linux-kernel - Re: JFFS2 using 'private' zlib header (was [RFC] LZO de/compression support

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Date:	Wed, 30 May 2007 19:00:02 +0530
From:	"Satyam Sharma" <satyam.sharma@...il.com>
To:	"Mark Adler" <madler@...mni.caltech.edu>
Cc:	"Daniel Hazelton" <dhazelton@...er.net>,
	"Michael-Luke Jones" <mlj28@....ac.uk>,
	lkml <linux-kernel@...r.kernel.org>, dwmw2@...radead.org,
	jloup@...p.org
Subject: Re: JFFS2 using 'private' zlib header (was [RFC] LZO de/compression support - take 6)

Hi Mark,

On 5/30/07, Mark Adler <madler@...mni.caltech.edu> wrote:
> On May 29, 2007, at 8:15 AM, Satyam Sharma wrote:
> > skipping some checksum calculation if some
> > flag (PRESET_DICT) is absent from the input stream about to
> > be decompressed ...
>
> You don't need to dissect the header manually to look for that bit.
> If you feed inflate() at least the first two bytes, it will return
> immediately with the Z_NEED_DICT return code if a preset dictionary
> is requested.  You can force inflate() to return immediately after
> decoding the two byte header even if a preset dictionary is not
> requested by using the Z_BLOCK flush code.

Thanks for replying, unfortunately I don't know either zlib or jffs2 code
deeply enough to actually understand what you meant here. Are you
saying that the if-else block in question [1] in
fs/jffs2/compr_zlib.c:jffs2_zlib_decompress() is unnecessary and can
be done away with?

Thanks,
Satyam

[1] For your reference, here is the user code in question:

	inf_strm.next_in = data_in;
	inf_strm.avail_in = srclen;
	inf_strm.total_in = 0;
	inf_strm.next_out = cpage_out;
	inf_strm.avail_out = destlen;
	inf_strm.total_out = 0;

	int wbits = MAX_WBITS;

	/* If it's deflate, and it's got no preset dictionary, then
	   we can tell zlib to skip the adler32 check. */

        if (srclen > 2 && !(data_in[1] & PRESET_DICT) &&
	    ((data_in[0] & 0x0f) == Z_DEFLATED) &&
	    !(((data_in[0]<<8) + data_in[1]) % 31)) {
		D2(printk(KERN_DEBUG "inflate skipping adler32\n"));
		wbits = -((data_in[0] >> 4) + 8);
		inf_strm.next_in += 2;
		inf_strm.avail_in -= 2;
	} else {
		/* Let this remain D1 for now -- it should never happen */
		D1(printk(KERN_DEBUG "inflate not skipping adler32\n"));
	}

	if (zlib_inflateInit2(&inf_strm, wbits) != Z_OK) {
		printk(KERN_WARNING "inflateInit failed\n");
		return 1;
	}
	while((ret = zlib_inflate(&inf_strm, Z_FINISH)) == Z_OK)
		;
	if (ret != Z_STREAM_END)
		printk(KERN_NOTICE "inflate returned %d\n", ret);
	zlib_inflateEnd(&inf_strm);
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