Documentation/doc-guide/sphinx.rst - linux - Git at Google

 .. _sphinxdoc:

 =====================================
 Using Sphinx for kernel documentation
 =====================================

 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.

 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_install:

 Sphinx Install
 ==============

 The ReST markups currently used by the Documentation/ files are meant to be
 built with ``Sphinx`` version 1.7 or higher.

 There's a script that checks for the Sphinx requirements. Please see
 :ref:`sphinx-pre-install` for further details.

 Most distributions are shipped with Sphinx, but its toolchain is fragile,
 and it is not uncommon that upgrading it or some other Python packages
 on your machine would cause the documentation build to break.

 A way to avoid that is to use a different version than the one shipped
 with your distributions. In order to do so, it is recommended to install
 Sphinx inside a virtual environment, using ``virtualenv-3``
 or ``virtualenv``, depending on how your distribution packaged Python 3.

 .. note::

    #) It is recommended to use the RTD theme for html output. Depending
       on the Sphinx version, it should be installed separately,
       with ``pip install sphinx_rtd_theme``.

 In summary, if you want to install Sphinx version 2.4.4, you should do::

        $ virtualenv sphinx_2.4.4
        $ . sphinx_2.4.4/bin/activate
        (sphinx_2.4.4) $ pip install -r Documentation/sphinx/requirements.txt

 After running ``. sphinx_2.4.4/bin/activate``, the prompt will change,
 in order to indicate that you're using the new environment. If you
 open a new shell, you need to rerun this command to enter again at
 the virtual environment before building the documentation.

 Image output
 ------------

 The kernel documentation build system contains an extension that
 handles images on both GraphViz and SVG formats (see
 :ref:`sphinx_kfigure`).

 For it to work, you need to install both GraphViz and ImageMagick
 packages. If those packages are not installed, the build system will
 still build the documentation, but won't include any images at the
 output.

 PDF and LaTeX builds
 --------------------

 Such builds are currently supported only with Sphinx versions 2.4 and higher.

 For PDF and LaTeX output, you'll also need ``XeLaTeX`` version 3.14159265.

 Depending on the distribution, you may also need to install a series of
 ``texlive`` packages that provide the minimal set of functionalities
 required for ``XeLaTeX`` to work.

 Math Expressions in HTML
 ------------------------

 Some ReST pages contain math expressions. Due to the way Sphinx works,
 those expressions are written using LaTeX notation.
 There are two options for Sphinx to render math expressions in html output.
 One is an extension called `imgmath`_ which converts math expressions into
 images and embeds them in html pages.
 The other is an extension called `mathjax`_ which delegates math rendering
 to JavaScript capable web browsers.
 The former was the only option for pre-6.1 kernel documentation and it
 requires quite a few texlive packages including amsfonts and amsmath among
 others.

 Since kernel release 6.1, html pages with math expressions can be built
 without installing any texlive packages. See `Choice of Math Renderer`_ for
 further info.

 .. _imgmath: https://www.sphinx-doc.org/en/master/usage/extensions/math.html#module-sphinx.ext.imgmath
 .. _mathjax: https://www.sphinx-doc.org/en/master/usage/extensions/math.html#module-sphinx.ext.mathjax

 .. _sphinx-pre-install:

 Checking for Sphinx dependencies
 --------------------------------

 There's a script that automatically check for Sphinx dependencies. If it can
 recognize your distribution, it will also give a hint about the install
 command line options for your distro::

 	$ ./scripts/sphinx-pre-install
 	Checking if the needed tools for Fedora release 26 (Twenty Six) are available
 	Warning: better to also install "texlive-luatex85".
 	You should run:

 		sudo dnf install -y texlive-luatex85
 		/usr/bin/virtualenv sphinx_2.4.4
 		. sphinx_2.4.4/bin/activate
 		pip install -r Documentation/sphinx/requirements.txt

 	Can't build as 1 mandatory dependency is missing at ./scripts/sphinx-pre-install line 468.

