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 163 imply BAZ 164 165 config BAZ 166 tristate 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 y n * N 178 === === ============= ============== 179 180 This is useful e.g. with multiple drivers that want to indicate their 181 ability to hook into a secondary subsystem while allowing the user to 182 configure that subsystem out without also having to unset these drivers. 183 184 Note: If the combination of FOO=y and BAR=m causes a link error, 185 you can guard the function call with IS_REACHABLE():: 186 187 foo_init() 188 { 189 if (IS_REACHABLE(CONFIG_BAZ)) 190 baz_register(&foo); 191 ... 192 } 193 194- limiting menu display: "visible if" <expr> 195 196 This attribute is only applicable to menu blocks, if the condition is 197 false, the menu block is not displayed to the user (the symbols 198 contained there can still be selected by other symbols, though). It is 199 similar to a conditional "prompt" attribute for individual menu 200 entries. Default value of "visible" is true. 201 202- numerical ranges: "range" <symbol> <symbol> ["if" <expr>] 203 204 This allows to limit the range of possible input values for int 205 and hex symbols. The user can only input a value which is larger than 206 or equal to the first symbol and smaller than or equal to the second 207 symbol. 208 209- help text: "help" 210 211 This defines a help text. The end of the help text is determined by 212 the indentation level, this means it ends at the first line which has 213 a smaller indentation than the first line of the help text. 214 215- misc options: "option" <symbol>[=<value>] 216 217 Various less common options can be defined via this option syntax, 218 which can modify the behaviour of the menu entry and its config 219 symbol. These options are currently possible: 220 221 - "defconfig_list" 222 This declares a list of default entries which can be used when 223 looking for the default configuration (which is used when the main 224 .config doesn't exists yet.) 225 226 - "modules" 227 This declares the symbol to be used as the MODULES symbol, which 228 enables the third modular state for all config symbols. 229 At most one symbol may have the "modules" option set. 230 231 - "allnoconfig_y" 232 This declares the symbol as one that should have the value y when 233 using "allnoconfig". Used for symbols that hide other symbols. 234 235Menu dependencies 236----------------- 237 238Dependencies define the visibility of a menu entry and can also reduce 239the input range of tristate symbols. The tristate logic used in the 240expressions uses one more state than normal boolean logic to express the 241module state. Dependency expressions have the following syntax:: 242 243 <expr> ::= <symbol> (1) 244 <symbol> '=' <symbol> (2) 245 <symbol> '!=' <symbol> (3) 246 <symbol1> '<' <symbol2> (4) 247 <symbol1> '>' <symbol2> (4) 248 <symbol1> '<=' <symbol2> (4) 249 <symbol1> '>=' <symbol2> (4) 250 '(' <expr> ')' (5) 251 '!' <expr> (6) 252 <expr> '&&' <expr> (7) 253 <expr> '||' <expr> (8) 254 255Expressions are listed in decreasing order of precedence. 256 257(1) Convert the symbol into an expression. Boolean and tristate symbols 258 are simply converted into the respective expression values. All 259 other symbol types result in 'n'. 260(2) If the values of both symbols are equal, it returns 'y', 261 otherwise 'n'. 262(3) If the values of both symbols are equal, it returns 'n', 263 otherwise 'y'. 264(4) If value of <symbol1> is respectively lower, greater, lower-or-equal, 265 or greater-or-equal than value of <symbol2>, it returns 'y', 266 otherwise 'n'. 267(5) Returns the value of the expression. Used to override precedence. 268(6) Returns the result of (2-/expr/). 269(7) Returns the result of min(/expr/, /expr/). 270(8) Returns the result of max(/expr/, /expr/). 271 272An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2 273respectively for calculations). A menu entry becomes visible when its 274expression evaluates to 'm' or 'y'. 275 276There are two types of symbols: constant and non-constant symbols. 277Non-constant symbols are the most common ones and are defined with the 278'config' statement. Non-constant symbols consist entirely of alphanumeric 279characters or underscores. 280Constant symbols are only part of expressions. Constant symbols are 281always surrounded by single or double quotes. Within the quote, any 282other character is allowed and the quotes can be escaped using '\'. 