1Introduction 2------------ 3 4The configuration database is a collection of configuration options 5organized 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 20Every entry has its own dependencies. These dependencies are used 21to determine the visibility of an entry. Any child entry is only 22visible if its parent entry is also visible. 23 24Menu entries 25------------ 26 27Most entries define a config option; all other entries help to organize 28them. A single configuration option is defined like this: 29 30config 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 37Every line starts with a key word and can be followed by multiple 38arguments. "config" starts a new config entry. The following lines 39define attributes for this config option. Attributes can be the type of 40the config option, input prompt, dependencies, help text and default 41values. A config option can be defined multiple times with the same 42name, but every definition can have only a single input prompt and the 43type must not conflict. 44 45Menu attributes 46--------------- 47 48A menu entry can have a number of attributes. Not all of them are 49applicable 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 192Menu dependencies 193----------------- 194 195Dependencies define the visibility of a menu entry and can also reduce 196the input range of tristate symbols. The tristate logic used in the 197expressions uses one more state than normal boolean logic to express the 198module 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 208Expressions 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 222An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2 223respectively for calculations). A menu entry becomes visible when its 224expression evaluates to 'm' or 'y'. 225 226There are two types of symbols: constant and non-constant symbols. 227Non-constant symbols are the most common ones and are defined with the 228'config' statement. Non-constant symbols consist entirely of alphanumeric 229characters or underscores. 230Constant symbols are only part of expressions. Constant symbols are 231always surrounded by single or double quotes. Within the quote, any 232other character is allowed and the quotes can be escaped using '\'. 233 234Menu structure 235-------------- 236 237The position of a menu entry in the tree is determined in two ways. First 238it can be specified explicitly: 239 240menu "Network device support" 241 depends on NET 242 243config NETDEVICES 244 ... 245 246endmenu 247 248All entries within the "menu" ... "endmenu" block become a submenu of 249"Network device support". All subentries inherit the dependencies from 250the menu entry, e.g. this means the dependency "NET" is added to the 251dependency list of the config option NETDEVICES. 252 253The other way to generate the menu structure is done by analyzing the 254dependencies. If a menu entry somehow depends on the previous entry, it 255can be made a submenu of it. First, the previous (parent) symbol must 256be part of the dependency list and then one of these two conditions 257must 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 261config MODULES 262 bool "Enable loadable module support" 263 264config MODVERSIONS 265 bool "Set version information on all module symbols" 266 depends on MODULES 267 268comment "module support disabled" 269 depends on !MODULES 270 271MODVERSIONS directly depends on MODULES, this means it's only visible if 272MODULES is different from 'n'. The comment on the other hand is only 273visible when MODULES is set to 'n'. 274 275 276Kconfig syntax 277-------------- 278 279The configuration file describes a series of menu entries, where every 280line starts with a keyword (except help texts). The following keywords 281end a menu entry: 282- config 283- menuconfig 284- choice/endchoice 285- comment 286- menu/endmenu 287- if/endif 288- source 289The first five also start the definition of a menu entry. 290 291config: 292 293 "config" <symbol> 294 <config options> 295 296This defines a config symbol <symbol> and accepts any of above 297attributes as options. 298 299menuconfig: 300 "menuconfig" <symbol> 301 <config options> 302 303This is similar to the simple config entry above, but it also gives a 304hint to front ends, that all suboptions should be displayed as a 305separate list of options. To make sure all the suboptions will really 306show up under the menuconfig entry and not outside of it, every item 307from the <config options> list must depend on the menuconfig symbol. 308In practice, this is achieved by using one of the next two constructs: 309 310(1): 311menuconfig M 312if M 313 config C1 314 config C2 315endif 316 317(2): 318menuconfig M 319config C1 320 depends on M 321config C2 322 depends on M 323 324In the following examples (3) and (4), C1 and C2 still have the M 325dependency, but will not appear under menuconfig M anymore, because 326of C0, which doesn't depend on M: 327 328(3): 329menuconfig M 330 config C0 331if M 332 config C1 333 config C2 334endif 335 336(4): 337menuconfig M 338config C0 339config C1 340 depends on M 341config C2 342 depends on M 343 344choices: 345 346 "choice" [symbol] 347 <choice options> 348 <choice block> 349 "endchoice" 350 351This defines a choice group and accepts any of the above attributes as 352options. A choice can only be of type bool or tristate. If no type is 353specified for a choice, it's type will be determined by the type of 354the first choice element in the group or remain unknown if none of the 355choice elements have a type specified, as well. 356 357While a boolean choice only allows a single config entry to be 358selected, a tristate choice also allows any number of config entries 359to be set to 'm'. This can be used if multiple drivers for a single 360hardware exists and only a single driver can be compiled/loaded into 361the kernel, but all drivers can be compiled as modules. 