5. DPDK Documentation Guidelines

This document outlines the guidelines for writing the DPDK Guides and API documentation in RST and Doxygen format.

It also explains the structure of the DPDK documentation and how to build it.

5.1. Structure of the Documentation

The DPDK source code repository contains input files to build the API documentation and User Guides.

The main directories that contain files related to documentation are shown below:

lib
|-- acl
|-- cfgfile
|-- cmdline
|-- eal
|   |-- ...
...
doc
|-- api
+-- guides
    |-- freebsd_gsg
    |-- linux_gsg
    |-- prog_guide
    |-- sample_app_ug
    |-- guidelines
    |-- testpmd_app_ug
    |-- rel_notes
    |-- nics
    |-- ...

The API documentation is built from Doxygen comments in the header files. These files are mainly in the lib/* directories although some of the Poll Mode Drivers in drivers/net are also documented with Doxygen.

The configuration files that are used to control the Doxygen output are in the doc/api directory.

The user guides such as The Programmers Guide and the FreeBSD and Linux Getting Started Guides are generated from RST markup text files using the Sphinx Documentation Generator.

These files are included in the doc/guides/ directory. The output is controlled by the doc/guides/conf.py file.

5.2. Role of the Documentation

The following items outline the roles of the different parts of the documentation and when they need to be updated or added to by the developer.

Release Notes

The Release Notes document which features have been added in the current and previous releases of DPDK and highlight any known issues. The Releases Notes also contain notifications of features that will change ABI compatibility in the next release.

Developers should include updates to the Release Notes with patch sets that relate to any of the following sections:
- New Features
- Resolved Issues (see below)
- Known Issues
- API Changes
- ABI Changes
- Shared Library Versions
Resolved Issues should only include issues from previous releases that have been resolved in the current release. Issues that are introduced and then fixed within a release cycle do not have to be included here.

Refer to the Release Notes from the previous DPDK release for the correct format of each section.
API documentation

The API documentation explains how to use the public DPDK functions. The API index page shows the generated API documentation with related groups of functions.

The API documentation should be updated via Doxygen comments when new functions are added.
Getting Started Guides

The Getting Started Guides show how to install and configure DPDK and how to run DPDK based applications on different OSes.

A Getting Started Guide should be added when DPDK is ported to a new OS.
The Programmers Guide

The Programmers Guide explains how the API components of DPDK such as the EAL, Memzone, Rings and the Hash Library work. It also explains how some higher level functionality such as Packet Distributor, Packet Framework and KNI work. It also shows the build system and explains how to add applications.

The Programmers Guide should be expanded when new functionality is added to DPDK.
App Guides

The app guides document the DPDK applications in the app directory such as testpmd.

The app guides should be updated if functionality is changed or added.
Sample App Guides

The sample app guides document the DPDK example applications in the examples directory. Generally they demonstrate a major feature such as L2 or L3 Forwarding, Multi Process or Power Management. They explain the purpose of the sample application, how to run it and step through some of the code to explain the major functionality.

A new sample application should be accompanied by a new sample app guide. The guide for the Skeleton Forwarding app is a good starting reference.
Network Interface Controller Drivers

The NIC Drivers document explains the features of the individual Poll Mode Drivers, such as software requirements, configuration and initialization.

New documentation should be added for new Poll Mode Drivers.
Guidelines

The guideline documents record community process, expectations and design directions.

They can be extended, amended or discussed by submitting a patch and getting community approval.

5.3. Building the Documentation

5.3.1. Dependencies

The following dependencies must be installed to build the documentation:

Doxygen.
Sphinx (also called python-sphinx).

Doxygen generates documentation from commented source code. It can be installed as follows:

# Ubuntu/Debian.
sudo apt-get -y install doxygen

# Red Hat/Fedora.
sudo dnf     -y install doxygen

Sphinx is a Python documentation tool for converting RST files to HTML. For full support with figure and table captioning the latest version of Sphinx can be installed as follows:

# Ubuntu/Debian.
sudo apt-get -y install python3-sphinx python3-sphinx-rtd-theme

# Red Hat/Fedora.
sudo dnf     -y install python3-sphinx python3-sphinx_rtd_theme

For further information on getting started with Sphinx see the Sphinx Getting Started.

