xz.txt: standardize document format

Each text file under Documentation follows a different
format. Some doesn't even have titles!

Change its representation to follow the adopted standard,
using ReST markups for it to be parseable by Sphinx:
- Use marks for titles;
- Adjust indentation.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/xz.txt

-
+============================
 XZ data compression in Linux
 ============================
 
 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.
+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
-    <http://tukaani.org/xz/embedded.html>, where you can find the
-    latest version and also information about using the code outside
-    the Linux kernel.
+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
+<http://tukaani.org/xz/embedded.html>, where you can find the
+latest version and also 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
-    <http://tukaani.org/xz/>.
+For userspace, XZ Utils provide a zlib-like compression library
+and a gzip-like command line tool. XZ Utils can be downloaded from
+<http://tukaani.org/xz/>.
 
 XZ related components in the kernel
-
-    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, 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 $(call if_needed,xzmisc)
-    which will use no BCJ filter and 1 MiB LZMA2 dictionary.
+===================================
+
+The xz_dec module provides XZ decompressor with single-call (buffer
+to buffer) and multi-call (stateful) APIs. The usage of the xz_dec
+module is documented in include/linux/xz.h.
+
+The xz_dec_test module is for testing xz_dec. xz_dec_test is not
+useful unless you are hacking the XZ decompressor. xz_dec_test
+allocates a char device major dynamically to which one can write
+.xz files from userspace. The decompressed output is thrown away.
+Keep an eye on dmesg to see diagnostics printed by xz_dec_test.
+See the xz_dec_test source code for the details.
+
+For decompressing the kernel image, initramfs, and initrd, there
+is a wrapper function in lib/decompress_unxz.c. Its API is the
+same as in other decompress_*.c files, which is defined in
+include/linux/decompress/generic.h.
+
+scripts/xz_wrap.sh is a wrapper for the xz command line tool found
+from XZ Utils. The wrapper sets compression options to values suitable
+for compressing the kernel image.
+
+For kernel makefiles, two commands are provided for use with
+$(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.
 
 Notes on compression options
+============================
 
-    Since the XZ Embedded supports only streams with no integrity check or
-    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.
-
-    Using CRC32 is strongly recommended unless there is some other layer
-    which will verify the integrity of the uncompressed data anyway.
-    Double checking the integrity would probably be waste of CPU cycles.
-    Note that the headers will always have a CRC32 which will be validated
-    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:
+Since the XZ Embedded supports only streams with no integrity check or
+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.
+
+Using CRC32 is strongly recommended unless there is some other layer
+which will verify the integrity of the uncompressed data anyway.
+Double checking the integrity would probably be waste of CPU cycles.
+Note that the headers will always have a CRC32 which will be validated
+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.
+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.
+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.
+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.
+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 <http://tukaani.org/xz/embedded.html> to get the
-    latest code.
+Before reporting a bug, please check that it's not fixed already
+at upstream. See <http://tukaani.org/xz/embedded.html> to get the
+latest code.
 
-    Report bugs to <lasse.collin@tukaani.org> 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.
+Report bugs to <lasse.collin@tukaani.org> 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.
+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.