1================ 2Kconfig Language 3================ 4 5Introduction 6------------ 7 8The configuration database is a collection of configuration options 9organized in a tree structure:: 10 11 +- Code maturity level options 12 | +- Prompt for development and/or incomplete code/drivers 13 +- General setup 14 | +- Networking support 15 | +- System V IPC 16 | +- BSD Process Accounting 17 | +- Sysctl support 18 +- Loadable module support 19 | +- Enable loadable module support 20 | +- Set version information on all module symbols 21 | +- Kernel module loader 22 +- ... 23 24Every entry has its own dependencies. These dependencies are used 25to determine the visibility of an entry. Any child entry is only 26visible if its parent entry is also visible. 27 28Menu entries 29------------ 30 31Most entries define a config option; all other entries help to organize 32them. A single configuration option is defined like this:: 33 34 config MODVERSIONS 35 bool "Set version information on all module symbols" 36 depends on MODULES 37 help 38 Usually, modules have to be recompiled whenever you switch to a new 39 kernel. ... 40 41Every line starts with a key word and can be followed by multiple 42arguments. "config" starts a new config entry. The following lines 43define attributes for this config option. Attributes can be the type of 44the config option, input prompt, dependencies, help text and default 45values. A config option can be defined multiple times with the same 46name, but every definition can have only a single input prompt and the 47type must not conflict. 48 49Menu attributes 50--------------- 51 52A menu entry can have a number of attributes. Not all of them are 53applicable everywhere (see syntax). 54 55- type definition: "bool"/"tristate"/"string"/"hex"/"int" 56 57 Every config option must have a type. There are only two basic types: 58 tristate and string; the other types are based on these two. The type 59 definition optionally accepts an input prompt, so these two examples 60 are equivalent:: 61 62 bool "Networking support" 63 64 and:: 65 66 bool 67 prompt "Networking support" 68 69- input prompt: "prompt" <prompt> ["if" <expr>] 70 71 Every menu entry can have at most one prompt, which is used to display 72 to the user. Optionally dependencies only for this prompt can be added 73 with "if". 74 75- default value: "default" <expr> ["if" <expr>] 76 77 A config option can have any number of default values. If multiple 78 default values are visible, only the first defined one is active. 79 Default values are not limited to the menu entry where they are 80 defined. This means the default can be defined somewhere else or be 81 overridden by an earlier definition. 82 The default value is only assigned to the config symbol if no other 83 value was set by the user (via the input prompt above). If an input 84 prompt is visible the default value is presented to the user and can 85 be overridden by him. 86 Optionally, dependencies only for this default value can be added with 87 "if". 88 89 The default value deliberately defaults to 'n' in order to avoid bloating the 90 build. With few exceptions, new config options should not change this. The 91 intent is for "make oldconfig" to add as little as possible to the config from 92 release to release. 93 94 Note: 95 Things that merit "default y/m" include: 96 97 a) A new Kconfig option for something that used to always be built 98 should be "default y". 99 100 b) A new gatekeeping Kconfig option that hides/shows other Kconfig 101 options (but does not generate any code of its own), should be 102 "default y" so people will see those other options. 103 104 c) Sub-driver behavior or similar options for a driver that is 105 "default n". This allows you to provide sane defaults. 106 107 d) Hardware or infrastructure that everybody expects, such as CONFIG_NET 108 or CONFIG_BLOCK. These are rare exceptions. 109 110- type definition + default value:: 111 112 "def_bool"/"def_tristate" <expr> ["if" <expr>] 113 114 This is a shorthand notation for a type definition plus a value. 115 Optionally dependencies for this default value can be added with "if". 