 By default, it checks all the requirements for both html and PDF, including
 the requirements for images, math expressions and LaTeX build, and assumes
 that a virtual Python environment will be used. The ones needed for html
 builds are assumed to be mandatory; the others to be optional.

 It supports two optional parameters:

 ``--no-pdf``
 	Disable checks for PDF;

 ``--no-virtualenv``
 	Use OS packaging for Sphinx instead of Python virtual environment.


 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 you'll also need
 ``XeLaTeX`` and ``convert(1)`` from ImageMagick
 (https://www.imagemagick.org).\ [#ink]_
 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.

 It is also possible to pass an extra DOCS_CSS overlay file, in order to customize
 the html layout, by using the ``DOCS_CSS`` make variable.

 By default, the build will try to use the Read the Docs sphinx theme:

     https://github.com/readthedocs/sphinx_rtd_theme

 If the theme is not available, it will fall-back to the classic one.

 The Sphinx theme can be overridden by using the ``DOCS_THEME`` make variable.

 There is another make variable ``SPHINXDIRS``, which is useful when test
 building a subset of documentation.  For example, you can build documents
 under ``Documentation/doc-guide`` by running
 ``make SPHINXDIRS=doc-guide htmldocs``.
 The documentation section of ``make help`` will show you the list of
 subdirectories you can specify.

 To remove the generated documentation, run ``make cleandocs``.

 .. [#ink] Having ``inkscape(1)`` from Inkscape (https://inkscape.org)
 	  as well would improve the quality of images embedded in PDF
 	  documents, especially for kernel releases 5.18 and later.

 Choice of Math Renderer
 -----------------------

 Since kernel release 6.1, mathjax works as a fallback math renderer for
 html output.\ [#sph1_8]_

 Math renderer is chosen depending on available commands as shown below:

 .. table:: Math Renderer Choices for HTML

     ============= ================= ============
     Math renderer Required commands Image format
     ============= ================= ============
     imgmath       latex, dvipng     PNG (raster)
     mathjax
     ============= ================= ============

 The choice can be overridden by setting an environment variable
 ``SPHINX_IMGMATH`` as shown below:

 .. table:: Effect of Setting ``SPHINX_IMGMATH``

     ====================== ========
     Setting                Renderer
     ====================== ========
     ``SPHINX_IMGMATH=yes`` imgmath
     ``SPHINX_IMGMATH=no``  mathjax
     ====================== ========

 .. [#sph1_8] Fallback of math renderer requires Sphinx >=1.8.


 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. For the most part the documentation should be plain text with
   just enough consistency in formatting that it can be converted to
   other formats.

 * Please keep the formatting changes minimal when converting existing
   documentation to reStructuredText.

 * Also update the content, not just the formatting, when converting
   documentation.

 * 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.

 * For inserting fixed width text blocks (for code examples, use case
   examples, etc.), use ``::`` for anything that doesn't really benefit
   from syntax highlighting, especially short snippets. Use
   ``.. code-block:: <language>`` for longer code blocks that benefit
   from highlighting. For a short snippet of code embedded in the text, use \`\`.


 the C domain
 ------------

 The **Sphinx C Domain** (name c) is suited for documentation of C API. E.g. a
 function prototype:

 .. code-block:: rst

     .. c:function:: int ioctl( int fd, int request )

 The C domain of the kernel-doc has some additional features. E.g. you can
 *rename* the reference name of a function with a common name like ``open`` or
 ``ioctl``:

 .. code-block:: rst

      .. c:function:: int ioctl( int fd, int request )
         :name: VIDIOC_LOG_STATUS

 The func-name (e.g. ioctl) remains in the output but the ref-name changed from
 ``ioctl`` to ``VIDIOC_LOG_STATUS``. The index entry for this function is also
 changed to ``VIDIOC_LOG_STATUS``.

