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diff --git a/Documentation/kernel-documentation.rst b/Documentation/kernel-documentation.rst new file mode 100644 index 000000000000..ad3506c13b11 --- /dev/null +++ b/Documentation/kernel-documentation.rst @@ -0,0 +1,562 @@ +========================== +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. |
