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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 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	- weak reverse dependencies: "imply" <symbol> ["if" <expr>]
117	  This is similar to "select" as it enforces a lower limit on another
118	  symbol except that the "implied" symbol's value may still be set to n
119	  from a direct dependency or with a visible prompt.
120	
121	  Given the following example:
122	
123	  config FOO
124		tristate
125		imply BAZ
126	
127	  config BAZ
128		tristate
129		depends on BAR
130	
131	  The following values are possible:
132	
133		FOO		BAR		BAZ's default	choice for BAZ
134		---		---		-------------	--------------
135		n		y		n		N/m/y
136		m		y		m		M/y/n
137		y		y		y		Y/n
138		y		n		*		N
139	
140	  This is useful e.g. with multiple drivers that want to indicate their
141	  ability to hook into a secondary subsystem while allowing the user to
142	  configure that subsystem out without also having to unset these drivers.
143	
144	- limiting menu display: "visible if" <expr>
145	  This attribute is only applicable to menu blocks, if the condition is
146	  false, the menu block is not displayed to the user (the symbols
147	  contained there can still be selected by other symbols, though). It is
148	  similar to a conditional "prompt" attribute for individual menu
149	  entries. Default value of "visible" is true.
150	
151	- numerical ranges: "range" <symbol> <symbol> ["if" <expr>]
152	  This allows to limit the range of possible input values for int
153	  and hex symbols. The user can only input a value which is larger than
154	  or equal to the first symbol and smaller than or equal to the second
155	  symbol.
156	
157	- help text: "help" or "---help---"
158	  This defines a help text. The end of the help text is determined by
159	  the indentation level, this means it ends at the first line which has
160	  a smaller indentation than the first line of the help text.
161	  "---help---" and "help" do not differ in behaviour, "---help---" is
162	  used to help visually separate configuration logic from help within
163	  the file as an aid to developers.
164	
165	- misc options: "option" <symbol>[=<value>]
166	  Various less common options can be defined via this option syntax,
167	  which can modify the behaviour of the menu entry and its config
168	  symbol. These options are currently possible:
169	
170	  - "defconfig_list"
171	    This declares a list of default entries which can be used when
172	    looking for the default configuration (which is used when the main
173	    .config doesn't exists yet.)
174	
175	  - "modules"
176	    This declares the symbol to be used as the MODULES symbol, which
177	    enables the third modular state for all config symbols.
178	    At most one symbol may have the "modules" option set.
179	
180	  - "env"=<value>
181	    This imports the environment variable into Kconfig. It behaves like
182	    a default, except that the value comes from the environment, this
183	    also means that the behaviour when mixing it with normal defaults is
184	    undefined at this point. The symbol is currently not exported back
185	    to the build environment (if this is desired, it can be done via
186	    another symbol).
187	
188	  - "allnoconfig_y"
189	    This declares the symbol as one that should have the value y when
190	    using "allnoconfig". Used for symbols that hide other symbols.
191	
192	Menu dependencies
193	-----------------
194	
195	Dependencies define the visibility of a menu entry and can also reduce
196	the input range of tristate symbols. The tristate logic used in the
197	expressions uses one more state than normal boolean logic to express the
198	module state. Dependency expressions have the following syntax:
199	
200	<expr> ::= <symbol>                             (1)
201	           <symbol> '=' <symbol>                (2)
202	           <symbol> '!=' <symbol>               (3)
203	           '(' <expr> ')'                       (4)
204	           '!' <expr>                           (5)
205	           <expr> '&&' <expr>                   (6)
206	           <expr> '||' <expr>                   (7)
207	
208	Expressions are listed in decreasing order of precedence. 
209	
210	(1) Convert the symbol into an expression. Boolean and tristate symbols
211	    are simply converted into the respective expression values. All
212	    other symbol types result in 'n'.
213	(2) If the values of both symbols are equal, it returns 'y',
214	    otherwise 'n'.
215	(3) If the values of both symbols are equal, it returns 'n',
216	    otherwise 'y'.
217	(4) Returns the value of the expression. Used to override precedence.
218	(5) Returns the result of (2-/expr/).
219	(6) Returns the result of min(/expr/, /expr/).
220	(7) Returns the result of max(/expr/, /expr/).
221	
222	An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2
223	respectively for calculations). A menu entry becomes visible when its
224	expression evaluates to 'm' or 'y'.
225	
226	There are two types of symbols: constant and non-constant symbols.
227	Non-constant symbols are the most common ones and are defined with the
228	'config' statement. Non-constant symbols consist entirely of alphanumeric
229	characters or underscores.
230	Constant symbols are only part of expressions. Constant symbols are
231	always surrounded by single or double quotes. Within the quote, any
232	other character is allowed and the quotes can be escaped using '\'.