116 117- dependencies: "depends on" <expr> 118 119 This defines a dependency for this menu entry. If multiple 120 dependencies are defined, they are connected with '&&'. Dependencies 121 are applied to all other options within this menu entry (which also 122 accept an "if" expression), so these two examples are equivalent:: 123 124 bool "foo" if BAR 125 default y if BAR 126 127 and:: 128 129 depends on BAR 130 bool "foo" 131 default y 132 133- reverse dependencies: "select" <symbol> ["if" <expr>] 134 135 While normal dependencies reduce the upper limit of a symbol (see 136 below), reverse dependencies can be used to force a lower limit of 137 another symbol. The value of the current menu symbol is used as the 138 minimal value <symbol> can be set to. If <symbol> is selected multiple 139 times, the limit is set to the largest selection. 140 Reverse dependencies can only be used with boolean or tristate 141 symbols. 142 143 Note: 144 select should be used with care. select will force 145 a symbol to a value without visiting the dependencies. 146 By abusing select you are able to select a symbol FOO even 147 if FOO depends on BAR that is not set. 148 In general use select only for non-visible symbols 149 (no prompts anywhere) and for symbols with no dependencies. 150 That will limit the usefulness but on the other hand avoid 151 the illegal configurations all over. 152 153- weak reverse dependencies: "imply" <symbol> ["if" <expr>] 154 155 This is similar to "select" as it enforces a lower limit on another 156 symbol except that the "implied" symbol's value may still be set to n 157 from a direct dependency or with a visible prompt. 158 159 Given the following example:: 160 161 config FOO 162 tristate "foo" 163 imply BAZ 164 165 config BAZ 166 tristate "baz" 167 depends on BAR 168 169 The following values are possible: 170 171 === === ============= ============== 172 FOO BAR BAZ's default choice for BAZ 173 === === ============= ============== 174 n y n N/m/y 175 m y m M/y/n 176 y y y Y/m/n 177 n m n N/m 178 m m m M/n 179 y m n M/n 180 y n * N 181 === === ============= ============== 182 183 This is useful e.g. with multiple drivers that want to indicate their 184 ability to hook into a secondary subsystem while allowing the user to 185 configure that subsystem out without also having to unset these drivers. 186 187 Note: If the combination of FOO=y and BAR=m causes a link error, 188 you can guard the function call with IS_REACHABLE():: 189 190 foo_init() 191 { 192 if (IS_REACHABLE(CONFIG_BAZ)) 193 baz_register(&foo); 194 ... 195 } 196 197 Note: If the feature provided by BAZ is highly desirable for FOO, 198 FOO should imply not only BAZ, but also its dependency BAR:: 199 200 config FOO 201 tristate "foo" 202 imply BAR 203 imply BAZ 204 205- limiting menu display: "visible if" <expr> 206 207 This attribute is only applicable to menu blocks, if the condition is 208 false, the menu block is not displayed to the user (the symbols 209 contained there can still be selected by other symbols, though). It is 210 similar to a conditional "prompt" attribute for individual menu 211 entries. Default value of "visible" is true. 212 213- numerical ranges: "range" <symbol> <symbol> ["if" <expr>] 214 215 This allows to limit the range of possible input values for int 216 and hex symbols. The user can only input a value which is larger than 217 or equal to the first symbol and smaller than or equal to the second 218 symbol. 219 220- help text: "help" 221 222 This defines a help text. The end of the help text is determined by 223 the indentation level, this means it ends at the first line which has 224 a smaller indentation than the first line of the help text. 225 226- misc options: "option" <symbol>[=<value>] 227 228 Various less common options can be defined via this option syntax, 229 which can modify the behaviour of the menu entry and its config 230 symbol. These options are currently possible: 231 232 - "defconfig_list" 233 This declares a list of default entries which can be used when 234 looking for the default configuration (which is used when the main 235 .config doesn't exists yet.) 236 237 - "modules" 238 This declares the symbol to be used as the MODULES symbol, which 239 enables the third modular state for all config symbols. 240 At most one symbol may have the "modules" option set. 241 242 - "allnoconfig_y" 243 This declares the symbol as one that should have the value y when 244 using "allnoconfig". Used for symbols that hide other symbols. 245 246Menu dependencies 247----------------- 248 249Dependencies define the visibility of a menu entry and can also reduce 250the input range of tristate symbols. The tristate logic used in the 251expressions uses one more state than normal boolean logic to express the 252module state. Dependency expressions have the following syntax:: 253 254 <expr> ::= <symbol> (1) 255 <symbol> '=' <symbol> (2) 256 <symbol> '!