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  1 Introduction
  2 ------------
  4 The configuration database is a collection of configuration options
  5 organized in a tree structure:
  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         +- ...
 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.
 24 Menu entries
 25 ------------
 27 Most entries define a config option; all other entries help to organize
 28 them. A single configuration option is defined like this:
 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.  ...
 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.
 45 Menu attributes
 46 ---------------
 48 A menu entry can have a number of attributes. Not all of them are
 49 applicable everywhere (see syntax).
 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:
 57         bool "Networking support"
 58   and
 59         bool
 60         prompt "Networking support"
 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".
 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".
 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".
 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:
 91         bool "foo" if BAR
 92         default y if BAR
 93   and
 94         depends on BAR
 95         bool "foo"
 96         default y
 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.
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.
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.
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.
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:
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.)
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.
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).
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.
164 Menu dependencies
165 -----------------
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:
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)
180 Expressions are listed in decreasing order of precedence. 
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/).
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'.
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 '\'.
206 Menu structure
207 --------------
209 The position of a menu entry in the tree is determined in two ways. First
210 it can be specified explicitly:
212 menu "Network device support"
213         depends on NET
215 config NETDEVICES
216         ...
218 endmenu
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.
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
233 config MODULES
234         bool "Enable loadable module support"
236 config MODVERSIONS
237         bool "Set version information on all module symbols"
238         depends on MODULES
240 comment "module support disabled"
241         depends on !MODULES
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 always
245 visible when MODULES is visible (the (empty) dependency of MODULES is
246 also part of the comment dependencies).
249 Kconfig syntax
250 --------------
252 The configuration file describes a series of menu entries, where every
253 line starts with a keyword (except help texts). The following keywords
254 end a menu entry:
255 - config
256 - menuconfig
257 - choice/endchoice
258 - comment
259 - menu/endmenu
260 - if/endif
261 - source
262 The first five also start the definition of a menu entry.
264 config:
266         "config" <symbol>
267         <config options>
269 This defines a config symbol <symbol> and accepts any of above
270 attributes as options.
272 menuconfig:
273         "menuconfig" <symbol>
274         <config options>
276 This is similar to the simple config entry above, but it also gives a
277 hint to front ends, that all suboptions should be displayed as a
278 separate list of options.
280 choices:
282         "choice" [symbol]
283         <choice options>
284         <choice block>
285         "endchoice"
287 This defines a choice group and accepts any of the above attributes as
288 options. A choice can only be of type bool or tristate, while a boolean
289 choice only allows a single config entry to be selected, a tristate
290 choice also allows any number of config entries to be set to 'm'. This
291 can be used if multiple drivers for a single hardware exists and only a
292 single driver can be compiled/loaded into the kernel, but all drivers
293 can be compiled as modules.
294 A choice accepts another option "optional", which allows to set the
295 choice to 'n' and no entry needs to be selected.
296 If no [symbol] is associated with a choice, then you can not have multiple
297 definitions of that choice. If a [symbol] is associated to the choice,
298 then you may define the same choice (ie. with the same entries) in another
299 place.
301 comment:
303         "comment" <prompt>
304         <comment options>
306 This defines a comment which is displayed to the user during the
307 configuration process and is also echoed to the output files. The only
308 possible options are dependencies.
310 menu:
312         "menu" <prompt>
313         <menu options>
314         <menu block>
315         "endmenu"
317 This defines a menu block, see "Menu structure" above for more
318 information. The only possible options are dependencies and "visible"
319 attributes.
321 if:
323         "if" <expr>
324         <if block>
325         "endif"
327 This defines an if block. The dependency expression <expr> is appended
328 to all enclosed menu entries.
330 source:
332         "source" <prompt>
334 This reads the specified configuration file. This file is always parsed.
336 mainmenu:
338         "mainmenu" <prompt>
340 This sets the config program's title bar if the config program chooses
341 to use it. It should be placed at the top of the configuration, before any
342 other statement.
345 Kconfig hints
346 -------------
347 This is a collection of Kconfig tips, most of which aren't obvious at
348 first glance and most of which have become idioms in several Kconfig
349 files.
351 Adding common features and make the usage configurable
352 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
353 It is a common idiom to implement a feature/functionality that are
354 relevant for some architectures but not all.
355 The recommended way to do so is to use a config variable named HAVE_*
356 that is defined in a common Kconfig file and selected by the relevant
357 architectures.
358 An example is the generic IOMAP functionality.
360 We would in lib/Kconfig see:
362 # Generic IOMAP is used to ...
365 config GENERIC_IOMAP
366         depends on HAVE_GENERIC_IOMAP && FOO
368 And in lib/Makefile we would see:
369 obj-$(CONFIG_GENERIC_IOMAP) += iomap.o
371 For each architecture using the generic IOMAP functionality we would see:
373 config X86
374         select ...
375         select HAVE_GENERIC_IOMAP
376         select ...
378 Note: we use the existing config option and avoid creating a new
379 config variable to select HAVE_GENERIC_IOMAP.
381 Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is
382 introduced to overcome the limitation of select which will force a
383 config option to 'y' no matter the dependencies.
384 The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the
385 situation where select forces a symbol equals to 'y'.
387 Build as module only
388 ~~~~~~~~~~~~~~~~~~~~
389 To restrict a component build to module-only, qualify its config symbol
390 with "depends on m".  E.g.:
392 config FOO
393         depends on BAR && m
395 limits FOO to module (=m) or disabled (=n).

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