doc: add compression level guide for ISA-L

This expands the ISA-L documentation to give more a detailed explanation
of the mapping between the compressdev API levels and PMD levels.

Signed-off-by: Lee Daly <lee.daly@intel.com>
Acked-by: Marko Kovacevic <marko.kovacevic@intel.com>

diff --git a/doc/guides/compressdevs/isal.rst b/doc/guides/compressdevs/isal.rst
index 2024437..276ff06 100644 (file)
--- a/doc/guides/compressdevs/isal.rst
+++ b/doc/guides/compressdevs/isal.rst
@@ -6,7 +6,8 @@ ISA-L Compression Poll Mode Driver
 
 The ISA-L PMD (**librte_pmd_isal_comp**) provides poll mode compression &
 decompression driver support for utilizing Intel ISA-L library,
-which implements the deflate algorithm for both compression and decompression
+which implements the deflate algorithm for both Deflate(compression) and Inflate(decompression).
+
 
 Features
 --------
@@ -15,25 +16,73 @@ ISA-L PMD has support for:
 
 Compression/Decompression algorithm:
 
-* DEFLATE
+    * DEFLATE
 
 Huffman code type:
 
-* FIXED
-* DYNAMIC
+    * FIXED
+    * DYNAMIC
 
 Window size support:
 
-* 32K
+    * 32K
+
+Level guide:
+
+The ISA-L levels have been mapped to somewhat correspond to the same ZLIB level,
+i.e. ZLIB L1 gives a compression ratio similar to ISA-L L1.
+Compressdev level 0 enables "No Compression", which passes the uncompressed
+data to the output buffer, plus deflate headers.
+The ISA-L library does not support this, therefore compressdev level 0 is not supported.
+
+The compressdev API has 10 levels, 0-9. ISA-L has 4 levels of compression, 0-3.
+As a result the level mappings from the API to the PMD are shown below.
+
+.. _table_ISA-L_compression_levels:
+
+.. table:: Level mapping from Compressdev to ISA-L PMD.
+
+   +-------------+----------------------------------------------+-----------------------------------------------+
+   | Compressdev | PMD Functionality                            | Internal ISA-L                                |
+   | API Level   |                                              | Level                                         |
+   +=============+==============================================+===============================================+
+   | 0           | No compression, Not Supported                | ---                                           |
+   +-------------+----------------------------------------------+-----------------------------------------------+
+   | 1           | Dynamic (Fast compression)                   | 1                                             |
+   +-------------+----------------------------------------------+-----------------------------------------------+
+   | 2           | Dynamic                                      | 2                                             |
+   |             | (Higher compression ratio)                   |                                               |
+   +-------------+----------------------------------------------+-----------------------------------------------+
+   | 3           | Dynamic                                      | 3                                             |
+   |             | (Best compression ratio)                     | (Level 2 if                                   |
+   |             |                                              | no AVX512/AVX2)                               |
+   +-------------+----------------------------------------------+-----------------------------------------------+
+   | 4           | Dynamic (Best compression ratio)             | Same as above                                 |
+   +-------------+----------------------------------------------+-----------------------------------------------+
+   | 5           | Dynamic (Best compression ratio)             | Same as above                                 |
+   +-------------+----------------------------------------------+-----------------------------------------------+
+   | 6           | Dynamic (Best compression ratio)             | Same as above                                 |
+   +-------------+----------------------------------------------+-----------------------------------------------+
+   | 7           | Dynamic (Best compression ratio)             | Same as above                                 |
+   +-------------+----------------------------------------------+-----------------------------------------------+
+   | 8           | Dynamic (Best compression ratio)             | Same as above                                 |
+   +-------------+----------------------------------------------+-----------------------------------------------+
+   | 9           | Dynamic (Best compression ratio)             | Same as above                                 |
+   +-------------+----------------------------------------------+-----------------------------------------------+
+
+.. Note::
+
+ The above table only shows mapping when API calls for dynamic compression.
+ For fixed compression, regardless of API level, internally ISA-L level 0 is always used.
 
 Limitations
 -----------
 
-* Chained mbufs are not supported, for future release.
+* Chained mbufs will not be supported until a future release, meaning max data size being passed to PMD through a single mbuf is 64K - 1. If data is larger than this it will need to be split up and sent as multiple operations.
 
-* Compressdev level 0, no compression, is not supported. ISA-L level 0 used for fixed huffman codes.
+* Compressdev level 0, no compression, is not supported.
 
-* Checksums are not supported, for future release.
+* Checksums will not be supported until future release.
 
 Installation
 ------------