Note

To get full support for Figure and Table numbering it is best to install Sphinx 1.3.1 or later.

5.3.2. Build commands

The documentation is built using the standard DPDK build system.

To build the documentation:

ninja -C build doc

See Compiling the DPDK Target from Source for more detail on compiling DPDK with meson.

The output is generated in the directories build/doc/html/{api,guides}.

Note

Make sure to fix any Sphinx or Doxygen warnings when adding or updating documentation.

5.4. Document Guidelines

Here are some guidelines in relation to the style of the documentation:

Document the obvious as well as the obscure since it won’t always be obvious to the reader. For example an instruction like “Set up 64 2MB Hugepages” is better when followed by a sample commandline or a link to the appropriate section of the documentation.
Use American English spellings throughout. This can be checked using the aspell utility:
```
aspell --lang=en_US --check doc/guides/sample_app_ug/mydoc.rst
```

5.5. RST Guidelines

The RST (reStructuredText) format is a plain text markup format that can be converted to HTML or other formats. It is most closely associated with Python but it can be used to document any language. It is used in DPDK to document everything apart from the API.

The Sphinx documentation contains a very useful RST Primer which is a good place to learn the minimal set of syntax required to format a document.

The official reStructuredText website contains the specification for the RST format and also examples of how to use it. However, for most developers the RST Primer is a better resource.

The most common guidelines for writing RST text are detailed in the Documenting Python guidelines. The additional guidelines below reiterate or expand upon those guidelines.

5.5.1. Line Length

Lines in sentences should be less than 80 characters and wrapped at words. Multiple sentences which are not separated by a blank line are joined automatically into paragraphs.

Lines in literal blocks should be less than 80 characters since they are not wrapped by the document formatters.

Long literal command lines can be shown wrapped with backslashes. For example:

dpdk-testpmd -l 2-3 -n 4 \
        --vdev=virtio_user0,path=/dev/vhost-net,queues=2,queue_size=1024 \
        -- -i --tx-offloads=0x0000002c --enable-lro --txq=2 --rxq=2 \
        --txd=1024 --rxd=1024

5.5.2. Whitespace

Standard RST indentation is 3 spaces. Code can be indented 4 spaces, especially if it is copied from source files.
No tabs. Convert tabs in embedded code to 4 or 8 spaces.
No trailing whitespace.
Add 2 blank lines before each section header.
Add 1 blank line after each section header.
Add 1 blank line between each line of a list.

5.5.3. Section Headers

Section headers should use the following underline formats:

Level 1 Heading
===============

Level 2 Heading
---------------

Level 3 Heading
~~~~~~~~~~~~~~~

Level 4 Heading
^^^^^^^^^^^^^^^

Level 4 headings should be used sparingly.
The underlines should match the length of the text.
In general, the heading should be less than 80 characters, for conciseness.
As noted above:
- Add 2 blank lines before each section header.
- Add 1 blank line after each section header.

5.5.4. Lists

Bullet lists should be formatted with a leading * as follows:

* Item one.

* Item two is a long line that is wrapped and then indented to match
  the start of the previous line.

* One space character between the bullet and the text is preferred.

Numbered lists can be formatted with a leading number but the preference is to use #. which will give automatic numbering. This is more convenient when adding or removing items:
```
#. Item one.

#. Item two is a long line that is wrapped and then indented to match
   the start of the previous line.