362 363A choice accepts another option "optional", which allows to set the 364choice to 'n' and no entry needs to be selected. 365If no [symbol] is associated with a choice, then you can not have multiple 366definitions of that choice. If a [symbol] is associated to the choice, 367then you may define the same choice (ie. with the same entries) in another 368place. 369 370comment: 371 372 "comment" <prompt> 373 <comment options> 374 375This defines a comment which is displayed to the user during the 376configuration process and is also echoed to the output files. The only 377possible options are dependencies. 378 379menu: 380 381 "menu" <prompt> 382 <menu options> 383 <menu block> 384 "endmenu" 385 386This defines a menu block, see "Menu structure" above for more 387information. The only possible options are dependencies and "visible" 388attributes. 389 390if: 391 392 "if" <expr> 393 <if block> 394 "endif" 395 396This defines an if block. The dependency expression <expr> is appended 397to all enclosed menu entries. 398 399source: 400 401 "source" <prompt> 402 403This reads the specified configuration file. This file is always parsed. 404 405mainmenu: 406 407 "mainmenu" <prompt> 408 409This sets the config program's title bar if the config program chooses 410to use it. It should be placed at the top of the configuration, before any 411other statement. 412 413 414Kconfig hints 415------------- 416This is a collection of Kconfig tips, most of which aren't obvious at 417first glance and most of which have become idioms in several Kconfig 418files. 419 420Adding common features and make the usage configurable 421~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 422It is a common idiom to implement a feature/functionality that are 423relevant for some architectures but not all. 424The recommended way to do so is to use a config variable named HAVE_* 425that is defined in a common Kconfig file and selected by the relevant 426architectures. 427An example is the generic IOMAP functionality. 428 429We would in lib/Kconfig see: 430 431# Generic IOMAP is used to ... 432config HAVE_GENERIC_IOMAP 433 434config GENERIC_IOMAP 435 depends on HAVE_GENERIC_IOMAP && FOO 436 437And in lib/Makefile we would see: 438obj-$(CONFIG_GENERIC_IOMAP) += iomap.o 439 440For each architecture using the generic IOMAP functionality we would see: 441 442config X86 443 select ... 444 select HAVE_GENERIC_IOMAP 445 select ... 446 447Note: we use the existing config option and avoid creating a new 448config variable to select HAVE_GENERIC_IOMAP. 449 450Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is 451introduced to overcome the limitation of select which will force a 452config option to 'y' no matter the dependencies. 453The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the 454situation where select forces a symbol equals to 'y'. 455 456Build as module only 457~~~~~~~~~~~~~~~~~~~~ 458To restrict a component build to module-only, qualify its config symbol 459with "depends on m". E.g.: 460 461config FOO 462 depends on BAR && m 463 464limits FOO to module (=m) or disabled (=n). 465 466Kconfig recursive dependency limitations 467~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 468 469If you've hit the Kconfig error: "recursive dependency detected" you've run 470into a recursive dependency issue with Kconfig, a recursive dependency can be 471summarized as a circular dependency. The kconfig tools need to ensure that 472Kconfig files comply with specified configuration requirements. In order to do 473that kconfig must determine the values that are possible for all Kconfig 474symbols, this is currently not possible if there is a circular relation 475between two or more Kconfig symbols. For more details refer to the "Simple 476Kconfig recursive issue" subsection below. Kconfig does not do recursive 477dependency resolution; this has a few implications for Kconfig file writers. 478We'll first explain why this issues exists and then provide an example 479technical limitation which this brings upon Kconfig developers. Eager 480developers wishing to try to address this limitation should read the next 481subsections. 482 483Simple Kconfig recursive issue 484~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 485 486Read: Documentation/kbuild/Kconfig.recursion-issue-01 487 488Test with: 489 490make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-01 allnoconfig 491 492Cumulative Kconfig recursive issue 493~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 494 495Read: Documentation/kbuild/Kconfig.recursion-issue-02 496 497Test with: 498 499make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-02 allnoconfig 500 501Practical solutions to kconfig recursive issue 502~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 503 504Developers who run into the recursive Kconfig issue have three options 505at their disposal. We document them below and also provide a list of 506historical 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 514The resolution to a) can be tested with the sample Kconfig file 515Documentation/kbuild/Kconfig.recursion-issue-01 through the removal 516of the "select CORE" from CORE_BELL_A_ADVANCED as that is implicit already 517since CORE_BELL_A depends on CORE. At times it may not be possible to remove 518some dependency criteria, for such cases you can work with solution b). 