[PATCH 04/11] xz: Documentation/staging/xz.rst: Revise thoroughly

[Lasse Collin]

Add SPDX license identifier.

Omit xz_dec_test info. That isn't relevant to developers of non-XZ code.

Revise the docs about xzkern and add xzkern_with_size. The latter was
added to scripts/Makefile.lib in the commit 7ce7e984ab2b218d ("kbuild:
rename cmd_{bzip2,lzma,lzo,lz4,xzkern,zstd22}").

Omit contact info as MAINTAINERS has it.

Omit other info that is outdated or not relevant in the kernel context.

Co-developed-by: Jia Tan <jiat0218@xxxxxxxxx>
Signed-off-by: Jia Tan <jiat0218@xxxxxxxxx>
Signed-off-by: Lasse Collin <lasse.collin@xxxxxxxxxxx>
---
 Documentation/staging/xz.rst | 130 +++++++++++++----------------------
 1 file changed, 46 insertions(+), 84 deletions(-)

diff --git a/Documentation/staging/xz.rst b/Documentation/staging/xz.rst
index b2f5ff12a161..301b1eeff411 100644
--- a/Documentation/staging/xz.rst
+++ b/Documentation/staging/xz.rst
@@ -1,3 +1,5 @@
+.. SPDX-License-Identifier: 0BSD
+
 ============================
 XZ data compression in Linux
 ============================
@@ -6,21 +8,17 @@ Introduction
 ============
 
 XZ is a general purpose data compression format with high compression
-ratio and relatively fast decompression. The primary compression
-algorithm (filter) is LZMA2. Additional filters can be used to improve
-compression ratio even further. E.g. Branch/Call/Jump (BCJ) filters
-improve compression ratio of executable data.
-
-The XZ decompressor in Linux is called XZ Embedded. It supports
-the LZMA2 filter and optionally also BCJ filters. CRC32 is supported
-for integrity checking. The home page of XZ Embedded is at
-<https://tukaani.org/xz/embedded.html>, where you can find the
-latest version and also information about using the code outside
-the Linux kernel.
+ratio. The XZ decompressor in Linux is called XZ Embedded. It supports
+the LZMA2 filter and optionally also Branch/Call/Jump (BCJ) filters
+for executable code. CRC32 is supported for integrity checking. The
+home page of XZ Embedded is at <https://xz.tukaani.org/xz-embedded/>.
+There you can find the latest version which includes a few optional
+extra features that aren't required in the Linux kernel and information
+about using the code outside the Linux kernel.
 
 For userspace, XZ Utils provide a zlib-like compression library
 and a gzip-like command line tool. XZ Utils can be downloaded from
-<https://tukaani.org/xz/>.
+<https://xz.tukaani.org/xz-utils/>.
 
 XZ related components in the kernel
 ===================================
@@ -29,29 +27,24 @@ The xz_dec module provides XZ decompressor with single-call (buffer
 to buffer) and multi-call (stateful) APIs. The usage of the xz_dec
 module is documented in include/linux/xz.h.
 
-The xz_dec_test module is for testing xz_dec. xz_dec_test is not
-useful unless you are hacking the XZ decompressor. xz_dec_test
-allocates a char device major dynamically to which one can write
-.xz files from userspace. The decompressed output is thrown away.
-Keep an eye on dmesg to see diagnostics printed by xz_dec_test.
-See the xz_dec_test source code for the details.
-
 For decompressing the kernel image, initramfs, and initrd, there
 is a wrapper function in lib/decompress_unxz.c. Its API is the
 same as in other decompress_*.c files, which is defined in
 include/linux/decompress/generic.h.
 
-scripts/xz_wrap.sh is a wrapper for the xz command line tool found
-from XZ Utils. The wrapper sets compression options to values suitable
-for compressing the kernel image.
+For kernel makefiles, three commands are provided for use with
+$(call if_changed). They require the xz tool from XZ Utils.
+
+  - $(call if_changed,xzkern) is for compressing the kernel image.
+    It runs the script scripts/xz_wrap.sh which uses arch-optimized
+    options and a big LZMA2 dictionary.
 