 Please note that there is no need to use ``c:func:`` to generate cross
 references to function documentation.  Due to some Sphinx extension magic,
 the documentation build system will automatically turn a reference to
 ``function()`` into a cross reference if an index entry for the given
 function name exists.  If you see ``c:func:`` use in a kernel document,
 please feel free to remove it.


 list tables
 -----------

 The list-table formats can be useful for tables that are not easily laid
 out in the usual Sphinx ASCII-art formats.  These formats are nearly
 impossible for readers of the plain-text documents to understand, though,
 and should be avoided in the absence of a strong justification for their
 use.

 The ``flat-table`` is a double-stage list similar to the ``list-table`` with
 some additional features:

 * column-span: with the role ``cspan`` a cell can be extended through
   additional columns

 * row-span: with the role ``rspan`` a cell can be extended through
   additional rows

 * auto span rightmost cell of a table row over the missing cells on the right
   side of that table-row.  With Option ``:fill-cells:`` this behavior can
   changed from *auto span* to *auto fill*, which automatically inserts (empty)
   cells instead of spanning the last cell.

 options:

 * ``:header-rows:``   [int] count of header rows
 * ``:stub-columns:``  [int] count of stub columns
 * ``:widths:``        [[int] [int] ... ] widths of columns
 * ``:fill-cells:``    instead of auto-spanning missing cells, insert missing cells

 roles:

 * ``:cspan:`` [int] additional columns (*morecols*)
 * ``:rspan:`` [int] additional rows (*morerows*)

 The example below shows how to use this markup.  The first level of the staged
 list is the *table-row*. In the *table-row* there is only one markup allowed,
 the list of the cells in this *table-row*. Exceptions are *comments* ( ``..`` )
 and *targets* (e.g. a ref to ``:ref:`last row <last row>``` / :ref:`last row
 <last row>`).

 .. code-block:: rst

    .. flat-table:: table title
       :widths: 2 1 1 3

       * - head col 1
         - head col 2
         - head col 3
         - head col 4

       * - row 1
         - field 1.1
         - field 1.2 with autospan

       * - row 2
         - field 2.1
         - :rspan:`1` :cspan:`1` field 2.2 - 3.3

       * .. _`last row`:

         - row 3

 Rendered as:

    .. flat-table:: table title
       :widths: 2 1 1 3

       * - head col 1
         - head col 2
         - head col 3
         - head col 4

       * - row 1
         - field 1.1
         - field 1.2 with autospan

       * - row 2
         - field 2.1
         - :rspan:`1` :cspan:`1` field 2.2 - 3.3

       * .. _`last row`:

         - row 3

 Cross-referencing
 -----------------

 Cross-referencing from one documentation page to another can be done simply by
 writing the path to the document file, no special syntax required. The path can
 be either absolute or relative. For absolute paths, start it with
 "Documentation/". For example, to cross-reference to this page, all the
 following are valid options, depending on the current document's directory (note
 that the ``.rst`` extension is required)::

     See Documentation/doc-guide/sphinx.rst. This always works.
     Take a look at sphinx.rst, which is at this same directory.
     Read ../sphinx.rst, which is one directory above.

 If you want the link to have a different rendered text other than the document's
 title, you need to use Sphinx's ``doc`` role. For example::

     See :doc:`my custom link text for document sphinx <sphinx>`.

 For most use cases, the former is preferred, as it is cleaner and more suited
 for people reading the source files. If you come across a ``:doc:`` usage that
 isn't adding any value, please feel free to convert it to just the document
 path.

 For information on cross-referencing to kernel-doc functions or types, see
 Documentation/doc-guide/kernel-doc.rst.

 .. _sphinx_kfigure:

 Figures & Images
 ================

 If you want to add an image, you should use the ``kernel-figure`` and
 ``kernel-image`` directives. E.g. to insert a figure with a scalable
 image format, use SVG (:ref:`svg_image_example`)::

     .. kernel-figure::  svg_image.svg
        :alt:    simple SVG image

        SVG image example

 .. _svg_image_example:

 .. kernel-figure::  svg_image.svg
    :alt:    simple SVG image

    SVG image example

 The kernel figure (and image) directive supports **DOT** formatted files, see

 * DOT: http://graphviz.org/pdf/dotguide.pdf
 * Graphviz: http://www.graphviz.org/content/dot-language