#. Item three.
```

Definition lists can be written with or without a bullet:

* Item one.

  Some text about item one.

* Item two.

  Some text about item two.

All lists, and sub-lists, must be separated from the preceding text by a blank line. This is a syntax requirement.
All list items should be separated by a blank line for readability.

5.5.5. Code and Literal block sections

Inline text that is required to be rendered with a fixed width font should be enclosed in backquotes like this: ``text``, so that it appears like this: text.
Fixed width, literal blocks of texts should be indented at least 3 spaces and prefixed with :: like this:
```
Here is some fixed width text::

   0x0001 0x0001 0x00FF 0x00FF
```

It is also possible to specify an encoding for a literal block using the .. code-block:: directive so that syntax highlighting can be applied. Examples of supported highlighting are:

.. code-block:: console
.. code-block:: c
.. code-block:: python
.. code-block:: diff
.. code-block:: none

That can be applied as follows:

.. code-block:: c

   #include<stdio.h>

   int main() {

      printf("Hello World\n");

      return 0;
   }

Which would be rendered as:

#include<stdio.h>

int main() {

   printf("Hello World\n");

   return 0;
}

Code snippets can also be included directly from the code using the literalinclude block. Using this block instead of a code block will ensure that the code snippets shown in the documentation are always up to date with the code.

The following will include a snippet from the skeleton sample app:

.. literalinclude:: ../../../examples/skeleton/basicfwd.c
   :language: c
   :start-after: Display the port MAC address.
   :end-before: Enable RX in promiscuous mode for the Ethernet device.
   :dedent: 1

This would be rendered as:

struct rte_ether_addr addr;
retval = rte_eth_macaddr_get(port, &addr);
if (retval != 0)
	return retval;

printf("Port %u MAC: %02" PRIx8 " %02" PRIx8 " %02" PRIx8
		   " %02" PRIx8 " %02" PRIx8 " %02" PRIx8 "\n",
		port,
		addr.addr_bytes[0], addr.addr_bytes[1],
		addr.addr_bytes[2], addr.addr_bytes[3],
		addr.addr_bytes[4], addr.addr_bytes[5]);

Specifying :language: will enable syntax highlighting for the specified language. :dedent: is used in this example to remove 1 leading tab from each line of the snippet.

start-after and end-before can use any text within a given file, however it may be difficult to find unique text within your code to mark the start and end of your snippets. In these cases, it is recommended to include explicit tags in your code to denote these locations for documentation purposes. The accepted format for these comments is:
- Before the code snippet, create a new comment which is a sentence explaining what the code snippet contains. The comment is terminated with a scissors 8<.
- After the code snippet, create another new comment which starts with a scissors >8, then End of and the first comment repeated.
- The scissors should be orientated as shown to make it clear what code is being snipped.
This can be done as follows:
```
/* Example feature being documented. 8< */
foo(bar);
/* >8 End of example feature being documented. */
```
foo(bar); could then be included in the docs using:
```
.. literalinclude:: ../../../examples/sample_app/main.c
   :language: c
   :start-after: Example feature being documented. 8<
   :end-before: >8 End of example feature being documented.
```
If a multiline comment is needed before the snippet, then the last line of the multiline comment should be in the same format as the first comment shown in the example.
More information about the literalinclude block can be found within the Sphinx Documentation.
The default encoding for a literal block using the simplified :: directive is none.

