Linux/Documentation/kbuild/kconfig-language.txt

  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"  ["if" ]
 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"  ["if" ]
 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"  ["if" ]
 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" 
 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"  ["if" ]
 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  can be set to. If  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 - numerical ranges: "range"   ["if" ]
117   This allows to limit the range of possible input values for int
118   and hex symbols. The user can only input a value which is larger than
119   or equal to the first symbol and smaller than or equal to the second
120   symbol.
121 
122 - help text: "help" or "---help---"
123   This defines a help text. The end of the help text is determined by
124   the indentation level, this means it ends at the first line which has
125   a smaller indentation than the first line of the help text.
126   "---help---" and "help" do not differ in behaviour, "---help---" is
127   used to help visually separate configuration logic from help within
128   the file as an aid to developers.
129 
130 - misc options: "option" [=]
131   Various less common options can be defined via this option syntax,
132   which can modify the behaviour of the menu entry and its config
133   symbol. These options are currently possible:
134 
135   - "defconfig_list"
136     This declares a list of default entries which can be used when
137     looking for the default configuration (which is used when the main
138     .config doesn't exists yet.)
139 
140   - "modules"
141     This declares the symbol to be used as the MODULES symbol, which
142     enables the third modular state for all config symbols.
143 
144   - "env"=
145     This imports the environment variable into Kconfig. It behaves like
146     a default, except that the value comes from the environment, this
147     also means that the behaviour when mixing it with normal defaults is
148     undefined at this point. The symbol is currently not exported back
149     to the build environment (if this is desired, it can be done via
150     another symbol).
151 
152 Menu dependencies
153 -----------------
154 
155 Dependencies define the visibility of a menu entry and can also reduce
156 the input range of tristate symbols. The tristate logic used in the
157 expressions uses one more state than normal boolean logic to express the
158 module state. Dependency expressions have the following syntax:
159 
160  ::=                              (1)
161             '='                 (2)
162             '!='                (3)
163            '('  ')'                       (4)
164            '!'                            (5)
165             '&&'                    (6)
166             '||'                    (7)
167 
168 Expressions are listed in decreasing order of precedence. 
169 
170 (1) Convert the symbol into an expression. Boolean and tristate symbols
171     are simply converted into the respective expression values. All
172     other symbol types result in 'n'.
173 (2) If the values of both symbols are equal, it returns 'y',
174     otherwise 'n'.
175 (3) If the values of both symbols are equal, it returns 'n',
176     otherwise 'y'.
177 (4) Returns the value of the expression. Used to override precedence.
178 (5) Returns the result of (2-/expr/).
179 (6) Returns the result of min(/expr/, /expr/).
180 (7) Returns the result of max(/expr/, /expr/).
181 
182 An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2
183 respectively for calculations). A menu entry becomes visible when its
184 expression evaluates to 'm' or 'y'.
185 
186 There are two types of symbols: constant and non-constant symbols.
187 Non-constant symbols are the most common ones and are defined with the
188 'config' statement. Non-constant symbols consist entirely of alphanumeric
189 characters or underscores.
190 Constant symbols are only part of expressions. Constant symbols are
191 always surrounded by single or double quotes. Within the quote, any
192 other character is allowed and the quotes can be escaped using '/'.
193 
194 Menu structure
195 --------------
196 
197 The position of a menu entry in the tree is determined in two ways. First
198 it can be specified explicitly:
199 
200 menu "Network device support"
201         depends on NET
202 
203 config NETDEVICES
204         ...
205 
206 endmenu
207 
208 All entries within the "menu" ... "endmenu" block become a submenu of
209 "Network device support". All subentries inherit the dependencies from
210 the menu entry, e.g. this means the dependency "NET" is added to the
211 dependency list of the config option NETDEVICES.
212 
213 The other way to generate the menu structure is done by analyzing the
214 dependencies. If a menu entry somehow depends on the previous entry, it
215 can be made a submenu of it. First, the previous (parent) symbol must
216 be part of the dependency list and then one of these two conditions
217 must be true:
218 - the child entry must become invisible, if the parent is set to 'n'
219 - the child entry must only be visible, if the parent is visible
220 
221 config MODULES
222         bool "Enable loadable module support"
223 
224 config MODVERSIONS
225         bool "Set version information on all module symbols"
226         depends on MODULES
227 
228 comment "module support disabled"
229         depends on !MODULES
230 
231 MODVERSIONS directly depends on MODULES, this means it's only visible if
232 MODULES is different from 'n'. The comment on the other hand is always
233 visible when MODULES is visible (the (empty) dependency of MODULES is
234 also part of the comment dependencies).