 A simple example (:ref:`hello_dot_file`)::

   .. kernel-figure::  hello.dot
      :alt:    hello world

      DOT's hello world example

 .. _hello_dot_file:

 .. kernel-figure::  hello.dot
    :alt:    hello world

    DOT's hello world example

 Embedded *render* markups (or languages) like Graphviz's **DOT** are provided by the
 ``kernel-render`` directives.::

   .. kernel-render:: DOT
      :alt: foobar digraph
      :caption: Embedded **DOT** (Graphviz) code

      digraph foo {
       "bar" -> "baz";
      }

 How this will be rendered depends on the installed tools. If Graphviz is
 installed, you will see a vector image. If not, the raw markup is inserted as
 *literal-block* (:ref:`hello_dot_render`).

 .. _hello_dot_render:

 .. kernel-render:: DOT
    :alt: foobar digraph
    :caption: Embedded **DOT** (Graphviz) code

    digraph foo {
       "bar" -> "baz";
    }

 The *render* directive has all the options known from the *figure* directive,
 plus option ``caption``.  If ``caption`` has a value, a *figure* node is
 inserted. If not, an *image* node is inserted. A ``caption`` is also needed, if
 you want to refer to it (:ref:`hello_svg_render`).

 Embedded **SVG**::

   .. kernel-render:: SVG
      :caption: Embedded **SVG** markup
      :alt: so-nw-arrow

      <?xml version="1.0" encoding="UTF-8"?>
      <svg xmlns="http://www.w3.org/2000/svg" version="1.1" ...>
         ...
      </svg>

 .. _hello_svg_render:

 .. kernel-render:: SVG
    :caption: Embedded **SVG** markup
    :alt: so-nw-arrow

    <?xml version="1.0" encoding="UTF-8"?>
    <svg xmlns="http://www.w3.org/2000/svg"
      version="1.1" baseProfile="full" width="70px" height="40px" viewBox="0 0 700 400">
    <line x1="180" y1="370" x2="500" y2="50" stroke="black" stroke-width="15px"/>
    <polygon points="585 0 525 25 585 50" transform="rotate(135 525 25)"/>
    </svg>
	.. _sphinxdoc:

	=====================================
	Using Sphinx for kernel documentation
	=====================================

	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.

	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_install:

	Sphinx Install
	==============

	The ReST markups currently used by the Documentation/ files are meant to be
	built with ``Sphinx`` version 1.7 or higher.

	There's a script that checks for the Sphinx requirements. Please see
	:ref:`sphinx-pre-install` for further details.

	Most distributions are shipped with Sphinx, but its toolchain is fragile,
	and it is not uncommon that upgrading it or some other Python packages
	on your machine would cause the documentation build to break.

	A way to avoid that is to use a different version than the one shipped
	with your distributions. In order to do so, it is recommended to install
	Sphinx inside a virtual environment, using ``virtualenv-3``
	or ``virtualenv``, depending on how your distribution packaged Python 3.

	.. note::

	#) It is recommended to use the RTD theme for html output. Depending
	on the Sphinx version, it should be installed separately,
	with ``pip install sphinx_rtd_theme``.

	In summary, if you want to install Sphinx version 2.4.4, you should do::

	$ virtualenv sphinx_2.4.4
	$ . sphinx_2.4.4/bin/activate
	(sphinx_2.4.4) $ pip install -r Documentation/sphinx/requirements.txt

	After running ``. sphinx_2.4.4/bin/activate``, the prompt will change,
	in order to indicate that you're using the new environment. If you
	open a new shell, you need to rerun this command to enter again at
	the virtual environment before building the documentation.

	Image output
	------------

	The kernel documentation build system contains an extension that
	handles images on both GraphViz and SVG formats (see
	:ref:`sphinx_kfigure`).

	For it to work, you need to install both GraphViz and ImageMagick
	packages. If those packages are not installed, the build system will
	still build the documentation, but won't include any images at the
	output.