233	
234	Menu structure
235	--------------
236	
237	The position of a menu entry in the tree is determined in two ways. First
238	it can be specified explicitly:
239	
240	menu "Network device support"
241		depends on NET
242	
243	config NETDEVICES
244		...
245	
246	endmenu
247	
248	All entries within the "menu" ... "endmenu" block become a submenu of
249	"Network device support". All subentries inherit the dependencies from
250	the menu entry, e.g. this means the dependency "NET" is added to the
251	dependency list of the config option NETDEVICES.
252	
253	The other way to generate the menu structure is done by analyzing the
254	dependencies. If a menu entry somehow depends on the previous entry, it
255	can be made a submenu of it. First, the previous (parent) symbol must
256	be part of the dependency list and then one of these two conditions
257	must be true:
258	- the child entry must become invisible, if the parent is set to 'n'
259	- the child entry must only be visible, if the parent is visible
260	
261	config MODULES
262		bool "Enable loadable module support"
263	
264	config MODVERSIONS
265		bool "Set version information on all module symbols"
266		depends on MODULES
267	
268	comment "module support disabled"
269		depends on !MODULES
270	
271	MODVERSIONS directly depends on MODULES, this means it's only visible if
272	MODULES is different from 'n'. The comment on the other hand is only
273	visible when MODULES is set to 'n'.
274	
275	
276	Kconfig syntax
277	--------------
278	
279	The configuration file describes a series of menu entries, where every
280	line starts with a keyword (except help texts). The following keywords
281	end a menu entry:
282	- config
283	- menuconfig
284	- choice/endchoice
285	- comment
286	- menu/endmenu
287	- if/endif
288	- source
289	The first five also start the definition of a menu entry.
290	
291	config:
292	
293		"config" <symbol>
294		<config options>
295	
296	This defines a config symbol <symbol> and accepts any of above
297	attributes as options.
298	
299	menuconfig:
300		"menuconfig" <symbol>
301		<config options>
302	
303	This is similar to the simple config entry above, but it also gives a
304	hint to front ends, that all suboptions should be displayed as a
305	separate list of options. To make sure all the suboptions will really
306	show up under the menuconfig entry and not outside of it, every item
307	from the <config options> list must depend on the menuconfig symbol.
308	In practice, this is achieved by using one of the next two constructs:
309	
310	(1):
311	menuconfig M
312	if M
313	    config C1
314	    config C2
315	endif
316	
317	(2):
318	menuconfig M
319	config C1
320	    depends on M
321	config C2
322	    depends on M
323	
324	In the following examples (3) and (4), C1 and C2 still have the M
325	dependency, but will not appear under menuconfig M anymore, because
326	of C0, which doesn't depend on M:
327	
328	(3):
329	menuconfig M
330	    config C0
331	if M
332	    config C1
333	    config C2
334	endif
335	
336	(4):
337	menuconfig M
338	config C0
339	config C1
340	    depends on M
341	config C2
342	    depends on M
343	
344	choices:
345	
346		"choice" [symbol]
347		<choice options>
348		<choice block>
349		"endchoice"
350	
351	This defines a choice group and accepts any of the above attributes as
352	options. A choice can only be of type bool or tristate.  If no type is
353	specified for a choice, it's type will be determined by the type of
354	the first choice element in the group or remain unknown if none of the
355	choice elements have a type specified, as well.
356	
357	While a boolean choice only allows a single config entry to be
358	selected, a tristate choice also allows any number of config entries
359	to be set to 'm'. This can be used if multiple drivers for a single
360	hardware exists and only a single driver can be compiled/loaded into
361	the kernel, but all drivers can be compiled as modules.
362	
363	A choice accepts another option "optional", which allows to set the
364	choice to 'n' and no entry needs to be selected.
365	If no [symbol] is associated with a choice, then you can not have multiple
366	definitions of that choice. If a [symbol] is associated to the choice,
367	then you may define the same choice (ie. with the same entries) in another
368	place.
369	
370	comment:
371	
372		"comment" <prompt>
373		<comment options>
374	
375	This defines a comment which is displayed to the user during the
376	configuration process and is also echoed to the output files. The only
377	possible options are dependencies.
378	
379	menu:
380	
381		"menu" <prompt>
382		<menu options>
383		<menu block>
384		"endmenu"
385	
386	This defines a menu block, see "Menu structure" above for more
387	information. The only possible options are dependencies and "visible"
388	attributes.
389	
390	if:
391	
392		"if" <expr>
393		<if block>
394		"endif"
395	
396	This defines an if block. The dependency expression <expr> is appended
397	to all enclosed menu entries.
398	
399	source:
400	
401		"source" <prompt>
402	
403	This reads the specified configuration file. This file is always parsed.
404	
405	mainmenu:
406	
407		"mainmenu" <prompt>
408	
409	This sets the config program's title bar if the config program chooses
410	to use it. It should be placed at the top of the configuration, before any
411	other statement.