235 
236 
237 Kconfig syntax
238 --------------
239 
240 The configuration file describes a series of menu entries, where every
241 line starts with a keyword (except help texts). The following keywords
242 end a menu entry:
243 - config
244 - menuconfig
245 - choice/endchoice
246 - comment
247 - menu/endmenu
248 - if/endif
249 - source
250 The first five also start the definition of a menu entry.
251 
252 config:
253 
254         "config" 
255         
256 
257 This defines a config symbol  and accepts any of above
258 attributes as options.
259 
260 menuconfig:
261         "menuconfig" 
262         
263 
264 This is similar to the simple config entry above, but it also gives a
265 hint to front ends, that all suboptions should be displayed as a
266 separate list of options.
267 
268 choices:
269 
270         "choice" [symbol]
271         
272         
273         "endchoice"
274 
275 This defines a choice group and accepts any of the above attributes as
276 options. A choice can only be of type bool or tristate, while a boolean
277 choice only allows a single config entry to be selected, a tristate
278 choice also allows any number of config entries to be set to 'm'. This
279 can be used if multiple drivers for a single hardware exists and only a
280 single driver can be compiled/loaded into the kernel, but all drivers
281 can be compiled as modules.
282 A choice accepts another option "optional", which allows to set the
283 choice to 'n' and no entry needs to be selected.
284 If no [symbol] is associated with a choice, then you can not have multiple
285 definitions of that choice. If a [symbol] is associated to the choice,
286 then you may define the same choice (ie. with the same entries) in another
287 place.
288 
289 comment:
290 
291         "comment" 
292         
293 
294 This defines a comment which is displayed to the user during the
295 configuration process and is also echoed to the output files. The only
296 possible options are dependencies.
297 
298 menu:
299 
300         "menu" 
301         
302         
303         "endmenu"
304 
305 This defines a menu block, see "Menu structure" above for more
306 information. The only possible options are dependencies.
307 
308 if:
309 
310         "if" 
311         
312         "endif"
313 
314 This defines an if block. The dependency expression  is appended
315 to all enclosed menu entries.
316 
317 source:
318 
319         "source" 
320 
321 This reads the specified configuration file. This file is always parsed.
322 
323 mainmenu:
324 
325         "mainmenu" 
326 
327 This sets the config program's title bar if the config program chooses
328 to use it. It should be placed at the top of the configuration, before any
329 other statement.
330 
331 
332 Kconfig hints
333 -------------
334 This is a collection of Kconfig tips, most of which aren't obvious at
335 first glance and most of which have become idioms in several Kconfig
336 files.
337 
338 Adding common features and make the usage configurable
339 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
340 It is a common idiom to implement a feature/functionality that are
341 relevant for some architectures but not all.
342 The recommended way to do so is to use a config variable named HAVE_*
343 that is defined in a common Kconfig file and selected by the relevant
344 architectures.
345 An example is the generic IOMAP functionality.
346 
347 We would in lib/Kconfig see:
348 
349 # Generic IOMAP is used to ...
350 config HAVE_GENERIC_IOMAP
351 
352 config GENERIC_IOMAP
353         depends on HAVE_GENERIC_IOMAP && FOO
354 
355 And in lib/Makefile we would see:
356 obj-$(CONFIG_GENERIC_IOMAP) += iomap.o
357 
358 For each architecture using the generic IOMAP functionality we would see:
359 
360 config X86
361         select ...
362         select HAVE_GENERIC_IOMAP
363         select ...
364 
365 Note: we use the existing config option and avoid creating a new
366 config variable to select HAVE_GENERIC_IOMAP.
367 
368 Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is
369 introduced to overcome the limitation of select which will force a
370 config option to 'y' no matter the dependencies.
371 The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the
372 situation where select forces a symbol equals to 'y'.
373 
374 Build as module only
375 ~~~~~~~~~~~~~~~~~~~~
376 To restrict a component build to module-only, qualify its config symbol
377 with "depends on m".  E.g.:
378 
379 config FOO
380         depends on BAR && m
381 
382 limits FOO to module (=m) or disabled (=n).
383