	PDF and LaTeX builds
	--------------------

	Such builds are currently supported only with Sphinx versions 2.4 and higher.

	For PDF and LaTeX output, you'll also need ``XeLaTeX`` version 3.14159265.

	Depending on the distribution, you may also need to install a series of
	``texlive`` packages that provide the minimal set of functionalities
	required for ``XeLaTeX`` to work.

	Math Expressions in HTML
	------------------------

	Some ReST pages contain math expressions. Due to the way Sphinx works,
	those expressions are written using LaTeX notation.
	There are two options for Sphinx to render math expressions in html output.
	One is an extension called `imgmath`_ which converts math expressions into
	images and embeds them in html pages.
	The other is an extension called `mathjax`_ which delegates math rendering
	to JavaScript capable web browsers.
	The former was the only option for pre-6.1 kernel documentation and it
	requires quite a few texlive packages including amsfonts and amsmath among
	others.

	Since kernel release 6.1, html pages with math expressions can be built
	without installing any texlive packages. See `Choice of Math Renderer`_ for
	further info.

	.. _imgmath: https://www.sphinx-doc.org/en/master/usage/extensions/math.html#module-sphinx.ext.imgmath
	.. _mathjax: https://www.sphinx-doc.org/en/master/usage/extensions/math.html#module-sphinx.ext.mathjax

	.. _sphinx-pre-install:

	Checking for Sphinx dependencies
	--------------------------------

	There's a script that automatically check for Sphinx dependencies. If it can
	recognize your distribution, it will also give a hint about the install
	command line options for your distro::

	$ ./scripts/sphinx-pre-install
	Checking if the needed tools for Fedora release 26 (Twenty Six) are available
	Warning: better to also install "texlive-luatex85".
	You should run:

	sudo dnf install -y texlive-luatex85
	/usr/bin/virtualenv sphinx_2.4.4
	. sphinx_2.4.4/bin/activate
	pip install -r Documentation/sphinx/requirements.txt

	Can't build as 1 mandatory dependency is missing at ./scripts/sphinx-pre-install line 468.

	By default, it checks all the requirements for both html and PDF, including
	the requirements for images, math expressions and LaTeX build, and assumes
	that a virtual Python environment will be used. The ones needed for html
	builds are assumed to be mandatory; the others to be optional.

	It supports two optional parameters:

	``--no-pdf``
	Disable checks for PDF;

	``--no-virtualenv``
	Use OS packaging for Sphinx instead of Python virtual environment.


	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 you'll also need
	``XeLaTeX`` and ``convert(1)`` from ImageMagick
	(https://www.imagemagick.org).\ [#ink]_
	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.

	It is also possible to pass an extra DOCS_CSS overlay file, in order to customize
	the html layout, by using the ``DOCS_CSS`` make variable.

	By default, the build will try to use the Read the Docs sphinx theme:

	https://github.com/readthedocs/sphinx_rtd_theme

	If the theme is not available, it will fall-back to the classic one.

	The Sphinx theme can be overridden by using the ``DOCS_THEME`` make variable.

	There is another make variable ``SPHINXDIRS``, which is useful when test
	building a subset of documentation. For example, you can build documents
	under ``Documentation/doc-guide`` by running
	``make SPHINXDIRS=doc-guide htmldocs``.
	The documentation section of ``make help`` will show you the list of
	subdirectories you can specify.

	To remove the generated documentation, run ``make cleandocs``.

	.. [#ink] Having ``inkscape(1)`` from Inkscape (https://inkscape.org)
	as well would improve the quality of images embedded in PDF
	documents, especially for kernel releases 5.18 and later.

	Choice of Math Renderer
	-----------------------

	Since kernel release 6.1, mathjax works as a fallback math renderer for
	html output.\ [#sph1_8]_

	Math renderer is chosen depending on available commands as shown below:

	.. table:: Math Renderer Choices for HTML

	============= ================= ============
	Math renderer Required commands Image format
	============= ================= ============
	imgmath latex, dvipng PNG (raster)
	mathjax
	============= ================= ============

	The choice can be overridden by setting an environment variable
	``SPHINX_IMGMATH`` as shown below:

	.. table:: Effect of Setting ``SPHINX_IMGMATH``

	====================== ========
	Setting Renderer
	====================== ========
	``SPHINX_IMGMATH=yes`` imgmath
	``SPHINX_IMGMATH=no`` mathjax
	====================== ========

	.. [#sph1_8] Fallback of math renderer requires Sphinx >=1.8.


