma wrote:
> > Right, actually, zlib could be switched over to [using a common
> > directory].
> > Because zlib_deflate/ and zlib_inflate/ too share a private header
> > zutil.h which has unfortunately been stuck into include/linux/ with
> > a big
> > /* WARNING: this file should *not* be used by applications. */
> > comment ...
>
> Well, unfortunately zutil.h is currently being used in-kernel by fs/
> jffs2/compr_zlib.c despite the "WARNING: this file should *not* be
> used by applications" notice.

Hmmm, either jffs2 thinks the zlib interfaces exposed through
include/linux/zlib.h are insufficient, or it isn't using zlib properly
(or at least the way it was supposed to be used :-)

Looking at fs/jffs2/compr_zlib.c, however, it just seems to be
an optimization -- skipping some checksum calculation if some
flag (PRESET_DICT) is absent from the input stream about to
be decompressed ...

This /looks/ like an optimization that might make sense for other
users of zlib too, in which case that entire check and
skip-the-checksum thing could actually be put into zlib proper. (?)

> So moving this header to a truly private location isn't possible
> right now, unfortunately,
>
> Michael-Luke Jones
> [added David Whitehouse to cc]

On 5/29/07, Daniel Hazelton <[email protected]> wrote:
I've looked at that code and it seems to need that file for one constant.
Perhaps it'd be better for jffs2/compr_zlib.c to define that constant itself
(or use it as a "Magic Number") rather than include the zlib private header.
Another possibility would be to move that constant out of zutil.h and into
zconf.h or zlih.b - doing any of those would allow the zlib private header to
be moved such that zlib could be changed to use a common directory *and* have
said private header in that directory.

Moving PRESET_DICT to zlib.h robs lib/zlib_deflate/deflate.c from
being able to resolve that macro. FWIW, a dirty hack could be to
simply duplicate it in zlib.h. But then, implementation and interface
are decoupled for good reasons ...

Satyam
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