Documentation: add meta-documentation for Sphinx and kernel-doc

Describe Sphinx, reStructuredText, the kernel-doc extension, the
kernel-doc structured documentation comments, etc.

The kernel-doc parts are based on kernel-doc-nano-HOWTO.txt, by Tim
<[e-mail]>.

Signed-off-by: Jani Nikula <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/index.rst

···
 .. toctree::
    :maxdepth: 2
 
+   kernel-documentation
 
 Indices and tables
 ==================

Documentation/kernel-documentation.rst

···
+==========================
+Linux Kernel Documentation
+==========================
+
+Introduction
+============
+
+The Linux kernel uses `Sphinx`_ to generate pretty documentation from
+`reStructuredText`_ files under ``Documentation``. To build the documentation in
+HTML or PDF formats, use ``make htmldocs`` or ``make pdfdocs``. The generated
+documentation is placed in ``Documentation/output``.
+
+.. _Sphinx: http://www.sphinx-doc.org/
+.. _reStructuredText: http://docutils.sourceforge.net/rst.html
+
+The reStructuredText files may contain directives to include structured
+documentation comments, or kernel-doc comments, from source files. Usually these
+are used to describe the functions and types and design of the code. The
+kernel-doc comments have some special structure and formatting, but beyond that
+they are also treated as reStructuredText.
+
+There is also the deprecated DocBook toolchain to generate documentation from
+DocBook XML template files under ``Documentation/DocBook``. The DocBook files
+are to be converted to reStructuredText, and the toolchain is slated to be
+removed.
+
+Finally, there are thousands of plain text documentation files scattered around
+``Documentation``. Some of these will likely be converted to reStructuredText
+over time, but the bulk of them will remain in plain text.
+
+Sphinx Build
+============
+
+The usual way to generate the documentation is to run ``make htmldocs`` or
+``make pdfdocs``. There are also other formats available, see the documentation
+section of ``make help``. The generated documentation is placed in
+format-specific subdirectories under ``Documentation/output``.
+
+To generate documentation, Sphinx (``sphinx-build``) must obviously be
+installed. For prettier HTML output, the Read the Docs Sphinx theme
+(``sphinx_rtd_theme``) is used if available. For PDF output, ``rst2pdf`` is also
+needed. All of these are widely available and packaged in distributions.
+
+To pass extra options to Sphinx, you can use the ``SPHINXOPTS`` make
+variable. For example, use ``make SPHINXOPTS=-v htmldocs`` to get more verbose
+output.
+
+To remove the generated documentation, run ``make cleandocs``.
+
+Writing Documentation
+=====================
+
+Adding new documentation can be as simple as:
+
+1. Add a new ``.rst`` file somewhere under ``Documentation``.
+2. Refer to it from the Sphinx main `TOC tree`_ in ``Documentation/index.rst``.
+
+.. _TOC tree: http://www.sphinx-doc.org/en/stable/markup/toctree.html
+
+This is usually good enough for simple documentation (like the one you're
+reading right now), but for larger documents it may be advisable to create a
+subdirectory (or use an existing one). For example, the graphics subsystem
+documentation is under ``Documentation/gpu``, split to several ``.rst`` files,
+and has a separate ``index.rst`` (with a ``toctree`` of its own) referenced from
+the main index.
+
+See the documentation for `Sphinx`_ and `reStructuredText`_ on what you can do
+with them. In particular, the Sphinx `reStructuredText Primer`_ is a good place
+to get started with reStructuredText. There are also some `Sphinx specific
+markup constructs`_.
+
+.. _reStructuredText Primer: http://www.sphinx-doc.org/en/stable/rest.html
+.. _Sphinx specific markup constructs: http://www.sphinx-doc.org/en/stable/markup/index.html
+
+Specific guidelines for the kernel documentation
+------------------------------------------------
+
+Here are some specific guidelines for the kernel documentation:
+
+* Please don't go overboard with reStructuredText markup. Keep it simple.