-For kernel makefiles, two commands are provided for use with
-$(call if_needed). The kernel image should be compressed with
-$(call if_needed,xzkern) which will use a BCJ filter and a big LZMA2
-dictionary. It will also append a four-byte trailer containing the
-uncompressed size of the file, which is needed by the boot code.
-Other things should be compressed with $(call if_needed,xzmisc)
-which will use no BCJ filter and 1 MiB LZMA2 dictionary.
+  - $(call if_changed,xzkern_with_size) is like xzkern above but this
+    also appends a four-byte trailer containing the uncompressed size
+    of the file, which is needed by the boot code on some archs.
+
+  - Other things can be compressed with $(call if_needed,xzmisc)
+    which will use no BCJ filter and 1 MiB LZMA2 dictionary.
 
 Notes on compression options
 ============================
@@ -61,7 +54,7 @@ CRC32, make sure that you don't use some other integrity check type
 when encoding files that are supposed to be decoded by the kernel. With
 liblzma, you need to use either LZMA_CHECK_NONE or LZMA_CHECK_CRC32
 when encoding. With the xz command line tool, use --check=none or
---check=crc32.
+--check=crc32 to override the default --check=crc64.
 
 Using CRC32 is strongly recommended unless there is some other layer
 which will verify the integrity of the uncompressed data anyway.
@@ -71,57 +64,26 @@ by the decoder; you can only change the integrity check type (or
 disable it) for the actual uncompressed data.
 
 In userspace, LZMA2 is typically used with dictionary sizes of several
-megabytes. The decoder needs to have the dictionary in RAM, thus big
-dictionaries cannot be used for files that are intended to be decoded
-by the kernel. 1 MiB is probably the maximum reasonable dictionary
-size for in-kernel use (maybe more is OK for initramfs). The presets
-in XZ Utils may not be optimal when creating files for the kernel,
-so don't hesitate to use custom settings. Example::
-
-	xz --check=crc32 --lzma2=dict=512KiB inputfile
-
-An exception to above dictionary size limitation is when the decoder
-is used in single-call mode. Decompressing the kernel itself is an
-example of this situation. In single-call mode, the memory usage
-doesn't depend on the dictionary size, and it is perfectly fine to
-use a big dictionary: for maximum compression, the dictionary should
-be at least as big as the uncompressed data itself.
-
-Future plans
-============
-
-Creating a limited XZ encoder may be considered if people think it is
-useful. LZMA2 is slower to compress than e.g. Deflate or LZO even at
-the fastest settings, so it isn't clear if LZMA2 encoder is wanted
-into the kernel.
-
-Support for limited random-access reading is planned for the
-decompression code. I don't know if it could have any use in the
-kernel, but I know that it would be useful in some embedded projects
-outside the Linux kernel.
-
-Conformance to the .xz file format specification
-================================================
-
-There are a couple of corner cases where things have been simplified
-at expense of detecting errors as early as possible. These should not
-matter in practice all, since they don't cause security issues. But
-it is good to know this if testing the code e.g. with the test files
-from XZ Utils.
-
-Reporting bugs
-==============
-
-Before reporting a bug, please check that it's not fixed already
-at upstream. See <https://tukaani.org/xz/embedded.html> to get the
-latest code.
-
-Report bugs to <lasse.collin@xxxxxxxxxxx> or visit #tukaani on
-Freenode and talk to Larhzu. I don't actively read LKML or other
-kernel-related mailing lists, so if there's something I should know,
-you should email to me personally or use IRC.
-
-Don't bother Igor Pavlov with questions about the XZ implementation
-in the kernel or about XZ Utils. While these two implementations
-include essential code that is directly based on Igor Pavlov's code,
-these implementations aren't maintained nor supported by him.
+megabytes. The decoder needs to have the dictionary in RAM:
+
+  - In multi-call mode the dictionary is allocated as part of the
+    decoder state. The reasonable maximum dictionary size for in-kernel
+    use will depend on the target hardware: a few megabytes should be
+    fine for desktop systems while 64 KiB to 1 MiB might be more
+    appropriate on some embedded systems.
+
+  - In single-call mode the output buffer is used as the dictionary
+    buffer. That is, the size of the dictionary doesn't affect the
+    decompressor memory usage at all. Only the base data structures
+    are allocated which take a little less than 30 KiB of memory.
+    For the best compression, the dictionary should be at least
+    as big as the uncompressed data. A notable example of single-call
+    mode is decompressing the kernel itself (except on PowerPC).
+
+The compression presets in XZ Utils may not be optimal when creating
+files for the kernel, so don't hesitate to use custom settings to,
+for example, set the dictionary size. Also, xz may produce a smaller
+file in single-threaded mode so setting that explicitly is recommended.
+Example::
+
+    xz --threads=1 --check=crc32 --lzma2=dict=512KiB inputfile
-- 
2.44.0