=' <symbol> (3) 257 <symbol1> '<' <symbol2> (4) 258 <symbol1> '>' <symbol2> (4) 259 <symbol1> '<=' <symbol2> (4) 260 <symbol1> '>=' <symbol2> (4) 261 '(' <expr> ')' (5) 262 '!' <expr> (6) 263 <expr> '&&' <expr> (7) 264 <expr> '||' <expr> (8) 265 266Expressions are listed in decreasing order of precedence. 267 268(1) Convert the symbol into an expression. Boolean and tristate symbols 269 are simply converted into the respective expression values. All 270 other symbol types result in 'n'. 271(2) If the values of both symbols are equal, it returns 'y', 272 otherwise 'n'. 273(3) If the values of both symbols are equal, it returns 'n', 274 otherwise 'y'. 275(4) If value of <symbol1> is respectively lower, greater, lower-or-equal, 276 or greater-or-equal than value of <symbol2>, it returns 'y', 277 otherwise 'n'. 278(5) Returns the value of the expression. Used to override precedence. 279(6) Returns the result of (2-/expr/). 280(7) Returns the result of min(/expr/, /expr/). 281(8) Returns the result of max(/expr/, /expr/). 282 283An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2 284respectively for calculations). A menu entry becomes visible when its 285expression evaluates to 'm' or 'y'. 286 287There are two types of symbols: constant and non-constant symbols. 288Non-constant symbols are the most common ones and are defined with the 289'config' statement. Non-constant symbols consist entirely of alphanumeric 290characters or underscores. 291Constant symbols are only part of expressions. Constant symbols are 292always surrounded by single or double quotes. Within the quote, any 293other character is allowed and the quotes can be escaped using '\'. 294 295Menu structure 296-------------- 297 298The position of a menu entry in the tree is determined in two ways. First 299it can be specified explicitly:: 300 301 menu "Network device support" 302 depends on NET 303 304 config NETDEVICES 305 ... 306 307 endmenu 308 309All entries within the "menu" ... "endmenu" block become a submenu of 310"Network device support". All subentries inherit the dependencies from 311the menu entry, e.g. this means the dependency "NET" is added to the 312dependency list of the config option NETDEVICES. 313 314The other way to generate the menu structure is done by analyzing the 315dependencies. If a menu entry somehow depends on the previous entry, it 316can be made a submenu of it. First, the previous (parent) symbol must 317be part of the dependency list and then one of these two conditions 318must be true: 319 320- the child entry must become invisible, if the parent is set to 'n' 321- the child entry must only be visible, if the parent is visible:: 322 323 config MODULES 324 bool "Enable loadable module support" 325 326 config MODVERSIONS 327 bool "Set version information on all module symbols" 328 depends on MODULES 329 330 comment "module support disabled" 331 depends on !MODULES 332 333MODVERSIONS directly depends on MODULES, this means it's only visible if 334MODULES is different from 'n'. The comment on the other hand is only 335visible when MODULES is set to 'n'. 336 337 338Kconfig syntax 339-------------- 340 341The configuration file describes a series of menu entries, where every 342line starts with a keyword (except help texts). The following keywords 343end a menu entry: 344 345- config 346- menuconfig 347- choice/endchoice 348- comment 349- menu/endmenu 350- if/endif 351- source 352 353The first five also start the definition of a menu entry. 354 355config:: 356 357 "config" <symbol> 358 <config options> 359 360This defines a config symbol <symbol> and accepts any of above 361attributes as options. 362 363menuconfig:: 364 365 "menuconfig" <symbol> 366 <config options> 367 368This is similar to the simple config entry above, but it also gives a 369hint to front ends, that all suboptions should be displayed as a 370separate list of options. To make sure all the suboptions will really 371show up under the menuconfig entry and not outside of it, every item 372from the <config options> list must depend on the menuconfig symbol. 