1Writing kernel-doc comments
2===========================
3
4The Linux kernel source files may contain structured documentation
5comments in the kernel-doc format to describe the functions, types
6and design of the code. It is easier to keep documentation up-to-date
7when it is embedded in source files.
8
9.. note:: The kernel-doc format is deceptively similar to javadoc,
10   gtk-doc or Doxygen, yet distinctively different, for historical
11   reasons. The kernel source contains tens of thousands of kernel-doc
12   comments. Please stick to the style described here.
13
14The kernel-doc structure is extracted from the comments, and proper
15`Sphinx C Domain`_ function and type descriptions with anchors are
16generated from them. The descriptions are filtered for special kernel-doc
17highlights and cross-references. See below for details.
18
19.. _Sphinx C Domain: http://www.sphinx-doc.org/en/stable/domains.html
20
21Every function that is exported to loadable modules using
22``EXPORT_SYMBOL`` or ``EXPORT_SYMBOL_GPL`` should have a kernel-doc
23comment. Functions and data structures in header files which are intended
24to be used by modules should also have kernel-doc comments.
25
26It is good practice to also provide kernel-doc formatted documentation
27for functions externally visible to other kernel files (not marked
28``static``). We also recommend providing kernel-doc formatted
29documentation for private (file ``static``) routines, for consistency of
30kernel source code layout. This is lower priority and at the discretion
31of the maintainer of that kernel source file.
32
33How to format kernel-doc comments
34---------------------------------
35
36The opening comment mark ``/**`` is used for kernel-doc comments. The
37``kernel-doc`` tool will extract comments marked this way. The rest of
38the comment is formatted like a normal multi-line comment with a column
39of asterisks on the left side, closing with ``*/`` on a line by itself.
40
41The function and type kernel-doc comments should be placed just before
42the function or type being described in order to maximise the chance
43that somebody changing the code will also change the documentation. The
44overview kernel-doc comments may be placed anywhere at the top indentation
45level.
46
47Running the ``kernel-doc`` tool with increased verbosity and without actual
48output generation may be used to verify proper formatting of the
49documentation comments. For example::
50
51	scripts/kernel-doc -v -none drivers/foo/bar.c
52
53The documentation format is verified by the kernel build when it is
54requested to perform extra gcc checks::
55
56	make W=n
57
58Function documentation
59----------------------
60
61The general format of a function and function-like macro kernel-doc comment is::
62
63  /**
64   * function_name() - Brief description of function.
65   * @arg1: Describe the first argument.
66   * @arg2: Describe the second argument.
67   *        One can provide multiple line descriptions
68   *        for arguments.
69   *
70   * A longer description, with more discussion of the function function_name()
71   * that might be useful to those using or modifying it. Begins with an
72   * empty comment line, and may include additional embedded empty
73   * comment lines.
74   *
75   * The longer description may have multiple paragraphs.
76   *
77   * Context: Describes whether the function can sleep, what locks it takes,
78   *          releases, or expects to be held. It can extend over multiple
79   *          lines.
80   * Return: Describe the return value of function_name.
81   *
82   * The return value description can also have multiple paragraphs, and should
83   * be placed at the end of the comment block.
84   */
85
86The brief description following the function name may span multiple lines, and
87ends with an argument description, a blank comment line, or the end of the
88comment block.
89
90Function parameters
91~~~~~~~~~~~~~~~~~~~
92
93Each function argument should be described in order, immediately following
94the short function description.  Do not leave a blank line between the
95function description and the arguments, nor between the arguments.
96
97Each ``@argument:`` description may span multiple lines.
98
99.. note::
100
101   If the ``@argument`` description has multiple lines, the continuation
102   of the description should start at the same column as the previous line::
103
104      * @argument: some long description
105      *            that continues on next lines
106
107   or::
108
109      * @argument:
110      *		some long description
111      *		that continues on next lines
112
113If a function has a variable number of arguments, its description should
114be written in kernel-doc notation as::
115
116      * @...: description
117
118Function context
119~~~~~~~~~~~~~~~~
120
121The context in which a function can be called should be described in a
122section named ``Context``. This should include whether the function
123sleeps or can be called from interrupt context, as well as what locks
124it takes, releases and expects to be held by its caller.
125
126Examples::
127
128  * Context: Any context.