+
+* Please stick to this order of heading adornments:
+
+  1. ``=`` with overline for document title::
+
+       ==============
+       Document title
+       ==============
+
+  2. ``=`` for chapters::
+
+       Chapters
+       ========
+
+  3. ``-`` for sections::
+
+       Section
+       -------
+
+  4. ``~`` for subsections::
+
+       Subsection
+       ~~~~~~~~~~
+
+  Although RST doesn't mandate a specific order ("Rather than imposing a fixed
+  number and order of section title adornment styles, the order enforced will be
+  the order as encountered."), having the higher levels the same overall makes
+  it easier to follow the documents.
+
+
+Including kernel-doc comments
+=============================
+
+The Linux kernel source files may contain structured documentation comments, or
+kernel-doc comments to describe the functions and types and design of the
+code. The documentation comments may be included to any of the reStructuredText
+documents using a dedicated kernel-doc Sphinx directive extension.
+
+The kernel-doc directive is of the format::
+
+  .. kernel-doc:: source
+     :option:
+
+The *source* is the path to a source file, relative to the kernel source
+tree. The following directive options are supported:
+
+export: *[source-pattern ...]*
+  Include documentation for all functions in *source* that have been exported
+  using ``EXPORT_SYMBOL`` or ``EXPORT_SYMBOL_GPL`` either in *source* or in any
+  of the files specified by *source-pattern*.
+
+  The *source-pattern* is useful when the kernel-doc comments have been placed
+  in header files, while ``EXPORT_SYMBOL`` and ``EXPORT_SYMBOL_GPL`` are next to
+  the function definitions.
+
+  Examples::
+
+    .. kernel-doc:: lib/bitmap.c
+       :export:
+
+    .. kernel-doc:: include/net/mac80211.h
+       :export: net/mac80211/*.c
+
+internal: *[source-pattern ...]*
+  Include documentation for all functions and types in *source* that have
+  **not** been exported using ``EXPORT_SYMBOL`` or ``EXPORT_SYMBOL_GPL`` either
+  in *source* or in any of the files specified by *source-pattern*.
+
+  Example::
+
+    .. kernel-doc:: drivers/gpu/drm/i915/intel_audio.c
+       :internal:
+
+doc: *title*
+  Include documentation for the ``DOC:`` paragraph identified by *title* in
+  *source*. Spaces are allowed in *title*; do not quote the *title*. The *title*
+  is only used as an identifier for the paragraph, and is not included in the
+  output. Please make sure to have an appropriate heading in the enclosing
+  reStructuredText document.
+
+  Example::
+
+    .. kernel-doc:: drivers/gpu/drm/i915/intel_audio.c
+       :doc: High Definition Audio over HDMI and Display Port
+
+functions: *function* *[...]*
+  Include documentation for each *function* in *source*.
+
+  Example::
+
+    .. kernel-doc:: lib/bitmap.c
+       :functions: bitmap_parselist bitmap_parselist_user
+
+Without options, the kernel-doc directive includes all documentation comments
+from the source file.
+
+The kernel-doc extension is included in the kernel source tree, at
+``Documentation/sphinx/kernel-doc.py``. Internally, it uses the
+``scripts/kernel-doc`` script to extract the documentation comments from the
+source.
+
+Writing kernel-doc comments
+===========================
+
+In order to provide embedded, "C" friendly, easy to maintain, but consistent and
+extractable overview, function and type documentation, the Linux kernel has
+adopted a consistent style for documentation comments. The format for this
+documentation is called the kernel-doc format, described below. This style
+embeds the documentation within the source files, using a few simple conventions
+for adding documentation paragraphs and documenting functions and their
+parameters, structures and unions and their members, enumerations, and typedefs.
+
+.. note:: The kernel-doc format is deceptively similar to gtk-doc or Doxygen,
+   yet distinctively different, for historical reasons. The kernel source
+   contains tens of thousands of kernel-doc comments. Please stick to the style
+   described here.
+
+The ``scripts/kernel-doc`` script is used by the Sphinx kernel-doc extension in
+the documentation build to extract this embedded documentation into the various
+HTML, PDF, and other format documents.
