lib/lzo: implement run-length encoding

[sfrench/cifs-2.6.git] / Documentation / lzo.txt

diff --git a/Documentation/lzo.txt b/Documentation/lzo.txt
index 6fa6a93d094971ec2721c47f5bf4d790d3f21cd9..306c60344ca7c3b5e2082d0bef11aa17f2f5e29c 100644 (file)
--- a/Documentation/lzo.txt
+++ b/Documentation/lzo.txt
@@ -78,16 +78,30 @@ Description
      is an implementation design choice independent on the algorithm or
      encoding.
 
+Versions
+
+0: Original version
+1: LZO-RLE
+
+Version 1 of LZO implements an extension to encode runs of zeros using run
+length encoding. This improves speed for data with many zeros, which is a
+common case for zram. This modifies the bitstream in a backwards compatible way
+(v1 can correctly decompress v0 compressed data, but v0 cannot read v1 data).
+
 Byte sequences
 ==============
 
   First byte encoding::
 
-      0..17   : follow regular instruction encoding, see below. It is worth
-                noting that codes 16 and 17 will represent a block copy from
-                the dictionary which is empty, and that they will always be
+      0..16   : follow regular instruction encoding, see below. It is worth
+                noting that code 16 will represent a block copy from the
+                dictionary which is empty, and that it will always be
                 invalid at this place.
 
+      17      : bitstream version. If the first byte is 17, the next byte
+                gives the bitstream version. If the first byte is not 17,
+                the bitstream version is 0.
+
       18..21  : copy 0..3 literals
                 state = (byte - 17) = 0..3  [ copy <state> literals ]
                 skip byte
@@ -140,6 +154,11 @@ Byte sequences
            state = S (copy S literals after this block)
            End of stream is reached if distance == 16384
 
+        In version 1, this instruction is also used to encode a run of zeros if
+        distance = 0xbfff, i.e. H = 1 and the D bits are all 1.
+           In this case, it is followed by a fourth byte, X.
+           run length = ((X << 3) | (0 0 0 0 0 L L L)) + 4.
+
       0 0 1 L L L L L  (32..63)
            Copy of small block within 16kB distance (preferably less than 34B)
            length = 2 + (L ?: 31 + (zero_bytes * 255) + non_zero_byte)
@@ -165,7 +184,9 @@ Authors
 =======
 
   This document was written by Willy Tarreau <w@1wt.eu> on 2014/07/19 during an
-  analysis of the decompression code available in Linux 3.16-rc5. The code is
-  tricky, it is possible that this document contains mistakes or that a few
-  corner cases were overlooked. In any case, please report any doubt, fix, or
-  proposed updates to the author(s) so that the document can be updated.
+  analysis of the decompression code available in Linux 3.16-rc5, and updated
+  by Dave Rodgman <dave.rodgman@arm.com> on 2018/10/30 to introduce run-length
+  encoding. The code is tricky, it is possible that this document contains
+  mistakes or that a few corner cases were overlooked. In any case, please
+  report any doubt, fix, or proposed updates to the author(s) so that the
+  document can be updated.