129  * Context: Any context. Takes and releases the RCU lock.
130  * Context: Any context. Expects <lock> to be held by caller.
131  * Context: Process context. May sleep if @gfp flags permit.
132  * Context: Process context. Takes and releases <mutex>.
133  * Context: Softirq or process context. Takes and releases <lock>, BH-safe.
134  * Context: Interrupt context.
135
136Return values
137~~~~~~~~~~~~~
138
139The return value, if any, should be described in a dedicated section
140named ``Return``.
141
142.. note::
143
144  #) The multi-line descriptive text you provide does *not* recognize
145     line breaks, so if you try to format some text nicely, as in::
146
147	* Return:
148	* 0 - OK
149	* -EINVAL - invalid argument
150	* -ENOMEM - out of memory
151
152     this will all run together and produce::
153
154	Return: 0 - OK -EINVAL - invalid argument -ENOMEM - out of memory
155
156     So, in order to produce the desired line breaks, you need to use a
157     ReST list, e. g.::
158
159      * Return:
160      * * 0		- OK to runtime suspend the device
161      * * -EBUSY	- Device should not be runtime suspended
162
163  #) If the descriptive text you provide has lines that begin with
164     some phrase followed by a colon, each of those phrases will be taken
165     as a new section heading, which probably won't produce the desired
166     effect.
167
168Structure, union, and enumeration documentation
169-----------------------------------------------
170
171The general format of a struct, union, and enum kernel-doc comment is::
172
173  /**
174   * struct struct_name - Brief description.
175   * @member1: Description of member1.
176   * @member2: Description of member2.
177   *           One can provide multiple line descriptions
178   *           for members.
179   *
180   * Description of the structure.
181   */
182
183You can replace the ``struct`` in the above example with ``union`` or
184``enum``  to describe unions or enums. ``member`` is used to mean struct
185and union member names as well as enumerations in an enum.
186
187The brief description following the structure name may span multiple
188lines, and ends with a member description, a blank comment line, or the
189end of the comment block.
190
191Members
192~~~~~~~
193
194Members of structs, unions and enums should be documented the same way
195as function parameters; they immediately succeed the short description
196and may be multi-line.
197
198Inside a struct or union description, you can use the ``private:`` and
199``public:`` comment tags. Structure fields that are inside a ``private:``
200area are not listed in the generated output documentation.
201
202The ``private:`` and ``public:`` tags must begin immediately following a
203``/*`` comment marker. They may optionally include comments between the
204``:`` and the ending ``*/`` marker.
205
206Example::
207
208  /**
209   * struct my_struct - short description
210   * @a: first member
211   * @b: second member
212   * @d: fourth member
213   *
214   * Longer description
215   */
216  struct my_struct {
217      int a;
218      int b;
219  /* private: internal use only */
220      int c;
221  /* public: the next one is public */
222      int d;
223  };
224
225Nested structs/unions
226~~~~~~~~~~~~~~~~~~~~~
227
228It is possible to document nested structs and unions, like::
229
230      /**
231       * struct nested_foobar - a struct with nested unions and structs
232       * @memb1: first member of anonymous union/anonymous struct
233       * @memb2: second member of anonymous union/anonymous struct
234       * @memb3: third member of anonymous union/anonymous struct
235       * @memb4: fourth member of anonymous union/anonymous struct
236       * @bar: non-anonymous union
237       * @bar.st1: struct st1 inside @bar
238       * @bar.st2: struct st2 inside @bar
239       * @bar.st1.memb1: first member of struct st1 on union bar
240       * @bar.st1.memb2: second member of struct st1 on union bar
241       * @bar.st2.memb1: first member of struct st2 on union bar
242       * @bar.st2.memb2: second member of struct st2 on union bar
243       */
244      struct nested_foobar {
245        /* Anonymous union/struct*/
246        union {
247          struct {
248            int memb1;
249            int memb2;
250        }
251          struct {
252            void *memb3;
253            int memb4;
254          }
255        }
256        union {
257          struct {
258            int memb1;
259            int memb2;
260          } st1;
261          struct {
262            void *memb1;
263            int memb2;
264          } st2;
265        } bar;
266      };
267
268.. note::
269
270   #) When documenting nested structs or unions, if the struct/union ``foo``
271      is named, the member ``bar`` inside it should be documented as
272      ``@foo.bar:``
273   #) When the nested struct/union is anonymous, the member ``bar`` in it
274      should be documented as ``@bar:``
275
276In-line member documentation comments
277~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
278
279The structure members may also be documented in-line within the definition.
