Commit 71179a5e authored by Enzo Matsumiya's avatar Enzo Matsumiya Committed by Steve French
Browse files

smb: client: compress: add code docs to lz77.c 



Document parts of the code, especially the apparently
non-sense parts.

Other:
- change pointer increment constants to sizeof() values

Signed-off-by: default avatarEnzo Matsumiya <ematsumiya@suse.de>
Signed-off-by: default avatarSteve French <stfrench@microsoft.com>
parent 4460e9c6

fs/smb/client/compress/lz77.c

+73 −8
Original line number Diff line number Diff line
@@ -15,6 +15,20 @@ 


/*

 * Compression parameters.

 *

 * LZ77_MATCH_MAX_DIST:		Farthest back a match can be from current position (can be 1 - 8K).

 * LZ77_HASH_LOG:

 * LZ77_HASH_SIZE:		ilog2 hash size (recommended to be 13 - 18, default 15 (hash size

 *				32k)).

 * LZ77_RSTEP_SIZE:		Number of bytes to read from input buffer for hashing and initial

 *				match check (default 4 bytes, this effectivelly makes this the min

 *				match len).

 * LZ77_MSTEP_SIZE:		Number of bytes to extend-compare a found match (default 8 bytes).

 * LZ77_SKIP_TRIGGER:		ilog2 value for adaptive skipping, i.e. to progressively skip input

 *				bytes when we can't find matches.  Default is 4.

 *				Higher values (>0) will decrease compression time, but will result

 *				in worse compression ratio.  Lower values will give better

 *				compression ratio (more matches found), but will increase time.

 */

#define LZ77_MATCH_MAX_DIST	SZ_8K

#define LZ77_HASH_LOG		15
@@ -86,6 +100,19 @@ static __always_inline u32 lz77_match_len(const void *match, const void *cur, co 
	return (cur - start);

}



/**

 * lz77_encode_match() - Match encoding.

 * @dst:	compressed buffer

 * @nib:	pointer to an address in @dst

 * @dist:	match distance

 * @len:	match length

 *

 * Assumes all args were previously checked.

 *

 * Return: @dst advanced to new position

 *

 * Ref: MS-XCA 2.3.4 "Plain LZ77 Compression Algorithm Details" - "Processing"

 */

static __always_inline void *lz77_encode_match(void *dst, void **nib, u16 dist, u32 len)

{

	len -= 3;
@@ -95,12 +122,12 @@ static __always_inline void *lz77_encode_match(void *dst, void **nib, u16 dist, 
	if (len < 7) {

		lz77_write16(dst, dist + len);



		return dst + 2;

		return dst + sizeof(u16);

	}



	dist |= 7;

	lz77_write16(dst, dist);

	dst += 2;

	dst += sizeof(u16);

	len -= 7;



	if (!*nib) {
@@ -130,16 +157,32 @@ static __always_inline void *lz77_encode_match(void *dst, void **nib, u16 dist, 
	if (len <= 0xffff) {

		lz77_write16(dst, len);



		return dst + 2;

		return dst + sizeof(u16);

	}



	lz77_write16(dst, 0);

	dst += 2;

	dst += sizeof(u16);

	lz77_write32(dst, len);



	return dst + 4;

	return dst + sizeof(u32);

}



/**

 * lz77_encode_literals() - Literals encoding.

 * @start:	where to start copying literals (uncompressed buffer)

 * @end:	when to stop copying (uncompressed buffer)

 * @dst:	compressed buffer

 * @f:		pointer to current flag value

 * @fc:		pointer to current flag count

 * @fp:		pointer to current flag address

 *

 * Batch copy literals from @start to @dst, updating flag values accordingly.

 * Assumes all args were previously checked.

 *

 * Return: @dst advanced to new position

 *

 * MS-XCA 2.3.4 "Plain LZ77 Compression Algorithm Details" - "Processing"

 */

static __always_inline void *lz77_encode_literals(const void *start, const void *end, void *dst,

						  long *f, u32 *fc, void **fp)

{
@@ -160,7 +203,7 @@ static __always_inline void *lz77_encode_literals(const void *start, const void 
			lz77_write32(*fp, *f);

			*fc = 0;

			*fp = dst;

			dst += 4;

			dst += sizeof(u32);

		}

	} while (start < end);
@@ -188,7 +231,7 @@ noinline int lz77_compress(const void *src, const u32 slen, void *dst, u32 *dlen 
	rlim = end - LZ77_MSTEP_SIZE; /* read limit (for lz77_match_len()) */

	dstp = dst;

	flag_pos = dstp;

	dstp += 4;

	dstp += sizeof(u32);

	nib = NULL;



	htable = kvcalloc(LZ77_HASH_SIZE, sizeof(*htable), GFP_KERNEL);
@@ -197,6 +240,10 @@ noinline int lz77_compress(const void *src, const u32 slen, void *dst, u32 *dlen 


	LZ77_PREFETCH(srcp + LZ77_RSTEP_SIZE);



	/*

	 * Adjust @srcp so we don't get a false positive match on first iteration.

	 * Then prepare hash for first loop iteration (don't advance @srcp again).

	 */

	hash = lz77_hash(lz77_read32(srcp++));

	htable[hash] = 0;

	hash = lz77_hash(lz77_read32(srcp));
@@ -219,6 +266,14 @@ noinline int lz77_compress(const void *src, const u32 slen, void *dst, u32 *dlen 


			srcp = next;

			next += step;



			/*

			 * Adaptive skipping.

			 *

			 * Increment @step every (1 << LZ77_SKIP_TRIGGER, 16 in our case) bytes

			 * without a match.

			 * Reset to 1 when a match is found.

			 */

			step = (skip++ >> LZ77_SKIP_TRIGGER);

			if (unlikely(next > rlim))

				goto out;
@@ -229,6 +284,16 @@ noinline int lz77_compress(const void *src, const u32 slen, void *dst, u32 *dlen 
		} while (likely(match + LZ77_MATCH_MAX_DIST < srcp) ||

			 lz77_read32(match) != lz77_read32(srcp));



		/*

		 * Match found.  Warm/cold path; begin parsing @srcp and writing to @dstp:

		 * - flush literals

		 * - compute match length (*)

		 * - encode match

		 *

		 * (*) Current minimum match length is defined by the memory read size above, so

		 * here we already know that we have 4 matching bytes, but it's just faster to

		 * redundantly compute it again in lz77_match_len() than to adjust pointers/len.

		 */

		dstp = lz77_encode_literals(anchor, srcp, dstp, &flag, &flag_count, &flag_pos);

		len = lz77_match_len(match, srcp, end);

		dstp = lz77_encode_match(dstp, &nib, srcp - match, len);
@@ -243,7 +308,7 @@ noinline int lz77_compress(const void *src, const u32 slen, void *dst, u32 *dlen 
			lz77_write32(flag_pos, flag);

			flag_count = 0;

			flag_pos = dstp;

			dstp += 4;

			dstp += sizeof(u32);

		}



		if (unlikely(srcp > rlim))