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 .. _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``.
 
    #) Some ReST pages contain math expressions. Due to the way Sphinx works,
       those expressions are written using LaTeX notation. It needs texlive
       installed with amsfonts and amsmath in order to evaluate them.
 
 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.
 
 .. _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.
 
 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>

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