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.

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

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 Checking for Sphinx dependencies 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

1. 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 Figures & Images).

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.

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).[1] 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.

[1]

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.[2]

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

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:

Effect of Setting SPHINX_IMGMATH

Setting	Renderer
SPHINX_IMGMATH=yes	imgmath
SPHINX_IMGMATH=no	mathjax

[2]

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.

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.

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:

.. 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:

.. 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>` / last row).

.. 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:

table title

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	field 2.2 - 3.3
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 Writing kernel-doc comments.

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 (SVG image example):

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

   SVG image example

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 (DOT’s hello world example):

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

   DOT's hello world example

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 (Embedded DOT (Graphviz) code).

Embedded DOT (Graphviz) code

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 (Embedded SVG markup).

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>

Embedded SVG markup