+
+In order to provide good documentation of kernel functions and data structures,
+please use the following conventions to format your kernel-doc comments in the
+Linux kernel source.
+
+How to format kernel-doc comments
+---------------------------------
+
+The opening comment mark ``/**`` is reserved for kernel-doc comments. Only
+comments so marked will be considered by the ``kernel-doc`` tool. Use it only
+for comment blocks that contain kernel-doc formatted comments. The usual ``*/``
+should be used as the closing comment marker. The lines in between should be
+prefixed by `` * `` (space star space).
+
+The function and type kernel-doc comments should be placed just before the
+function or type being described. The overview kernel-doc comments may be freely
+placed at the top indentation level.
+
+Example kernel-doc function comment::
+
+  /**
+   * foobar() - Brief description of foobar.
+   * @arg: Description of argument of foobar.
+   *
+   * Longer description of foobar.
+   *
+   * Return: Description of return value of foobar.
+   */
+  int foobar(int arg)
+
+The format is similar for documentation for structures, enums, paragraphs,
+etc. See the sections below for details.
+
+The kernel-doc structure is extracted from the comments, and proper `Sphinx C
+Domain`_ function and type descriptions with anchors are generated for them. The
+descriptions are filtered for special kernel-doc highlights and
+cross-references. See below for details.
+
+.. _Sphinx C Domain: http://www.sphinx-doc.org/en/stable/domains.html
+
+Highlights and cross-references
+-------------------------------
+
+The following special patterns are recognized in the kernel-doc comment
+descriptive text and converted to proper reStructuredText markup and `Sphinx C
+Domain`_ references.
+
+.. attention:: The below are **only** recognized within kernel-doc comments,
+	       **not** within normal reStructuredText documents.
+
+``funcname()``
+  Function reference.
+
+``@parameter``
+  Name of a function parameter. (No cross-referencing, just formatting.)
+
+``%CONST``
+  Name of a constant. (No cross-referencing, just formatting.)
+
+``$ENVVAR``
+  Name of an environment variable. (No cross-referencing, just formatting.)
+
+``&struct name``
+  Structure reference.
+
+``&enum name``
+  Enum reference.
+
+``&typedef name``
+  Typedef reference.
+
+``&struct_name->member`` or ``&struct_name.member``
+  Structure or union member reference. The cross-reference will be to the struct
+  or union definition, not the member directly.
+
+``&name``
+  A generic type reference. Prefer using the full reference described above
+  instead. This is mostly for legacy comments.
+
+Cross-referencing from reStructuredText
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+To cross-reference the functions and types defined in the kernel-doc comments
+from reStructuredText documents, please use the `Sphinx C Domain`_
+references. For example::
+
+  See function :c:func:`foo` and struct/union/enum/typedef :c:type:`bar`.
+
+While the type reference works with just the type name, without the
+struct/union/enum/typedef part in front, you may want to use::
+
+  See :c:type:`struct foo <foo>`.
+  See :c:type:`union bar <bar>`.
+  See :c:type:`enum baz <baz>`.
+  See :c:type:`typedef meh <meh>`.
+
+This will produce prettier links, and is in line with how kernel-doc does the
+cross-references.
+
+For further details, please refer to the `Sphinx C Domain`_ documentation.
+
+Function documentation
+----------------------
+
+The general format of a function and function-like macro kernel-doc comment is::
+
+  /**
+   * function_name() - Brief description of function.
+   * @arg1: Describe the first argument.
+   * @arg2: Describe the second argument.
+   *        One can provide multiple line descriptions
+   *        for arguments.
+   *
+   * A longer description, with more discussion of the function function_name()
+   * that might be useful to those using or modifying it. Begins with an
+   * empty comment line, and may include additional embedded empty
+   * comment lines.
+   *
+   * The longer description may have multiple paragraphs.
+   *
+   * Return: Describe the return value of foobar.
+   *
+   * The return value description can also have multiple paragraphs, and should
+   * be placed at the end of the comment block.