280There are two styles, single-line comments where both the opening ``/**`` and
281closing ``*/`` are on the same line, and multi-line comments where they are each
282on a line of their own, like all other kernel-doc comments::
283
284  /**
285   * struct foo - Brief description.
286   * @foo: The Foo member.
287   */
288  struct foo {
289        int foo;
290        /**
291         * @bar: The Bar member.
292         */
293        int bar;
294        /**
295         * @baz: The Baz member.
296         *
297         * Here, the member description may contain several paragraphs.
298         */
299        int baz;
300        union {
301                /** @foobar: Single line description. */
302                int foobar;
303        };
304        /** @bar2: Description for struct @bar2 inside @foo */
305        struct {
306                /**
307                 * @bar2.barbar: Description for @barbar inside @foo.bar2
308                 */
309                int barbar;
310        } bar2;
311  };
312
313Typedef documentation
314---------------------
315
316The general format of a typedef kernel-doc comment is::
317
318  /**
319   * typedef type_name - Brief description.
320   *
321   * Description of the type.
322   */
323
324Typedefs with function prototypes can also be documented::
325
326  /**
327   * typedef type_name - Brief description.
328   * @arg1: description of arg1
329   * @arg2: description of arg2
330   *
331   * Description of the type.
332   *
333   * Context: Locking context.
334   * Return: Meaning of the return value.
335   */
336   typedef void (*type_name)(struct v4l2_ctrl *arg1, void *arg2);
337
338Highlights and cross-references
339-------------------------------
340
341The following special patterns are recognized in the kernel-doc comment
342descriptive text and converted to proper reStructuredText markup and `Sphinx C
343Domain`_ references.
344
345.. attention:: The below are **only** recognized within kernel-doc comments,
346	       **not** within normal reStructuredText documents.
347
348``funcname()``
349  Function reference.
350
351``@parameter``
352  Name of a function parameter. (No cross-referencing, just formatting.)
353
354``%CONST``
355  Name of a constant. (No cross-referencing, just formatting.)
356
357````literal````
358  A literal block that should be handled as-is. The output will use a
359  ``monospaced font``.
360
361  Useful if you need to use special characters that would otherwise have some
362  meaning either by kernel-doc script or by reStructuredText.
363
364  This is particularly useful if you need to use things like ``%ph`` inside
365  a function description.
366
367``$ENVVAR``
368  Name of an environment variable. (No cross-referencing, just formatting.)
369
370``&struct name``
371  Structure reference.
372
373``&enum name``
374  Enum reference.
375
376``&typedef name``
377  Typedef reference.
378
379``&struct_name->member`` or ``&struct_name.member``
380  Structure or union member reference. The cross-reference will be to the struct
381  or union definition, not the member directly.
382
383``&name``
384  A generic type reference. Prefer using the full reference described above
385  instead. This is mostly for legacy comments.
386
387Cross-referencing from reStructuredText
388~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
389
390To cross-reference the functions and types defined in the kernel-doc comments
391from reStructuredText documents, please use the `Sphinx C Domain`_
392references. For example::
393
394  See function :c:func:`foo` and struct/union/enum/typedef :c:type:`bar`.
395
396While the type reference works with just the type name, without the
397struct/union/enum/typedef part in front, you may want to use::
398
399  See :c:type:`struct foo <foo>`.
400  See :c:type:`union bar <bar>`.
401  See :c:type:`enum baz <baz>`.
402  See :c:type:`typedef meh <meh>`.
403
404This will produce prettier links, and is in line with how kernel-doc does the
405cross-references.
406
407For further details, please refer to the `Sphinx C Domain`_ documentation.
408
409Overview documentation comments
410-------------------------------
411
412To facilitate having source code and comments close together, you can include
413kernel-doc documentation blocks that are free-form comments instead of being
414kernel-doc for functions, structures, unions, enums, or typedefs. This could be
415used for something like a theory of operation for a driver or library code, for
416example.
417
418This is done by using a ``DOC:`` section keyword with a section title.