373In practice, this is achieved by using one of the next two constructs:: 374 375 (1): 376 menuconfig M 377 if M 378 config C1 379 config C2 380 endif 381 382 (2): 383 menuconfig M 384 config C1 385 depends on M 386 config C2 387 depends on M 388 389In the following examples (3) and (4), C1 and C2 still have the M 390dependency, but will not appear under menuconfig M anymore, because 391of C0, which doesn't depend on M:: 392 393 (3): 394 menuconfig M 395 config C0 396 if M 397 config C1 398 config C2 399 endif 400 401 (4): 402 menuconfig M 403 config C0 404 config C1 405 depends on M 406 config C2 407 depends on M 408 409choices:: 410 411 "choice" [symbol] 412 <choice options> 413 <choice block> 414 "endchoice" 415 416This defines a choice group and accepts any of the above attributes as 417options. A choice can only be of type bool or tristate. If no type is 418specified for a choice, its type will be determined by the type of 419the first choice element in the group or remain unknown if none of the 420choice elements have a type specified, as well. 421 422While a boolean choice only allows a single config entry to be 423selected, a tristate choice also allows any number of config entries 424to be set to 'm'. This can be used if multiple drivers for a single 425hardware exists and only a single driver can be compiled/loaded into 426the kernel, but all drivers can be compiled as modules. 427 428A choice accepts another option "optional", which allows to set the 429choice to 'n' and no entry needs to be selected. 430If no [symbol] is associated with a choice, then you can not have multiple 431definitions of that choice. If a [symbol] is associated to the choice, 432then you may define the same choice (i.e. with the same entries) in another 433place. 434 435comment:: 436 437 "comment" <prompt> 438 <comment options> 439 440This defines a comment which is displayed to the user during the 441configuration process and is also echoed to the output files. The only 442possible options are dependencies. 443 444menu:: 445 446 "menu" <prompt> 447 <menu options> 448 <menu block> 449 "endmenu" 450 451This defines a menu block, see "Menu structure" above for more 452information. The only possible options are dependencies and "visible" 453attributes. 454 455if:: 456 457 "if" <expr> 458 <if block> 459 "endif" 460 461This defines an if block. The dependency expression <expr> is appended 462to all enclosed menu entries. 463 464source:: 465 466 "source" <prompt> 467 468This reads the specified configuration file. This file is always parsed. 469 470mainmenu:: 471 472 "mainmenu" <prompt> 473 474This sets the config program's title bar if the config program chooses 475to use it. It should be placed at the top of the configuration, before any 476other statement. 477 478'#' Kconfig source file comment: 479 480An unquoted '#' character anywhere in a source file line indicates 481the beginning of a source file comment. The remainder of that line 482is a comment. 483 484 485Kconfig hints 486------------- 487This is a collection of Kconfig tips, most of which aren't obvious at 488first glance and most of which have become idioms in several Kconfig 489files. 490 491Adding common features and make the usage configurable 492~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 493It is a common idiom to implement a feature/functionality that are 494relevant for some architectures but not all. 495The recommended way to do so is to use a config variable named HAVE_* 496that is defined in a common Kconfig file and selected by the relevant 497architectures. 498An example is the generic IOMAP functionality. 499 500We would in lib/Kconfig see:: 501 502 # Generic IOMAP is used to ... 503 config HAVE_GENERIC_IOMAP 504 505 config GENERIC_IOMAP 506 depends on HAVE_GENERIC_IOMAP && FOO 507 508And in lib/Makefile we would see:: 509 510 obj-$(CONFIG_GENERIC_IOMAP) += iomap.o 511 512For each architecture using the generic IOMAP functionality we would see:: 513 514 config X86 515 select ... 516 select HAVE_GENERIC_IOMAP 517 select ... 518 519Note: we use the existing config option and avoid creating a new 520config variable to select HAVE_GENERIC_IOMAP. 521 522Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is 523introduced to overcome the limitation of select which will force a 524config option to 'y' no matter the dependencies. 525The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the 526situation where select forces a symbol equals to 'y'. 