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Based on kernel version 4.9. Page generated on 2016-12-21 14:34 EST.

1	Introduction
2	------------
3	
4	The configuration database is a collection of configuration options
5	organized in a tree structure:
6	
7		+- Code maturity level options
8		|  +- Prompt for development and/or incomplete code/drivers
9		+- General setup
10		|  +- Networking support
11		|  +- System V IPC
12		|  +- BSD Process Accounting
13		|  +- Sysctl support
14		+- Loadable module support
15		|  +- Enable loadable module support
16		|     +- Set version information on all module symbols
17		|     +- Kernel module loader
18		+- ...
19	
20	Every entry has its own dependencies. These dependencies are used
21	to determine the visibility of an entry. Any child entry is only
22	visible if its parent entry is also visible.
23	
24	Menu entries
25	------------
26	
27	Most entries define a config option; all other entries help to organize
28	them. A single configuration option is defined like this:
29	
30	config MODVERSIONS
31		bool "Set version information on all module symbols"
32		depends on MODULES
33		help
34		  Usually, modules have to be recompiled whenever you switch to a new
35		  kernel.  ...
36	
37	Every line starts with a key word and can be followed by multiple
38	arguments.  "config" starts a new config entry. The following lines
39	define attributes for this config option. Attributes can be the type of
40	the config option, input prompt, dependencies, help text and default
41	values. A config option can be defined multiple times with the same
42	name, but every definition can have only a single input prompt and the
43	type must not conflict.
44	
45	Menu attributes
46	---------------
47	
48	A menu entry can have a number of attributes. Not all of them are
49	applicable everywhere (see syntax).
50	
51	- type definition: "bool"/"tristate"/"string"/"hex"/"int"
52	  Every config option must have a type. There are only two basic types:
53	  tristate and string; the other types are based on these two. The type
54	  definition optionally accepts an input prompt, so these two examples
55	  are equivalent:
56	
57		bool "Networking support"
58	  and
59		bool
60		prompt "Networking support"
61	
62	- input prompt: "prompt" <prompt> ["if" <expr>]
63	  Every menu entry can have at most one prompt, which is used to display
64	  to the user. Optionally dependencies only for this prompt can be added
65	  with "if".
66	
67	- default value: "default" <expr> ["if" <expr>]
68	  A config option can have any number of default values. If multiple
69	  default values are visible, only the first defined one is active.
70	  Default values are not limited to the menu entry where they are
71	  defined. This means the default can be defined somewhere else or be
72	  overridden by an earlier definition.
73	  The default value is only assigned to the config symbol if no other
74	  value was set by the user (via the input prompt above). If an input
75	  prompt is visible the default value is presented to the user and can
76	  be overridden by him.
77	  Optionally, dependencies only for this default value can be added with
78	  "if".
79	
80	- type definition + default value:
81		"def_bool"/"def_tristate" <expr> ["if" <expr>]
82	  This is a shorthand notation for a type definition plus a value.
83	  Optionally dependencies for this default value can be added with "if".
84	
85	- dependencies: "depends on" <expr>
86	  This defines a dependency for this menu entry. If multiple
87	  dependencies are defined, they are connected with '&&'. Dependencies
88	  are applied to all other options within this menu entry (which also
89	  accept an "if" expression), so these two examples are equivalent:
90	
91		bool "foo" if BAR
92		default y if BAR
93	  and
94		depends on BAR
95		bool "foo"
96		default y
97	
98	- reverse dependencies: "select" <symbol> ["if" <expr>]
99	  While normal dependencies reduce the upper limit of a symbol (see
100	  below), reverse dependencies can be used to force a lower limit of
101	  another symbol. The value of the current menu symbol is used as the
102	  minimal value <symbol> can be set to. If <symbol> is selected multiple
103	  times, the limit is set to the largest selection.
104	  Reverse dependencies can only be used with boolean or tristate
105	  symbols.
106	  Note:
107		select should be used with care. select will force
108		a symbol to a value without visiting the dependencies.
109		By abusing select you are able to select a symbol FOO even
110		if FOO depends on BAR that is not set.
111		In general use select only for non-visible symbols
112		(no prompts anywhere) and for symbols with no dependencies.
113		That will limit the usefulness but on the other hand avoid
114		the illegal configurations all over.
115	
116	- limiting menu display: "visible if" <expr>
117	  This attribute is only applicable to menu blocks, if the condition is
118	  false, the menu block is not displayed to the user (the symbols
119	  contained there can still be selected by other symbols, though). It is
120	  similar to a conditional "prompt" attribute for individual menu
121	  entries. Default value of "visible" is true.
