xref: /openbmc/linux/Documentation/doc-guide/sphinx.rst (revision 0e407915)

.. _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>