527 528Adding features that need compiler support 529~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 530 531There are several features that need compiler support. The recommended way 532to describe the dependency on the compiler feature is to use "depends on" 533followed by a test macro:: 534 535 config STACKPROTECTOR 536 bool "Stack Protector buffer overflow detection" 537 depends on $(cc-option,-fstack-protector) 538 ... 539 540If you need to expose a compiler capability to makefiles and/or C source files, 541`CC_HAS_` is the recommended prefix for the config option:: 542 543 config CC_HAS_ASM_GOTO 544 def_bool $(success,$(srctree)/scripts/gcc-goto.sh $(CC)) 545 546Build as module only 547~~~~~~~~~~~~~~~~~~~~ 548To restrict a component build to module-only, qualify its config symbol 549with "depends on m". E.g.:: 550 551 config FOO 552 depends on BAR && m 553 554limits FOO to module (=m) or disabled (=n). 555 556Kconfig recursive dependency limitations 557~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 558 559If you've hit the Kconfig error: "recursive dependency detected" you've run 560into a recursive dependency issue with Kconfig, a recursive dependency can be 561summarized as a circular dependency. The kconfig tools need to ensure that 562Kconfig files comply with specified configuration requirements. In order to do 563that kconfig must determine the values that are possible for all Kconfig 564symbols, this is currently not possible if there is a circular relation 565between two or more Kconfig symbols. For more details refer to the "Simple 566Kconfig recursive issue" subsection below. Kconfig does not do recursive 567dependency resolution; this has a few implications for Kconfig file writers. 568We'll first explain why this issues exists and then provide an example 569technical limitation which this brings upon Kconfig developers. Eager 570developers wishing to try to address this limitation should read the next 571subsections. 572 573Simple Kconfig recursive issue 574~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 575 576Read: Documentation/kbuild/Kconfig.recursion-issue-01 577 578Test with:: 579 580 make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-01 allnoconfig 581 582Cumulative Kconfig recursive issue 583~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 584 585Read: Documentation/kbuild/Kconfig.recursion-issue-02 586 587Test with:: 588 589 make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-02 allnoconfig 590 591Practical solutions to kconfig recursive issue 592~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 593 594Developers who run into the recursive Kconfig issue have two options 595at their disposal. We document them below and also provide a list of 596historical issues resolved through these different solutions. 597 598 a) Remove any superfluous "select FOO" or "depends on FOO" 599 b) Match dependency semantics: 600 601 b1) Swap all "select FOO" to "depends on FOO" or, 602 603 b2) Swap all "depends on FOO" to "select FOO" 604 605The resolution to a) can be tested with the sample Kconfig file 606Documentation/kbuild/Kconfig.recursion-issue-01 through the removal 607of the "select CORE" from CORE_BELL_A_ADVANCED as that is implicit already 608since CORE_BELL_A depends on CORE. At times it may not be possible to remove 609some dependency criteria, for such cases you can work with solution b). 610 611The two different resolutions for b) can be tested in the sample Kconfig file 612Documentation/kbuild/Kconfig.recursion-issue-02. 613 614Below is a list of examples of prior fixes for these types of recursive issues; 615all errors appear to involve one or more "select" statements and one or more 616"depends on". 617 618============ =================================== 619commit fix 620============ =================================== 62106b718c01208 select A -> depends on A 622c22eacfe82f9 depends on A -> depends on B 6236a91e854442c select A -> depends on A 624118c565a8f2e select A -> select B 625f004e5594705 select A -> depends on A 626c7861f37b4c6 depends on A -> (null) 62780c69915e5fb select A -> (null) (1) 628c2218e26c0d0 select A -> depends on A (1) 629d6ae99d04e1c select A -> depends on A 63095ca19cf8cbf select A -> depends on A 6318f057d7bca54 depends on A -> (null) 6328f057d7bca54 depends on A -> select A 633a0701f04846e select A -> depends on A 6340c8b92f7f259 depends on A -> (null) 635e4e9e0540928 select A -> depends on A (2) 6367453ea886e87 depends on A > (null) (1) 6377b1fff7e4fdf select A -> depends on A 63886c747d2a4f0 select A -> depends on A 639d9f9ab51e55e select A -> depends on A 6400c51a4d8abd6 depends on A -> select A (3) 641e98062ed6dc4 select A -> depends on A (3) 64291e5d284a7f1 select A -> (null) 643============ =================================== 644 645(1) Partial (or no) quote of error. 