122	
123	- numerical ranges: "range" <symbol> <symbol> ["if" <expr>]
124	  This allows to limit the range of possible input values for int
125	  and hex symbols. The user can only input a value which is larger than
126	  or equal to the first symbol and smaller than or equal to the second
127	  symbol.
128	
129	- help text: "help" or "---help---"
130	  This defines a help text. The end of the help text is determined by
131	  the indentation level, this means it ends at the first line which has
132	  a smaller indentation than the first line of the help text.
133	  "---help---" and "help" do not differ in behaviour, "---help---" is
134	  used to help visually separate configuration logic from help within
135	  the file as an aid to developers.
136	
137	- misc options: "option" <symbol>[=<value>]
138	  Various less common options can be defined via this option syntax,
139	  which can modify the behaviour of the menu entry and its config
140	  symbol. These options are currently possible:
141	
142	  - "defconfig_list"
143	    This declares a list of default entries which can be used when
144	    looking for the default configuration (which is used when the main
145	    .config doesn't exists yet.)
146	
147	  - "modules"
148	    This declares the symbol to be used as the MODULES symbol, which
149	    enables the third modular state for all config symbols.
150	    At most one symbol may have the "modules" option set.
151	
152	  - "env"=<value>
153	    This imports the environment variable into Kconfig. It behaves like
154	    a default, except that the value comes from the environment, this
155	    also means that the behaviour when mixing it with normal defaults is
156	    undefined at this point. The symbol is currently not exported back
157	    to the build environment (if this is desired, it can be done via
158	    another symbol).
159	
160	  - "allnoconfig_y"
161	    This declares the symbol as one that should have the value y when
162	    using "allnoconfig". Used for symbols that hide other symbols.
163	
164	Menu dependencies
165	-----------------
166	
167	Dependencies define the visibility of a menu entry and can also reduce
168	the input range of tristate symbols. The tristate logic used in the
169	expressions uses one more state than normal boolean logic to express the
170	module state. Dependency expressions have the following syntax:
171	
172	<expr> ::= <symbol>                             (1)
173	           <symbol> '=' <symbol>                (2)
174	           <symbol> '!=' <symbol>               (3)
175	           '(' <expr> ')'                       (4)
176	           '!' <expr>                           (5)
177	           <expr> '&&' <expr>                   (6)
178	           <expr> '||' <expr>                   (7)
179	
180	Expressions are listed in decreasing order of precedence. 
181	
182	(1) Convert the symbol into an expression. Boolean and tristate symbols
183	    are simply converted into the respective expression values. All
184	    other symbol types result in 'n'.
185	(2) If the values of both symbols are equal, it returns 'y',
186	    otherwise 'n'.
187	(3) If the values of both symbols are equal, it returns 'n',
188	    otherwise 'y'.
189	(4) Returns the value of the expression. Used to override precedence.
190	(5) Returns the result of (2-/expr/).
191	(6) Returns the result of min(/expr/, /expr/).
192	(7) Returns the result of max(/expr/, /expr/).
193	
194	An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2
195	respectively for calculations). A menu entry becomes visible when its
196	expression evaluates to 'm' or 'y'.
197	
198	There are two types of symbols: constant and non-constant symbols.
199	Non-constant symbols are the most common ones and are defined with the
200	'config' statement. Non-constant symbols consist entirely of alphanumeric
201	characters or underscores.
202	Constant symbols are only part of expressions. Constant symbols are
203	always surrounded by single or double quotes. Within the quote, any
204	other character is allowed and the quotes can be escaped using '\'.
205	
206	Menu structure
207	--------------
208	
209	The position of a menu entry in the tree is determined in two ways. First
210	it can be specified explicitly:
211	
212	menu "Network device support"
213		depends on NET
214	
215	config NETDEVICES
216		...
217	
218	endmenu
219	
220	All entries within the "menu" ... "endmenu" block become a submenu of
221	"Network device support". All subentries inherit the dependencies from
222	the menu entry, e.g. this means the dependency "NET" is added to the
223	dependency list of the config option NETDEVICES.