	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. For the most part the documentation should be plain text with
	just enough consistency in formatting that it can be converted to
	other formats.

	* Please keep the formatting changes minimal when converting existing
	documentation to reStructuredText.

	* Also update the content, not just the formatting, when converting
	documentation.

	* 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.

	* For inserting fixed width text blocks (for code examples, use case
	examples, etc.), use ``::`` for anything that doesn't really benefit
	from syntax highlighting, especially short snippets. Use
	``.. code-block:: <language>`` for longer code blocks that benefit
	from highlighting. For a short snippet of code embedded in the text, use \`\`.


	the C domain
	------------

	The Sphinx C Domain (name c) is suited for documentation of C API. E.g. a
	function prototype:

	.. code-block:: rst

	.. c:function:: int ioctl( int fd, int request )

	The C domain of the kernel-doc has some additional features. E.g. you can
	rename the reference name of a function with a common name like ``open`` or
	``ioctl``:

	.. code-block:: rst

	.. c:function:: int ioctl( int fd, int request )
	:name: VIDIOC_LOG_STATUS

	The func-name (e.g. ioctl) remains in the output but the ref-name changed from
	``ioctl`` to ``VIDIOC_LOG_STATUS``. The index entry for this function is also
	changed to ``VIDIOC_LOG_STATUS``.

	Please note that there is no need to use ``c:func:`` to generate cross
	references to function documentation. Due to some Sphinx extension magic,
	the documentation build system will automatically turn a reference to
	``function()`` into a cross reference if an index entry for the given
	function name exists. If you see ``c:func:`` use in a kernel document,
	please feel free to remove it.


	list tables
	-----------

	The list-table formats can be useful for tables that are not easily laid
	out in the usual Sphinx ASCII-art formats. These formats are nearly
	impossible for readers of the plain-text documents to understand, though,
	and should be avoided in the absence of a strong justification for their
	use.

	The ``flat-table`` is a double-stage list similar to the ``list-table`` with
	some additional features:

	* column-span: with the role ``cspan`` a cell can be extended through
	additional columns

	* row-span: with the role ``rspan`` a cell can be extended through
	additional rows

	* auto span rightmost cell of a table row over the missing cells on the right
	side of that table-row. With Option ``:fill-cells:`` this behavior can
	changed from auto span to auto fill, which automatically inserts (empty)
	cells instead of spanning the last cell.

	options:

	* ``:header-rows:`` [int] count of header rows
	* ``:stub-columns:`` [int] count of stub columns
	* ``:widths:`` [[int] [int] ... ] widths of columns
	* ``:fill-cells:`` instead of auto-spanning missing cells, insert missing cells

	roles:

	* ``:cspan:`` [int] additional columns (morecols)
	* ``:rspan:`` [int] additional rows (morerows)

	The example below shows how to use this markup. The first level of the staged
	list is the table-row. In the table-row there is only one markup allowed,
	the list of the cells in this table-row. Exceptions are comments ( ``..`` )
	and targets (e.g. a ref to ``:ref:`last row <last row>``` / :ref:`last row
	<last row>`).

