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.

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.

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.

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 and the function can now referenced by:

:c:func:`VIDIOC_LOG_STATUS`

list tables

We recommend the use of list table formats. The list table formats are double-stage lists. Compared to the ASCII-art they might not be as comfortable for readers of the text files. Their advantage is that they are easy to create or modify and that the diff of a modification is much more meaningful, because it is limited to the modified content.

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

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

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

   * .. _`last row`:

     - column 3

Rendered as:

table title

head col 1	head col 2	head col 3	head col 4
column 1	field 1.1	field 1.2 with autospan
column 2	field 2.1	field 2.2 - 3.3
column 3