283 284Menu structure 285-------------- 286 287The position of a menu entry in the tree is determined in two ways. First 288it can be specified explicitly:: 289 290 menu "Network device support" 291 depends on NET 292 293 config NETDEVICES 294 ... 295 296 endmenu 297 298All entries within the "menu" ... "endmenu" block become a submenu of 299"Network device support". All subentries inherit the dependencies from 300the menu entry, e.g. this means the dependency "NET" is added to the 301dependency list of the config option NETDEVICES. 302 303The other way to generate the menu structure is done by analyzing the 304dependencies. If a menu entry somehow depends on the previous entry, it 305can be made a submenu of it. First, the previous (parent) symbol must 306be part of the dependency list and then one of these two conditions 307must be true: 308 309- the child entry must become invisible, if the parent is set to 'n' 310- the child entry must only be visible, if the parent is visible:: 311 312 config MODULES 313 bool "Enable loadable module support" 314 315 config MODVERSIONS 316 bool "Set version information on all module symbols" 317 depends on MODULES 318 319 comment "module support disabled" 320 depends on !MODULES 321 322MODVERSIONS directly depends on MODULES, this means it's only visible if 323MODULES is different from 'n'. The comment on the other hand is only 324visible when MODULES is set to 'n'. 325 326 327Kconfig syntax 328-------------- 329 330The configuration file describes a series of menu entries, where every 331line starts with a keyword (except help texts). The following keywords 332end a menu entry: 333 334- config 335- menuconfig 336- choice/endchoice 337- comment 338- menu/endmenu 339- if/endif 340- source 341 342The first five also start the definition of a menu entry. 343 344config:: 345 346 "config" <symbol> 347 <config options> 348 349This defines a config symbol <symbol> and accepts any of above 350attributes as options. 351 352menuconfig:: 353 354 "menuconfig" <symbol> 355 <config options> 356 357This is similar to the simple config entry above, but it also gives a 358hint to front ends, that all suboptions should be displayed as a 359separate list of options. To make sure all the suboptions will really 360show up under the menuconfig entry and not outside of it, every item 361from the <config options> list must depend on the menuconfig symbol. 362In practice, this is achieved by using one of the next two constructs:: 363 364 (1): 365 menuconfig M 366 if M 367 config C1 368 config C2 369 endif 370 371 (2): 372 menuconfig M 373 config C1 374 depends on M 375 config C2 376 depends on M 377 378In the following examples (3) and (4), C1 and C2 still have the M 379dependency, but will not appear under menuconfig M anymore, because 380of C0, which doesn't depend on M:: 381 382 (3): 383 menuconfig M 384 config C0 385 if M 386 config C1 387 config C2 388 endif 389 390 (4): 391 menuconfig M 392 config C0 393 config C1 394 depends on M 395 config C2 396 depends on M 397 398choices:: 399 400 "choice" [symbol] 401 <choice options> 402 <choice block> 403 "endchoice" 404 405This defines a choice group and accepts any of the above attributes as 406options. A choice can only be of type bool or tristate. If no type is 407specified for a choice, its type will be determined by the type of 408the first choice element in the group or remain unknown if none of the 409choice elements have a type specified, as well. 410 411While a boolean choice only allows a single config entry to be 412selected, a tristate choice also allows any number of config entries 413to be set to 'm'. This can be used if multiple drivers for a single 414hardware exists and only a single driver can be compiled/loaded into 415the kernel, but all drivers can be compiled as modules. 416 417A choice accepts another option "optional", which allows to set the 418choice to 'n' and no entry needs to be selected. 419If no [symbol] is associated with a choice, then you can not have multiple 420definitions of that choice. If a [symbol] is associated to the choice, 421then you may define the same choice (i.e. with the same entries) in another 422place. 423 424comment:: 425 426 "comment" <prompt> 427 <comment options> 428 429This defines a comment which is displayed to the user during the 430configuration process and is also echoed to the output files. The only 431possible options are dependencies. 