+   */
+
+The brief description following the function name may span multiple lines, and
+ends with an ``@argument:`` description, a blank comment line, or the end of the
+comment block.
+
+The kernel-doc function comments describe each parameter to the function, in
+order, with the ``@argument:`` descriptions. The ``@argument:`` descriptions
+must begin on the very next line following the opening brief function
+description line, with no intervening blank comment lines. The ``@argument:``
+descriptions may span multiple lines. The continuation lines may contain
+indentation. If a function parameter is ``...`` (varargs), it should be listed
+in kernel-doc notation as: ``@...:``.
+
+The return value, if any, should be described in a dedicated section at the end
+of the comment starting with "Return:".
+
+Structure, union, and enumeration documentation
+-----------------------------------------------
+
+The general format of a struct, union, and enum kernel-doc comment is::
+
+  /**
+   * struct struct_name - Brief description.
+   * @member_name: Description of member member_name.
+   *
+   * Description of the structure.
+   */
+
+Below, "struct" is used to mean structs, unions and enums, and "member" is used
+to mean struct and union members as well as enumerations in an enum.
+
+The brief description following the structure name may span multiple lines, and
+ends with a ``@member:`` description, a blank comment line, or the end of the
+comment block.
+
+The kernel-doc data structure comments describe each member of the structure, in
+order, with the ``@member:`` descriptions. The ``@member:`` descriptions must
+begin on the very next line following the opening brief function description
+line, with no intervening blank comment lines. The ``@member:`` descriptions may
+span multiple lines. The continuation lines may contain indentation.
+
+In-line member documentation comments
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The structure members may also be documented in-line within the definition::
+
+  /**
+   * struct foo - Brief description.
+   * @foo: The Foo member.
+   */
+  struct foo {
+        int foo;
+        /**
+         * @bar: The Bar member.
+         */
+        int bar;
+        /**
+         * @baz: The Baz member.
+         *
+         * Here, the member description may contain several paragraphs.
+         */
+        int baz;
+  }
+
+Private members
+~~~~~~~~~~~~~~~
+
+Inside a struct description, you can use the "private:" and "public:" comment
+tags. Structure fields that are inside a "private:" area are not listed in the
+generated output documentation.  The "private:" and "public:" tags must begin
+immediately following a ``/*`` comment marker.  They may optionally include
+comments between the ``:`` and the ending ``*/`` marker.
+
+Example::
+
+  /**
+   * struct my_struct - short description
+   * @a: first member
+   * @b: second member
+   *
+   * Longer description
+   */
+  struct my_struct {
+      int a;
+      int b;
+  /* private: internal use only */
+      int c;
+  };
+
+
+Typedef documentation
+---------------------
+
+The general format of a typedef kernel-doc comment is::
+
+  /**
+   * typedef type_name - Brief description.
+   *
+   * Description of the type.
+   */
+
+Overview documentation comments
+-------------------------------
+
+To facilitate having source code and comments close together, you can include
+kernel-doc documentation blocks that are free-form comments instead of being
+kernel-doc for functions, structures, unions, enums, or typedefs. This could be
+used for something like a theory of operation for a driver or library code, for
+example.
+
+This is done by using a ``DOC:`` section keyword with a section title.
+
+The general format of an overview or high-level documentation comment is::
+
+  /**
+   * DOC: Theory of Operation
+   *
+   * The whizbang foobar is a dilly of a gizmo. It can do whatever you
+   * want it to do, at any time. It reads your mind. Here's how it works.
+   *
+   * foo bar splat
+   *
+   * The only drawback to this gizmo is that is can sometimes damage
+   * hardware, software, or its subject(s).
+   */
+
+The title following ``DOC:`` acts as a heading within the source file, but also
+as an identifier for extracting the documentation comment. Thus, the title must
+be unique within the file.
+
+Recommendations
+---------------
+
+We definitely need kernel-doc formatted documentation for functions that are
+exported to loadable modules using ``EXPORT_SYMBOL`` or ``EXPORT_SYMBOL_GPL``.