519 520The two different resolutions for b) can be tested in the sample Kconfig file 521Documentation/kbuild/Kconfig.recursion-issue-02. 522 523Below is a list of examples of prior fixes for these types of recursive issues; 524all errors appear to involve one or more select's and one or more "depends on". 525 526commit fix 527====== === 52806b718c01208 select A -> depends on A 529c22eacfe82f9 depends on A -> depends on B 5306a91e854442c select A -> depends on A 531118c565a8f2e select A -> select B 532f004e5594705 select A -> depends on A 533c7861f37b4c6 depends on A -> (null) 53480c69915e5fb select A -> (null) (1) 535c2218e26c0d0 select A -> depends on A (1) 536d6ae99d04e1c select A -> depends on A 53795ca19cf8cbf select A -> depends on A 5388f057d7bca54 depends on A -> (null) 5398f057d7bca54 depends on A -> select A 540a0701f04846e select A -> depends on A 5410c8b92f7f259 depends on A -> (null) 542e4e9e0540928 select A -> depends on A (2) 5437453ea886e87 depends on A > (null) (1) 5447b1fff7e4fdf select A -> depends on A 54586c747d2a4f0 select A -> depends on A 546d9f9ab51e55e select A -> depends on A 5470c51a4d8abd6 depends on A -> select A (3) 548e98062ed6dc4 select A -> depends on A (3) 54991e5d284a7f1 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 555Future kconfig work 556~~~~~~~~~~~~~~~~~~~ 557 558Work on kconfig is welcomed on both areas of clarifying semantics and on 559evaluating the use of a full SAT solver for it. A full SAT solver can be 560desirable to enable more complex dependency mappings and / or queries, 561for instance on possible use case for a SAT solver could be that of handling 562the current known recursive dependency issues. It is not known if this would 563address such issues but such evaluation is desirable. If support for a full SAT 564solver proves too complex or that it cannot address recursive dependency issues 565Kconfig should have at least clear and well defined semantics which also 566addresses and documents limitations or requirements such as the ones dealing 567with recursive dependencies. 568 569Further work on both of these areas is welcomed on Kconfig. We elaborate 570on both of these in the next two subsections. 571 572Semantics of Kconfig 573~~~~~~~~~~~~~~~~~~~~ 574 575The use of Kconfig is broad, Linux is now only one of Kconfig's users: 576one study has completed a broad analysis of Kconfig use in 12 projects [0]. 577Despite its widespread use, and although this document does a reasonable job 578in documenting basic Kconfig syntax a more precise definition of Kconfig 579semantics is welcomed. One project deduced Kconfig semantics through 580the use of the xconfig configurator [1]. Work should be done to confirm if 581the deduced semantics matches our intended Kconfig design goals. 582 583Having well defined semantics can be useful for tools for practical 584evaluation of depenencies, for instance one such use known case was work to 585express in boolean abstraction of the inferred semantics of Kconfig to 586translate Kconfig logic into boolean formulas and run a SAT solver on this to 587find dead code / features (always inactive), 114 dead features were found in 588Linux using this methodology [1] (Section 8: Threats to validity). 589 590Confirming this could prove useful as Kconfig stands as one of the the leading 591industrial variability modeling languages [1] [2]. Its study would help 592evaluate practical uses of such languages, their use was only theoretical 593and real world requirements were not well understood. As it stands though 594only reverse engineering techniques have been used to deduce semantics from 595variability 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 602Full SAT solver for Kconfig 603~~~~~~~~~~~~~~~~~~~~~~~~~~~ 604 605Although SAT solvers [0] haven't yet been used by Kconfig directly, as noted in 606the previous subsection, work has been done however to express in boolean 607abstraction the inferred semantics of Kconfig to translate Kconfig logic into 608boolean formulas and run a SAT solver on it [1]. Another known related project 609is CADOS [2] (former VAMOS [3]) and the tools, mainly undertaker [4], which has 610been introduced first with [5]. The basic concept of undertaker is to exract 611variability models from Kconfig, and put them together with a propositional 612formula extracted from CPP #ifdefs and build-rules into a SAT solver in order 613to find dead code, dead files, and dead symbols. If using a SAT solver is 614desirable on Kconfig one approach would be to evaluate repurposing such efforts 615somehow on Kconfig. There is enough interest from mentors of existing projects 616to not only help advise how to integrate this work upstream but also help 617maintain it long term. Interested developers should visit: 618 619http://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 627