Lines in literal blocks should be less than 80 characters. For long literal lines, try to wrap the text at sensible locations. For example a long command line could be documented like this and still work if copied directly from the docs:

./<build_dir>/app/dpdk-testpmd -l 0-2 -n3 --vdev=net_pcap0,iface=eth0    \
                          --vdev=net_pcap1,iface=eth1     \
                          -- -i --nb-cores=2 --nb-ports=2 \
                             --total-num-mbufs=2048

Long lines that cannot be wrapped, such as application output, should be truncated to be less than 80 characters.

5.5.6. Images

All images should be in SVG scalar graphics format. They should be true SVG XML files and should not include binary formats embedded in a SVG wrapper.
The DPDK documentation contains some legacy images in PNG format. These will be converted to SVG in time.
Inkscape is the recommended graphics editor for creating the images. Use some of the older images in doc/guides/prog_guide/img/ as a template, for example mbuf1.svg or ring-enqueue1.svg.
The SVG images should include a copyright notice, as an XML comment.
Images in the documentation should be formatted as follows:
- The image should be preceded by a label in the format .. _figure_XXXX: with a leading underscore and where XXXX is a unique descriptive name.
- Images should be included using the .. figure:: directive and the file type should be set to * (not .svg). This allows the format of the image to be changed if required, without updating the documentation.
- Images must have a caption as part of the .. figure:: directive.

Here is an example of the previous three guidelines:

.. _figure_mempool:

.. figure:: img/mempool.*

   A mempool in memory with its associated ring.

Images can then be linked to using the :numref: directive:
```
The mempool layout is shown in :numref:`figure_mempool`.
```
This would be rendered as: The mempool layout is shown in Fig 6.3.

Note: The :numref: directive requires Sphinx 1.3.1 or later. With earlier versions it will still be rendered as a link but won’t have an automatically generated number.
The caption of the image can be generated, with a link, using the :ref: directive:
```
:ref:`figure_mempool`
```
This would be rendered as: A mempool in memory with its associated ring.

5.5.7. Tables

RST tables should be used sparingly. They are hard to format and to edit, and the same information can usually be shown just as clearly with a definition or bullet list.
Tables in the documentation should be formatted as follows:
- The table should be preceded by a label in the format .. _table_XXXX: with a leading underscore and where XXXX is a unique descriptive name.
- Tables should be included using the .. table:: directive and must have a caption.

Here is an example of the previous two guidelines:

.. _table_qos_pipes:

.. table:: Sample configuration for QOS pipes.

   +----------+----------+----------+
   | Header 1 | Header 2 | Header 3 |
   |          |          |          |
   +==========+==========+==========+
   | Text     | Text     | Text     |
   +----------+----------+----------+
   | ...      | ...      | ...      |
   +----------+----------+----------+

Tables can be linked to using the :numref: and :ref: directives, as shown in the previous section for images. For example:
```
The QOS configuration is shown in :numref:`table_qos_pipes`.
```

5.5.8. Hyperlinks

Links to external websites can be plain URLs. The following is rendered as https://dpdk.org:
```
https://dpdk.org
```
They can contain alternative text. The following is rendered as Check out DPDK:
```
`Check out DPDK <https://dpdk.org>`_
```
An internal link can be generated by placing labels in the document with the format .. _label_name.

The following links to the top of this section: Hyperlinks:

.. _links:

Hyperlinks
~~~~~~~~~~