419
420The general format of an overview or high-level documentation comment is::
421
422  /**
423   * DOC: Theory of Operation
424   *
425   * The whizbang foobar is a dilly of a gizmo. It can do whatever you
426   * want it to do, at any time. It reads your mind. Here's how it works.
427   *
428   * foo bar splat
429   *
430   * The only drawback to this gizmo is that is can sometimes damage
431   * hardware, software, or its subject(s).
432   */
433
434The title following ``DOC:`` acts as a heading within the source file, but also
435as an identifier for extracting the documentation comment. Thus, the title must
436be unique within the file.
437
438Including kernel-doc comments
439=============================
440
441The documentation comments may be included in any of the reStructuredText
442documents using a dedicated kernel-doc Sphinx directive extension.
443
444The kernel-doc directive is of the format::
445
446  .. kernel-doc:: source
447     :option:
448
449The *source* is the path to a source file, relative to the kernel source
450tree. The following directive options are supported:
451
452export: *[source-pattern ...]*
453  Include documentation for all functions in *source* that have been exported
454  using ``EXPORT_SYMBOL`` or ``EXPORT_SYMBOL_GPL`` either in *source* or in any
455  of the files specified by *source-pattern*.
456
457  The *source-pattern* is useful when the kernel-doc comments have been placed
458  in header files, while ``EXPORT_SYMBOL`` and ``EXPORT_SYMBOL_GPL`` are next to
459  the function definitions.
460
461  Examples::
462
463    .. kernel-doc:: lib/bitmap.c
464       :export:
465
466    .. kernel-doc:: include/net/mac80211.h
467       :export: net/mac80211/*.c
468
469internal: *[source-pattern ...]*
470  Include documentation for all functions and types in *source* that have
471  **not** been exported using ``EXPORT_SYMBOL`` or ``EXPORT_SYMBOL_GPL`` either
472  in *source* or in any of the files specified by *source-pattern*.
473
474  Example::
475
476    .. kernel-doc:: drivers/gpu/drm/i915/intel_audio.c
477       :internal:
478
479identifiers: *[ function/type ...]*
480  Include documentation for each *function* and *type* in *source*.
481  If no *function* is specified, the documentation for all functions
482  and types in the *source* will be included.
483
484  Examples::
485
486    .. kernel-doc:: lib/bitmap.c
487       :identifiers: bitmap_parselist bitmap_parselist_user
488
489    .. kernel-doc:: lib/idr.c
490       :identifiers:
491
492functions: *[ function/type ...]*
493  This is an alias of the 'identifiers' directive and deprecated.
494
495doc: *title*
496  Include documentation for the ``DOC:`` paragraph identified by *title* in
497  *source*. Spaces are allowed in *title*; do not quote the *title*. The *title*
498  is only used as an identifier for the paragraph, and is not included in the
499  output. Please make sure to have an appropriate heading in the enclosing
500  reStructuredText document.
501
502  Example::
503
504    .. kernel-doc:: drivers/gpu/drm/i915/intel_audio.c
505       :doc: High Definition Audio over HDMI and Display Port
506
507Without options, the kernel-doc directive includes all documentation comments
508from the source file.
509
510The kernel-doc extension is included in the kernel source tree, at
511``Documentation/sphinx/kerneldoc.py``. Internally, it uses the
512``scripts/kernel-doc`` script to extract the documentation comments from the
513source.
514
515.. _kernel_doc:
516
517How to use kernel-doc to generate man pages
518-------------------------------------------
519
520If you just want to use kernel-doc to generate man pages you can do this
521from the kernel git tree::
522
523  $ scripts/kernel-doc -man \
524    $(git grep -l '/\*\*' -- :^Documentation :^tools) \
525    | scripts/split-man.pl /tmp/man
526
527Some older versions of git do not support some of the variants of syntax for
528path exclusion.  One of the following commands may work for those versions::
529
530  $ scripts/kernel-doc -man \
531    $(git grep -l '/\*\*' -- . ':!Documentation' ':!tools') \
532    | scripts/split-man.pl /tmp/man
533
534  $ scripts/kernel-doc -man \
535    $(git grep -l '/\*\*' -- . ":(exclude)Documentation" ":(exclude)tools") \
536    | scripts/split-man.pl /tmp/man
537