224	
225	The other way to generate the menu structure is done by analyzing the
226	dependencies. If a menu entry somehow depends on the previous entry, it
227	can be made a submenu of it. First, the previous (parent) symbol must
228	be part of the dependency list and then one of these two conditions
229	must be true:
230	- the child entry must become invisible, if the parent is set to 'n'
231	- the child entry must only be visible, if the parent is visible
232	
233	config MODULES
234		bool "Enable loadable module support"
235	
236	config MODVERSIONS
237		bool "Set version information on all module symbols"
238		depends on MODULES
239	
240	comment "module support disabled"
241		depends on !MODULES
242	
243	MODVERSIONS directly depends on MODULES, this means it's only visible if
244	MODULES is different from 'n'. The comment on the other hand is only
245	visible when MODULES is set to 'n'.
246	
247	
248	Kconfig syntax
249	--------------
250	
251	The configuration file describes a series of menu entries, where every
252	line starts with a keyword (except help texts). The following keywords
253	end a menu entry:
254	- config
255	- menuconfig
256	- choice/endchoice
257	- comment
258	- menu/endmenu
259	- if/endif
260	- source
261	The first five also start the definition of a menu entry.
262	
263	config:
264	
265		"config" <symbol>
266		<config options>
267	
268	This defines a config symbol <symbol> and accepts any of above
269	attributes as options.
270	
271	menuconfig:
272		"menuconfig" <symbol>
273		<config options>
274	
275	This is similar to the simple config entry above, but it also gives a
276	hint to front ends, that all suboptions should be displayed as a
277	separate list of options. To make sure all the suboptions will really
278	show up under the menuconfig entry and not outside of it, every item
279	from the <config options> list must depend on the menuconfig symbol.
280	In practice, this is achieved by using one of the next two constructs:
281	
282	(1):
283	menuconfig M
284	if M
285	    config C1
286	    config C2
287	endif
288	
289	(2):
290	menuconfig M
291	config C1
292	    depends on M
293	config C2
294	    depends on M
295	
296	In the following examples (3) and (4), C1 and C2 still have the M
297	dependency, but will not appear under menuconfig M anymore, because
298	of C0, which doesn't depend on M:
299	
300	(3):
301	menuconfig M
302	    config C0
303	if M
304	    config C1
305	    config C2
306	endif
307	
308	(4):
309	menuconfig M
310	config C0
311	config C1
312	    depends on M
313	config C2
314	    depends on M
315	
316	choices:
317	
318		"choice" [symbol]
319		<choice options>
320		<choice block>
321		"endchoice"
322	
323	This defines a choice group and accepts any of the above attributes as
324	options. A choice can only be of type bool or tristate.  If no type is
325	specified for a choice, it's type will be determined by the type of
326	the first choice element in the group or remain unknown if none of the
327	choice elements have a type specified, as well.
328	
329	While a boolean choice only allows a single config entry to be
330	selected, a tristate choice also allows any number of config entries
331	to be set to 'm'. This can be used if multiple drivers for a single
332	hardware exists and only a single driver can be compiled/loaded into
333	the kernel, but all drivers can be compiled as modules.
334	
335	A choice accepts another option "optional", which allows to set the
336	choice to 'n' and no entry needs to be selected.
337	If no [symbol] is associated with a choice, then you can not have multiple
338	definitions of that choice. If a [symbol] is associated to the choice,
339	then you may define the same choice (ie. with the same entries) in another
340	place.
341	
342	comment:
343	
344		"comment" <prompt>
345		<comment options>
346	
347	This defines a comment which is displayed to the user during the
348	configuration process and is also echoed to the output files. The only
349	possible options are dependencies.
350	
351	menu:
352	
353		"menu" <prompt>
354		<menu options>
355		<menu block>
356		"endmenu"
357	
358	This defines a menu block, see "Menu structure" above for more
359	information. The only possible options are dependencies and "visible"
360	attributes.
361	
362	if:
363	
364		"if" <expr>
365		<if block>
366		"endif"
367	
368	This defines an if block. The dependency expression <expr> is appended
369	to all enclosed menu entries.
370	
371	source:
372	
373		"source" <prompt>
374	
375	This reads the specified configuration file. This file is always parsed.
376	
377	mainmenu:
378	
379		"mainmenu" <prompt>
380	
381	This sets the config program's title bar if the config program chooses
382	to use it. It should be placed at the top of the configuration, before any
383	other statement.
384	
385	
386	Kconfig hints
387	-------------
388	This is a collection of Kconfig tips, most of which aren't obvious at
389	first glance and most of which have become idioms in several Kconfig
390	files.
391	
392	Adding common features and make the usage configurable
393	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
394	It is a common idiom to implement a feature/functionality that are
395	relevant for some architectures but not all.
396	The recommended way to do so is to use a config variable named HAVE_*
397	that is defined in a common Kconfig file and selected by the relevant
398	architectures.