* The following links to the top of this section: :ref:`links`:

Note

The label must have a leading underscore but the reference to it must omit it. This is a frequent cause of errors and warnings.

The use of a label is preferred since it works across files and will still work if the header text changes.

5.6. Doxygen Guidelines

The DPDK API is documented using Doxygen comment annotations in the header files. Doxygen is a very powerful tool, it is extremely configurable and with a little effort can be used to create expressive documents. See the Doxygen website for full details on how to use it.

The following are some guidelines for use of Doxygen in the DPDK API documentation:

New libraries that are documented with Doxygen should be added to the Doxygen configuration file: doc/api/doxy-api.conf. It is only required to add the directory that contains the files. It isn’t necessary to explicitly name each file since the configuration matches all rte_*.h files in the directory.
Use proper capitalization and punctuation in the Doxygen comments since they will become sentences in the documentation. This in particular applies to single line comments, which is the case the is most often forgotten.
Use @ style Doxygen commands instead of \ style commands.

Add a general description of each library at the head of the main header files:

/**
 * @file
 * RTE Mempool.
 *
 * A memory pool is an allocator of fixed-size object. It is
 * identified by its name, and uses a ring to store free objects.
 * ...
 */

Document the purpose of a function, the parameters used and the return value:

/**
 * Try to take the lock.
 *
 * @param sl
 *   A pointer to the spinlock.
 * @return
 *   1 if the lock is successfully taken; 0 otherwise.
 */
int rte_spinlock_trylock(rte_spinlock_t *sl);

Doxygen supports Markdown style syntax such as bold, italics, fixed width text and lists. For example the second line in the devargs parameter in the previous example will be rendered as:

The strings should be a pci address like 0000:01:00.0 or virtual device name like net_pcap0.
Use - instead of * for lists within the Doxygen comment since the latter can get confused with the comment delimiter.
Add an empty line between the function description, the @params and @return for readability.
Place the @params description on separate line and indent it by 2 spaces. (It would be better to use no indentation since this is more common and also because checkpatch complains about leading whitespace in comments. However this is the convention used in the existing DPDK code.)

Documented functions can be linked to simply by adding () to the function name:

/**
 * The functions exported by the application Ethernet API to setup
 * a device designated by its port identifier must be invoked in
 * the following order:
 *     - rte_eth_dev_configure()
 *     - rte_eth_tx_queue_setup()
 *     - rte_eth_rx_queue_setup()
 *     - rte_eth_dev_start()
 */

In the API documentation the functions will be rendered as links, see the online section of the rte_ethdev.h docs that contains the above text.

The @see keyword can be used to create a see also link to another file or library. This directive should be placed on one line at the bottom of the documentation section.
```
/**
 * ...
 *
 * Some text that references mempools.
 *
 * @see eal_memzone.c
 */
```

Doxygen supports two types of comments for documenting variables, constants and members: prefix and postfix:

/** This is a prefix comment. */
#define RTE_FOO_ERROR  0x023.

#define RTE_BAR_ERROR  0x024. /**< This is a postfix comment. */

Postfix comments are preferred for struct members and constants if they can be documented in the same way:

struct rte_eth_stats {
    uint64_t ipackets; /**< Total number of received packets. */
    uint64_t opackets; /**< Total number of transmitted packets.*/
    uint64_t ibytes;   /**< Total number of received bytes. */
    uint64_t obytes;   /**< Total number of transmitted bytes. */
    uint64_t imissed;  /**< Total of RX missed packets. */
    uint64_t ibadcrc;  /**< Total of RX packets with CRC error. */
    uint64_t ibadlen;  /**< Total of RX packets with bad length. */
}

Note: postfix comments should be aligned with spaces not tabs in accordance with the DPDK Coding Style.

If a single comment type can’t be used, due to line length limitations then prefix comments should be preferred. For example this section of the code contains prefix comments, postfix comments on the same line and postfix comments on a separate line:

/** Number of elements in the elt_pa array. */
uint32_t    pg_num __rte_cache_aligned;
uint32_t    pg_shift;     /**< LOG2 of the physical pages. */
uintptr_t   pg_mask;      /**< Physical page mask value. */
uintptr_t   elt_va_start;
/**< Virtual address of the first mempool object. */
uintptr_t   elt_va_end;
/**< Virtual address of the <size + 1> mempool object. */
phys_addr_t elt_pa[MEMPOOL_PG_NUM_DEFAULT];
/**< Array of physical page addresses for the mempool buffer. */

This doesn’t have an effect on the rendered documentation but it is confusing for the developer reading the code. It this case it would be clearer to use prefix comments throughout:

/** Number of elements in the elt_pa array. */
uint32_t    pg_num __rte_cache_aligned;
/** LOG2 of the physical pages. */
uint32_t    pg_shift;
/** Physical page mask value. */
uintptr_t   pg_mask;
/** Virtual address of the first mempool object. */
uintptr_t   elt_va_start;
/** Virtual address of the <size + 1> mempool object. */
uintptr_t   elt_va_end;
/** Array of physical page addresses for the mempool buffer. */
phys_addr_t elt_pa[MEMPOOL_PG_NUM_DEFAULT];

Read the rendered section of the documentation that you have added for correctness, clarity and consistency with the surrounding text.