+
+We also look to provide kernel-doc formatted documentation for functions
+externally visible to other kernel files (not marked "static").
+
+We also recommend providing kernel-doc formatted documentation for private (file
+"static") routines, for consistency of kernel source code layout. But this is
+lower priority and at the discretion of the MAINTAINER of that kernel source
+file.
+
+Data structures visible in kernel include files should also be documented using
+kernel-doc formatted comments.
+
+DocBook XML [DEPRECATED]
+========================
+
+.. attention::
+
+   This section describes the deprecated DocBook XML toolchain. Please do not
+   create new DocBook XML template files. Please consider converting existing
+   DocBook XML templates files to Sphinx/reStructuredText.
+
+Converting DocBook to Sphinx
+----------------------------
+
+Over time, we expect all of the documents under ``Documentation/DocBook`` to be
+converted to Sphinx and reStructuredText. For most DocBook XML documents, a good
+enough solution is to use the simple ``Documentation/sphinx/tmplcvt`` script,
+which uses ``pandoc`` under the hood. For example::
+
+  $ cd Documentation/sphinx
+  $ ./tmplcvt ../DocBook/in.tmpl ../out.rst
+
+Then edit the resulting rst files to fix any remaining issues, and add the
+document in the ``toctree`` in ``Documentation/index.rst``.
+
+Components of the kernel-doc system
+-----------------------------------
+
+Many places in the source tree have extractable documentation in the form of
+block comments above functions. The components of this system are:
+
+- ``scripts/kernel-doc``
+
+  This is a perl script that hunts for the block comments and can mark them up
+  directly into reStructuredText, DocBook, man, text, and HTML. (No, not
+  texinfo.)
+
+- ``Documentation/DocBook/*.tmpl``
+
+  These are XML template files, which are normal XML files with special
+  place-holders for where the extracted documentation should go.
+
+- ``scripts/docproc.c``
+
+  This is a program for converting XML template files into XML files. When a
+  file is referenced it is searched for symbols exported (EXPORT_SYMBOL), to be
+  able to distinguish between internal and external functions.
+
+  It invokes kernel-doc, giving it the list of functions that are to be
+  documented.
+
+  Additionally it is used to scan the XML template files to locate all the files
+  referenced herein. This is used to generate dependency information as used by
+  make.
+
+- ``Makefile``
+
+  The targets 'xmldocs', 'psdocs', 'pdfdocs', and 'htmldocs' are used to build
+  DocBook XML files, PostScript files, PDF files, and html files in
+  Documentation/DocBook. The older target 'sgmldocs' is equivalent to 'xmldocs'.
+
+- ``Documentation/DocBook/Makefile``
+
+  This is where C files are associated with SGML templates.
+
+How to use kernel-doc comments in DocBook XML template files
+------------------------------------------------------------
+
+DocBook XML template files (\*.tmpl) are like normal XML files, except that they
+can contain escape sequences where extracted documentation should be inserted.
+
+``!E<filename>`` is replaced by the documentation, in ``<filename>``, for
+functions that are exported using ``EXPORT_SYMBOL``: the function list is
+collected from files listed in ``Documentation/DocBook/Makefile``.
+
+``!I<filename>`` is replaced by the documentation for functions that are **not**
+exported using ``EXPORT_SYMBOL``.
+
+``!D<filename>`` is used to name additional files to search for functions
+exported using ``EXPORT_SYMBOL``.
+
+``!F<filename> <function [functions...]>`` is replaced by the documentation, in
+``<filename>``, for the functions listed.
+
+``!P<filename> <section title>`` is replaced by the contents of the ``DOC:``
+section titled ``<section title>`` from ``<filename>``. Spaces are allowed in
+``<section title>``; do not quote the ``<section title>``.
+
+``!C<filename>`` is replaced by nothing, but makes the tools check that all DOC:
+sections and documented functions, symbols, etc. are used. This makes sense to
+use when you use ``!F`` or ``!P`` only and want to verify that all documentation
+is included.