399	An example is the generic IOMAP functionality.
400	
401	We would in lib/Kconfig see:
402	
403	# Generic IOMAP is used to ...
404	config HAVE_GENERIC_IOMAP
405	
406	config GENERIC_IOMAP
407		depends on HAVE_GENERIC_IOMAP && FOO
408	
409	And in lib/Makefile we would see:
410	obj-$(CONFIG_GENERIC_IOMAP) += iomap.o
411	
412	For each architecture using the generic IOMAP functionality we would see:
413	
414	config X86
415		select ...
416		select HAVE_GENERIC_IOMAP
417		select ...
418	
419	Note: we use the existing config option and avoid creating a new
420	config variable to select HAVE_GENERIC_IOMAP.
421	
422	Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is
423	introduced to overcome the limitation of select which will force a
424	config option to 'y' no matter the dependencies.
425	The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the
426	situation where select forces a symbol equals to 'y'.
427	
428	Build as module only
429	~~~~~~~~~~~~~~~~~~~~
430	To restrict a component build to module-only, qualify its config symbol
431	with "depends on m".  E.g.:
432	
433	config FOO
434		depends on BAR && m
435	
436	limits FOO to module (=m) or disabled (=n).
437	
438	Kconfig recursive dependency limitations
439	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
440	
441	If you've hit the Kconfig error: "recursive dependency detected" you've run
442	into a recursive dependency issue with Kconfig, a recursive dependency can be
443	summarized as a circular dependency. The kconfig tools need to ensure that
444	Kconfig files comply with specified configuration requirements. In order to do
445	that kconfig must determine the values that are possible for all Kconfig
446	symbols, this is currently not possible if there is a circular relation
447	between two or more Kconfig symbols. For more details refer to the "Simple
448	Kconfig recursive issue" subsection below. Kconfig does not do recursive
449	dependency resolution; this has a few implications for Kconfig file writers.
450	We'll first explain why this issues exists and then provide an example
451	technical limitation which this brings upon Kconfig developers. Eager
452	developers wishing to try to address this limitation should read the next
453	subsections.
454	
455	Simple Kconfig recursive issue
456	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
457	
458	Read: Documentation/kbuild/Kconfig.recursion-issue-01
459	
460	Test with:
461	
462	make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-01 allnoconfig
463	
464	Cumulative Kconfig recursive issue
465	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
466	
467	Read: Documentation/kbuild/Kconfig.recursion-issue-02
468	
469	Test with:
470	
471	make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-02 allnoconfig
472	
473	Practical solutions to kconfig recursive issue
474	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
475	
476	Developers who run into the recursive Kconfig issue have three options
477	at their disposal. We document them below and also provide a list of
478	historical issues resolved through these different solutions.
479	
480	  a) Remove any superfluous "select FOO" or "depends on FOO"
481	  b) Match dependency semantics:
482		b1) Swap all "select FOO" to "depends on FOO" or,
483		b2) Swap all "depends on FOO" to "select FOO"
484	
485	The resolution to a) can be tested with the sample Kconfig file
486	Documentation/kbuild/Kconfig.recursion-issue-01 through the removal
487	of the "select CORE" from CORE_BELL_A_ADVANCED as that is implicit already
488	since CORE_BELL_A depends on CORE. At times it may not be possible to remove
489	some dependency criteria, for such cases you can work with solution b).
490	
491	The two different resolutions for b) can be tested in the sample Kconfig file
492	Documentation/kbuild/Kconfig.recursion-issue-02.
493	
494	Below is a list of examples of prior fixes for these types of recursive issues;
495	all errors appear to involve one or more select's and one or more "depends on".
496	
497	commit          fix
498	======          ===
499	06b718c01208    select A -> depends on A
500	c22eacfe82f9    depends on A -> depends on B
501	6a91e854442c    select A -> depends on A
502	118c565a8f2e    select A -> select B
503	f004e5594705    select A -> depends on A
504	c7861f37b4c6    depends on A -> (null)
505	80c69915e5fb    select A -> (null)              (1)
506	c2218e26c0d0    select A -> depends on A        (1)
507	d6ae99d04e1c    select A -> depends on A
508	95ca19cf8cbf    select A -> depends on A
509	8f057d7bca54    depends on A -> (null)
510	8f057d7bca54    depends on A -> select A
511	a0701f04846e    select A -> depends on A
512	0c8b92f7f259    depends on A -> (null)
513	e4e9e0540928    select A -> depends on A        (2)
514	7453ea886e87    depends on A > (null)           (1)
515	7b1fff7e4fdf    select A -> depends on A
516	86c747d2a4f0    select A -> depends on A
517	d9f9ab51e55e    select A -> depends on A
518	0c51a4d8abd6    depends on A -> select A        (3)
519	e98062ed6dc4    select A -> depends on A        (3)
520	91e5d284a7f1    select A -> (null)
521	
522	(1) Partial (or no) quote of error.