412	
413	
414	Kconfig hints
415	-------------
416	This is a collection of Kconfig tips, most of which aren't obvious at
417	first glance and most of which have become idioms in several Kconfig
418	files.
419	
420	Adding common features and make the usage configurable
421	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
422	It is a common idiom to implement a feature/functionality that are
423	relevant for some architectures but not all.
424	The recommended way to do so is to use a config variable named HAVE_*
425	that is defined in a common Kconfig file and selected by the relevant
426	architectures.
427	An example is the generic IOMAP functionality.
428	
429	We would in lib/Kconfig see:
430	
431	# Generic IOMAP is used to ...
432	config HAVE_GENERIC_IOMAP
433	
434	config GENERIC_IOMAP
435		depends on HAVE_GENERIC_IOMAP && FOO
436	
437	And in lib/Makefile we would see:
438	obj-$(CONFIG_GENERIC_IOMAP) += iomap.o
439	
440	For each architecture using the generic IOMAP functionality we would see:
441	
442	config X86
443		select ...
444		select HAVE_GENERIC_IOMAP
445		select ...
446	
447	Note: we use the existing config option and avoid creating a new
448	config variable to select HAVE_GENERIC_IOMAP.
449	
450	Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is
451	introduced to overcome the limitation of select which will force a
452	config option to 'y' no matter the dependencies.
453	The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the
454	situation where select forces a symbol equals to 'y'.
455	
456	Build as module only
457	~~~~~~~~~~~~~~~~~~~~
458	To restrict a component build to module-only, qualify its config symbol
459	with "depends on m".  E.g.:
460	
461	config FOO
462		depends on BAR && m
463	
464	limits FOO to module (=m) or disabled (=n).
465	
466	Kconfig recursive dependency limitations
467	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
468	
469	If you've hit the Kconfig error: "recursive dependency detected" you've run
470	into a recursive dependency issue with Kconfig, a recursive dependency can be
471	summarized as a circular dependency. The kconfig tools need to ensure that
472	Kconfig files comply with specified configuration requirements. In order to do
473	that kconfig must determine the values that are possible for all Kconfig
474	symbols, this is currently not possible if there is a circular relation
475	between two or more Kconfig symbols. For more details refer to the "Simple
476	Kconfig recursive issue" subsection below. Kconfig does not do recursive
477	dependency resolution; this has a few implications for Kconfig file writers.
478	We'll first explain why this issues exists and then provide an example
479	technical limitation which this brings upon Kconfig developers. Eager
480	developers wishing to try to address this limitation should read the next
481	subsections.
482	
483	Simple Kconfig recursive issue
484	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
485	
486	Read: Documentation/kbuild/Kconfig.recursion-issue-01
487	
488	Test with:
489	
490	make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-01 allnoconfig
491	
492	Cumulative Kconfig recursive issue
493	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
494	
495	Read: Documentation/kbuild/Kconfig.recursion-issue-02
496	
497	Test with:
498	
499	make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-02 allnoconfig
500	
501	Practical solutions to kconfig recursive issue
502	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
503	
504	Developers who run into the recursive Kconfig issue have three options
505	at their disposal. We document them below and also provide a list of
506	historical issues resolved through these different solutions.
507	
508	  a) Remove any superfluous "select FOO" or "depends on FOO"
509	  b) Match dependency semantics:
510		b1) Swap all "select FOO" to "depends on FOO" or,
511		b2) Swap all "depends on FOO" to "select FOO"
512	  c) Consider the use of "imply" instead of "select"
513	
514	The resolution to a) can be tested with the sample Kconfig file
515	Documentation/kbuild/Kconfig.recursion-issue-01 through the removal
516	of the "select CORE" from CORE_BELL_A_ADVANCED as that is implicit already
517	since CORE_BELL_A depends on CORE. At times it may not be possible to remove
518	some dependency criteria, for such cases you can work with solution b).
519	
520	The two different resolutions for b) can be tested in the sample Kconfig file
521	Documentation/kbuild/Kconfig.recursion-issue-02.
522	
523	Below is a list of examples of prior fixes for these types of recursive issues;
524	all errors appear to involve one or more select's and one or more "depends on".
