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.

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.

  • 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.

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

  

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.

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.