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Commit 2a1f7c0c authored by Qu Wenruo's avatar Qu Wenruo Committed by David Sterba
Browse files

btrfs: lzo: document the compressed data format 



Although it's not that complex, but such comment could still save
several minutes for newer reader/reviewer instead of inferring that from
the code.

Signed-off-by: default avatarQu Wenruo <wqu@suse.com>
Reviewed-by: default avatarDavid Sterba <dsterba@suse.com>
[ minor wording updates ]
Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
parent d5c1d68f

fs/btrfs/lzo.c

+37 −0
Original line number Diff line number Diff line
@@ -17,6 +17,43 @@ 


#define LZO_LEN	4



/*

 * Btrfs LZO compression format

 *

 * Regular and inlined LZO compressed data extents consist of:

 *

 * 1.  Header

 *     Fixed size. LZO_LEN (4) bytes long, LE32.

 *     Records the total size (including the header) of compressed data.

 *

 * 2.  Segment(s)

 *     Variable size. Each segment includes one segment header, followd by data

 *     payload.

 *     One regular LZO compressed extent can have one or more segments.

 *     For inlined LZO compressed extent, only one segment is allowed.

 *     One segment represents at most one page of uncompressed data.

 *

 * 2.1 Segment header

 *     Fixed size. LZO_LEN (4) bytes long, LE32.

 *     Records the total size of the segment (not including the header).

 *     Segment header never crosses page boundary, thus it's possible to

 *     have at most 3 padding zeros at the end of the page.

 *

 * 2.2 Data Payload

 *     Variable size. Size up limit should be lzo1x_worst_compress(PAGE_SIZE)

 *     which is 4419 for a 4KiB page.

 *

 * Example:

 * Page 1:

 *          0     0x2   0x4   0x6   0x8   0xa   0xc   0xe     0x10

 * 0x0000   |  Header   | SegHdr 01 | Data payload 01 ...     |

 * ...

 * 0x0ff0   | SegHdr  N | Data payload  N     ...          |00|

 *                                                          ^^ padding zeros

 * Page 2:

 * 0x1000   | SegHdr N+1| Data payload N+1 ...                |

 */



struct workspace {

	void *mem;

	void *buf;	/* where decompressed data goes */