525	
526	commit          fix
527	======          ===
528	06b718c01208    select A -> depends on A
529	c22eacfe82f9    depends on A -> depends on B
530	6a91e854442c    select A -> depends on A
531	118c565a8f2e    select A -> select B
532	f004e5594705    select A -> depends on A
533	c7861f37b4c6    depends on A -> (null)
534	80c69915e5fb    select A -> (null)              (1)
535	c2218e26c0d0    select A -> depends on A        (1)
536	d6ae99d04e1c    select A -> depends on A
537	95ca19cf8cbf    select A -> depends on A
538	8f057d7bca54    depends on A -> (null)
539	8f057d7bca54    depends on A -> select A
540	a0701f04846e    select A -> depends on A
541	0c8b92f7f259    depends on A -> (null)
542	e4e9e0540928    select A -> depends on A        (2)
543	7453ea886e87    depends on A > (null)           (1)
544	7b1fff7e4fdf    select A -> depends on A
545	86c747d2a4f0    select A -> depends on A
546	d9f9ab51e55e    select A -> depends on A
547	0c51a4d8abd6    depends on A -> select A        (3)
548	e98062ed6dc4    select A -> depends on A        (3)
549	91e5d284a7f1    select A -> (null)
550	
551	(1) Partial (or no) quote of error.
552	(2) That seems to be the gist of that fix.
553	(3) Same error.
554	
555	Future kconfig work
556	~~~~~~~~~~~~~~~~~~~
557	
558	Work on kconfig is welcomed on both areas of clarifying semantics and on
559	evaluating the use of a full SAT solver for it. A full SAT solver can be
560	desirable to enable more complex dependency mappings and / or queries,
561	for instance on possible use case for a SAT solver could be that of handling
562	the current known recursive dependency issues. It is not known if this would
563	address such issues but such evaluation is desirable. If support for a full SAT
564	solver proves too complex or that it cannot address recursive dependency issues
565	Kconfig should have at least clear and well defined semantics which also
566	addresses and documents limitations or requirements such as the ones dealing
567	with recursive dependencies.
568	
569	Further work on both of these areas is welcomed on Kconfig. We elaborate
570	on both of these in the next two subsections.
571	
572	Semantics of Kconfig
573	~~~~~~~~~~~~~~~~~~~~
574	
575	The use of Kconfig is broad, Linux is now only one of Kconfig's users:
576	one study has completed a broad analysis of Kconfig use in 12 projects [0].
577	Despite its widespread use, and although this document does a reasonable job
578	in documenting basic Kconfig syntax a more precise definition of Kconfig
579	semantics is welcomed. One project deduced Kconfig semantics through
580	the use of the xconfig configurator [1]. Work should be done to confirm if
581	the deduced semantics matches our intended Kconfig design goals.
582	
583	Having well defined semantics can be useful for tools for practical
584	evaluation of depenencies, for instance one such use known case was work to
585	express in boolean abstraction of the inferred semantics of Kconfig to
586	translate Kconfig logic into boolean formulas and run a SAT solver on this to
587	find dead code / features (always inactive), 114 dead features were found in
588	Linux using this methodology [1] (Section 8: Threats to validity).
589	
590	Confirming this could prove useful as Kconfig stands as one of the the leading
591	industrial variability modeling languages [1] [2]. Its study would help
592	evaluate practical uses of such languages, their use was only theoretical
593	and real world requirements were not well understood. As it stands though
594	only reverse engineering techniques have been used to deduce semantics from
595	variability modeling languages such as Kconfig [3].
596	
597	[0] http://www.eng.uwaterloo.ca/~shshe/kconfig_semantics.pdf
598	[1] http://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf
599	[2] http://gsd.uwaterloo.ca/sites/default/files/ase241-berger_0.pdf
600	[3] http://gsd.uwaterloo.ca/sites/default/files/icse2011.pdf
601	
602	Full SAT solver for Kconfig
603	~~~~~~~~~~~~~~~~~~~~~~~~~~~
604	
605	Although SAT solvers [0] haven't yet been used by Kconfig directly, as noted in
606	the previous subsection, work has been done however to express in boolean
607	abstraction the inferred semantics of Kconfig to translate Kconfig logic into
608	boolean formulas and run a SAT solver on it [1]. Another known related project
609	is CADOS [2] (former VAMOS [3]) and the tools, mainly undertaker [4], which has
610	been introduced first with [5].  The basic concept of undertaker is to exract
611	variability models from Kconfig, and put them together with a propositional
612	formula extracted from CPP #ifdefs and build-rules into a SAT solver in order
613	to find dead code, dead files, and dead symbols. If using a SAT solver is
614	desirable on Kconfig one approach would be to evaluate repurposing such efforts
615	somehow on Kconfig. There is enough interest from mentors of existing projects
616	to not only help advise how to integrate this work upstream but also help
617	maintain it long term. Interested developers should visit:
618	
619	http://kernelnewbies.org/KernelProjects/kconfig-sat
620	
621	[0] http://www.cs.cornell.edu/~sabhar/chapters/SATSolvers-KR-Handbook.pdf
622	[1] http://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf
623	[2] https://cados.cs.fau.de
624	[3] https://vamos.cs.fau.de
625	[4] https://undertaker.cs.fau.de
626	[5] https://www4.cs.fau.de/Publications/2011/tartler_11_eurosys.pdf
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