646(2) That seems to be the gist of that fix. 647(3) Same error. 648 649Future kconfig work 650~~~~~~~~~~~~~~~~~~~ 651 652Work on kconfig is welcomed on both areas of clarifying semantics and on 653evaluating the use of a full SAT solver for it. A full SAT solver can be 654desirable to enable more complex dependency mappings and / or queries, 655for instance on possible use case for a SAT solver could be that of handling 656the current known recursive dependency issues. It is not known if this would 657address such issues but such evaluation is desirable. If support for a full SAT 658solver proves too complex or that it cannot address recursive dependency issues 659Kconfig should have at least clear and well defined semantics which also 660addresses and documents limitations or requirements such as the ones dealing 661with recursive dependencies. 662 663Further work on both of these areas is welcomed on Kconfig. We elaborate 664on both of these in the next two subsections. 665 666Semantics of Kconfig 667~~~~~~~~~~~~~~~~~~~~ 668 669The use of Kconfig is broad, Linux is now only one of Kconfig's users: 670one study has completed a broad analysis of Kconfig use in 12 projects [0]_. 671Despite its widespread use, and although this document does a reasonable job 672in documenting basic Kconfig syntax a more precise definition of Kconfig 673semantics is welcomed. One project deduced Kconfig semantics through 674the use of the xconfig configurator [1]_. Work should be done to confirm if 675the deduced semantics matches our intended Kconfig design goals. 676 677Having well defined semantics can be useful for tools for practical 678evaluation of dependencies, for instance one such case was work to 679express in boolean abstraction of the inferred semantics of Kconfig to 680translate Kconfig logic into boolean formulas and run a SAT solver on this to 681find dead code / features (always inactive), 114 dead features were found in 682Linux using this methodology [1]_ (Section 8: Threats to validity). 683 684Confirming this could prove useful as Kconfig stands as one of the leading 685industrial variability modeling languages [1]_ [2]_. Its study would help 686evaluate practical uses of such languages, their use was only theoretical 687and real world requirements were not well understood. As it stands though 688only reverse engineering techniques have been used to deduce semantics from 689variability modeling languages such as Kconfig [3]_. 690 691.. [0] https://www.eng.uwaterloo.ca/~shshe/kconfig_semantics.pdf 692.. [1] https://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf 693.. [2] https://gsd.uwaterloo.ca/sites/default/files/ase241-berger_0.pdf 694.. [3] https://gsd.uwaterloo.ca/sites/default/files/icse2011.pdf 695 696Full SAT solver for Kconfig 697~~~~~~~~~~~~~~~~~~~~~~~~~~~ 698 699Although SAT solvers [4]_ haven't yet been used by Kconfig directly, as noted 700in the previous subsection, work has been done however to express in boolean 701abstraction the inferred semantics of Kconfig to translate Kconfig logic into 702boolean formulas and run a SAT solver on it [5]_. Another known related project 703is CADOS [6]_ (former VAMOS [7]_) and the tools, mainly undertaker [8]_, which 704has been introduced first with [9]_. The basic concept of undertaker is to 705extract variability models from Kconfig and put them together with a 706propositional formula extracted from CPP #ifdefs and build-rules into a SAT 707solver in order to find dead code, dead files, and dead symbols. If using a SAT 708solver is desirable on Kconfig one approach would be to evaluate repurposing 709such efforts somehow on Kconfig. There is enough interest from mentors of 710existing projects to not only help advise how to integrate this work upstream 711but also help maintain it long term. Interested developers should visit: 712 713https://kernelnewbies.org/KernelProjects/kconfig-sat 714 715.. [4] https://www.cs.cornell.edu/~sabhar/chapters/SATSolvers-KR-Handbook.pdf 716.. [5] https://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf 717.. [6] https://cados.cs.fau.de 718.. [7] https://vamos.cs.fau.de 719.. [8] https://undertaker.cs.fau.de 720.. [9] https://www4.cs.fau.de/Publications/2011/tartler_11_eurosys.pdf 721