[PATCH v6 23/23] Documentation/zram: add documentation for algorithm parameters

[Sergey Senozhatsky]

Document brief description of compression algorithms' parameters:
compression level and pre-trained dictionary.

Signed-off-by: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
---
 Documentation/admin-guide/blockdev/zram.rst | 45 ++++++++++++++++-----
 1 file changed, 36 insertions(+), 9 deletions(-)

diff --git a/Documentation/admin-guide/blockdev/zram.rst b/Documentation/admin-guide/blockdev/zram.rst
index 091e8bb38887..e3cdbf4ba61f 100644
--- a/Documentation/admin-guide/blockdev/zram.rst
+++ b/Documentation/admin-guide/blockdev/zram.rst
@@ -102,15 +102,33 @@ Examples::
 	#select lzo compression algorithm
 	echo lzo > /sys/block/zram0/comp_algorithm
 
-For the time being, the `comp_algorithm` content does not necessarily
-show every compression algorithm supported by the kernel. We keep this
-list primarily to simplify device configuration and one can configure
-a new device with a compression algorithm that is not listed in
-`comp_algorithm`. The thing is that, internally, ZRAM uses Crypto API
-and, if some of the algorithms were built as modules, it's impossible
-to list all of them using, for instance, /proc/crypto or any other
-method. This, however, has an advantage of permitting the usage of
-custom crypto compression modules (implementing S/W or H/W compression).
+For the time being, the `comp_algorithm` content shows only compression
+algorithms that are supported by zram.
+
+It is also possible to pass algorithm specific configuration parameters::
+
+	#set compression level to 8
+	echo "zstd level=8" > /sys/block/zram0/comp_algorithm
+
+Note that `comp_algorithm` also supports `algo=name` format::
+
+	#set compression level to 8
+	echo "algo=zstd level=8" > /sys/block/zram0/comp_algorithm
+
+Certain compression algorithms support pre-trained dictionaries, which
+significantly change algorithms' characteristics. In order to configure
+compression algorithm to use external pre-trained dictionary, pass full
+path to the dictionary along with other parameters::
+
+	#pass path to pre-trained dictionary
+	echo "algo=zstd dict=/etc/dictioary" > /sys/block/zram0/comp_algorithm
+
+Parameters are algorithm specific: not all algorithms support pre-trained
+dictionaries, not all algorithms support 'level'. Furthermore, for certain
+algorithms 'level' controls the compression level (the higher the value the
+better the compression ratio, it even can take negatives values for some
+algorithms), for other algorithms 'level' is acceleration level (the higher
+the value the lower the compression ratio).
 
 4) Set Disksize
 ===============
@@ -442,6 +460,15 @@ configuration:::
 	#select deflate recompression algorithm, priority 2
 	echo "algo=deflate priority=2" > /sys/block/zramX/recomp_algorithm
 
+The `recomp_algorithm` also supports algorithm configuration parameters, e.g.
+compression level and pre-trained dircionary::
+
+	#pass compression level
+	echo "algo=zstd level=8" > /sys/block/zramX/recomp_algorithm
+
+	#pass path to pre-trained dictionary
+	echo "algo=zstd dict=/etc/dictioary" > /sys/block/zramX/recomp_algorithm
+
 Another device attribute that CONFIG_ZRAM_MULTI_COMP enables is recompress,
 which controls recompression.
 
-- 
2.45.2.993.g49e7a77208-goog