523	(2) That seems to be the gist of that fix.
524	(3) Same error.
525	
526	Future kconfig work
527	~~~~~~~~~~~~~~~~~~~
528	
529	Work on kconfig is welcomed on both areas of clarifying semantics and on
530	evaluating the use of a full SAT solver for it. A full SAT solver can be
531	desirable to enable more complex dependency mappings and / or queries,
532	for instance on possible use case for a SAT solver could be that of handling
533	the current known recursive dependency issues. It is not known if this would
534	address such issues but such evaluation is desirable. If support for a full SAT
535	solver proves too complex or that it cannot address recursive dependency issues
536	Kconfig should have at least clear and well defined semantics which also
537	addresses and documents limitations or requirements such as the ones dealing
538	with recursive dependencies.
539	
540	Further work on both of these areas is welcomed on Kconfig. We elaborate
541	on both of these in the next two subsections.
542	
543	Semantics of Kconfig
544	~~~~~~~~~~~~~~~~~~~~
545	
546	The use of Kconfig is broad, Linux is now only one of Kconfig's users:
547	one study has completed a broad analysis of Kconfig use in 12 projects [0].
548	Despite its widespread use, and although this document does a reasonable job
549	in documenting basic Kconfig syntax a more precise definition of Kconfig
550	semantics is welcomed. One project deduced Kconfig semantics through
551	the use of the xconfig configurator [1]. Work should be done to confirm if
552	the deduced semantics matches our intended Kconfig design goals.
553	
554	Having well defined semantics can be useful for tools for practical
555	evaluation of depenencies, for instance one such use known case was work to
556	express in boolean abstraction of the inferred semantics of Kconfig to
557	translate Kconfig logic into boolean formulas and run a SAT solver on this to
558	find dead code / features (always inactive), 114 dead features were found in
559	Linux using this methodology [1] (Section 8: Threats to validity).
560	
561	Confirming this could prove useful as Kconfig stands as one of the the leading
562	industrial variability modeling languages [1] [2]. Its study would help
563	evaluate practical uses of such languages, their use was only theoretical
564	and real world requirements were not well understood. As it stands though
565	only reverse engineering techniques have been used to deduce semantics from
566	variability modeling languages such as Kconfig [3].
567	
568	[0] http://www.eng.uwaterloo.ca/~shshe/kconfig_semantics.pdf
569	[1] http://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf
570	[2] http://gsd.uwaterloo.ca/sites/default/files/ase241-berger_0.pdf
571	[3] http://gsd.uwaterloo.ca/sites/default/files/icse2011.pdf
572	
573	Full SAT solver for Kconfig
574	~~~~~~~~~~~~~~~~~~~~~~~~~~~
575	
576	Although SAT solvers [0] haven't yet been used by Kconfig directly, as noted in
577	the previous subsection, work has been done however to express in boolean
578	abstraction the inferred semantics of Kconfig to translate Kconfig logic into
579	boolean formulas and run a SAT solver on it [1]. Another known related project
580	is CADOS [2] (former VAMOS [3]) and the tools, mainly undertaker [4], which has
581	been introduced first with [5].  The basic concept of undertaker is to exract
582	variability models from Kconfig, and put them together with a propositional
583	formula extracted from CPP #ifdefs and build-rules into a SAT solver in order
584	to find dead code, dead files, and dead symbols. If using a SAT solver is
585	desirable on Kconfig one approach would be to evaluate repurposing such efforts
586	somehow on Kconfig. There is enough interest from mentors of existing projects
587	to not only help advise how to integrate this work upstream but also help
588	maintain it long term. Interested developers should visit:
589	
590	http://kernelnewbies.org/KernelProjects/kconfig-sat
591	
592	[0] http://www.cs.cornell.edu/~sabhar/chapters/SATSolvers-KR-Handbook.pdf
593	[1] http://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf
594	[2] https://cados.cs.fau.de
595	[3] https://vamos.cs.fau.de
596	[4] https://undertaker.cs.fau.de
597	[5] https://www4.cs.fau.de/Publications/2011/tartler_11_eurosys.pdf
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