	.. code-block:: rst

	.. flat-table:: table title
	:widths: 2 1 1 3

	* - head col 1
	- head col 2
	- head col 3
	- head col 4

	* - row 1
	- field 1.1
	- field 1.2 with autospan

	* - row 2
	- field 2.1
	- :rspan:`1` :cspan:`1` field 2.2 - 3.3

	* .. _`last row`:

	- row 3

	Rendered as:

	.. flat-table:: table title
	:widths: 2 1 1 3

	* - head col 1
	- head col 2
	- head col 3
	- head col 4

	* - row 1
	- field 1.1
	- field 1.2 with autospan

	* - row 2
	- field 2.1
	- :rspan:`1` :cspan:`1` field 2.2 - 3.3

	* .. _`last row`:

	- row 3

	Cross-referencing
	-----------------

	Cross-referencing from one documentation page to another can be done simply by
	writing the path to the document file, no special syntax required. The path can
	be either absolute or relative. For absolute paths, start it with
	"Documentation/". For example, to cross-reference to this page, all the
	following are valid options, depending on the current document's directory (note
	that the ``.rst`` extension is required)::

	See Documentation/doc-guide/sphinx.rst. This always works.
	Take a look at sphinx.rst, which is at this same directory.
	Read ../sphinx.rst, which is one directory above.

	If you want the link to have a different rendered text other than the document's
	title, you need to use Sphinx's ``doc`` role. For example::

	See :doc:`my custom link text for document sphinx <sphinx>`.

	For most use cases, the former is preferred, as it is cleaner and more suited
	for people reading the source files. If you come across a ``:doc:`` usage that
	isn't adding any value, please feel free to convert it to just the document
	path.

	For information on cross-referencing to kernel-doc functions or types, see
	Documentation/doc-guide/kernel-doc.rst.

	.. _sphinx_kfigure:

	Figures & Images
	================

	If you want to add an image, you should use the ``kernel-figure`` and
	``kernel-image`` directives. E.g. to insert a figure with a scalable
	image format, use SVG (:ref:`svg_image_example`)::

	.. kernel-figure:: svg_image.svg
	:alt: simple SVG image

	SVG image example

	.. _svg_image_example:

	.. kernel-figure:: svg_image.svg
	:alt: simple SVG image

	SVG image example

	The kernel figure (and image) directive supports DOT formatted files, see

	* DOT: http://graphviz.org/pdf/dotguide.pdf
	* Graphviz: http://www.graphviz.org/content/dot-language

	A simple example (:ref:`hello_dot_file`)::

	.. kernel-figure:: hello.dot
	:alt: hello world

	DOT's hello world example

	.. _hello_dot_file:

	.. kernel-figure:: hello.dot
	:alt: hello world

	DOT's hello world example

	Embedded render markups (or languages) like Graphviz's DOT are provided by the
	``kernel-render`` directives.::

	.. kernel-render:: DOT
	:alt: foobar digraph
	:caption: Embedded DOT (Graphviz) code

	digraph foo {
	"bar" -> "baz";
	}

	How this will be rendered depends on the installed tools. If Graphviz is
	installed, you will see a vector image. If not, the raw markup is inserted as
	literal-block (:ref:`hello_dot_render`).

	.. _hello_dot_render:

	.. kernel-render:: DOT
	:alt: foobar digraph
	:caption: Embedded DOT (Graphviz) code

	digraph foo {
	"bar" -> "baz";
	}

	The render directive has all the options known from the figure directive,
	plus option ``caption``. If ``caption`` has a value, a figure node is
	inserted. If not, an image node is inserted. A ``caption`` is also needed, if
	you want to refer to it (:ref:`hello_svg_render`).

	Embedded SVG::

	.. kernel-render:: SVG
	:caption: Embedded SVG markup
	:alt: so-nw-arrow

	<?xml version="1.0" encoding="UTF-8"?>
	<svg xmlns="http://www.w3.org/2000/svg" version="1.1" ...>
	...
	</svg>

	.. _hello_svg_render:

	.. kernel-render:: SVG
	:caption: Embedded SVG markup
	:alt: so-nw-arrow

	<?xml version="1.0" encoding="UTF-8"?>
	<svg xmlns="http://www.w3.org/2000/svg"
	version="1.1" baseProfile="full" width="70px" height="40px" viewBox="0 0 700 400">
	<line x1="180" y1="370" x2="500" y2="50" stroke="black" stroke-width="15px"/>
	<polygon points="585 0 525 25 585 50" transform="rotate(135 525 25)"/>
	</svg>