432 433menu:: 434 435 "menu" <prompt> 436 <menu options> 437 <menu block> 438 "endmenu" 439 440This defines a menu block, see "Menu structure" above for more 441information. The only possible options are dependencies and "visible" 442attributes. 443 444if:: 445 446 "if" <expr> 447 <if block> 448 "endif" 449 450This defines an if block. The dependency expression <expr> is appended 451to all enclosed menu entries. 452 453source:: 454 455 "source" <prompt> 456 457This reads the specified configuration file. This file is always parsed. 458 459mainmenu:: 460 461 "mainmenu" <prompt> 462 463This sets the config program's title bar if the config program chooses 464to use it. It should be placed at the top of the configuration, before any 465other statement. 466 467'#' Kconfig source file comment: 468 469An unquoted '#' character anywhere in a source file line indicates 470the beginning of a source file comment. The remainder of that line 471is a comment. 472 473 474Kconfig hints 475------------- 476This is a collection of Kconfig tips, most of which aren't obvious at 477first glance and most of which have become idioms in several Kconfig 478files. 479 480Adding common features and make the usage configurable 481~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 482It is a common idiom to implement a feature/functionality that are 483relevant for some architectures but not all. 484The recommended way to do so is to use a config variable named HAVE_* 485that is defined in a common Kconfig file and selected by the relevant 486architectures. 487An example is the generic IOMAP functionality. 488 489We would in lib/Kconfig see:: 490 491 # Generic IOMAP is used to ... 492 config HAVE_GENERIC_IOMAP 493 494 config GENERIC_IOMAP 495 depends on HAVE_GENERIC_IOMAP && FOO 496 497And in lib/Makefile we would see:: 498 499 obj-$(CONFIG_GENERIC_IOMAP) += iomap.o 500 501For each architecture using the generic IOMAP functionality we would see:: 502 503 config X86 504 select ... 505 select HAVE_GENERIC_IOMAP 506 select ... 507 508Note: we use the existing config option and avoid creating a new 509config variable to select HAVE_GENERIC_IOMAP. 510 511Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is 512introduced to overcome the limitation of select which will force a 513config option to 'y' no matter the dependencies. 514The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the 515situation where select forces a symbol equals to 'y'. 516 517Adding features that need compiler support 518~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 519 520There are several features that need compiler support. The recommended way 521to describe the dependency on the compiler feature is to use "depends on" 522followed by a test macro:: 523 524 config STACKPROTECTOR 525 bool "Stack Protector buffer overflow detection" 526 depends on $(cc-option,-fstack-protector) 527 ... 528 529If you need to expose a compiler capability to makefiles and/or C source files, 530`CC_HAS_` is the recommended prefix for the config option:: 531 532 config CC_HAS_STACKPROTECTOR_NONE 533 def_bool $(cc-option,-fno-stack-protector) 534 535Build as module only 536~~~~~~~~~~~~~~~~~~~~ 537To restrict a component build to module-only, qualify its config symbol 538with "depends on m". E.g.:: 539 540 config FOO 541 depends on BAR && m 542 543limits FOO to module (=m) or disabled (=n). 544 545Kconfig recursive dependency limitations 546~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 547 548If you've hit the Kconfig error: "recursive dependency detected" you've run 549into a recursive dependency issue with Kconfig, a recursive dependency can be 550summarized as a circular dependency. The kconfig tools need to ensure that 551Kconfig files comply with specified configuration requirements. In order to do 552that kconfig must determine the values that are possible for all Kconfig 553symbols, this is currently not possible if there is a circular relation 554between two or more Kconfig symbols. For more details refer to the "Simple 555Kconfig recursive issue" subsection below. Kconfig does not do recursive 556dependency resolution; this has a few implications for Kconfig file writers. 557We'll first explain why this issues exists and then provide an example 558technical limitation which this brings upon Kconfig developers. Eager 559developers wishing to try to address this limitation should read the next 560subsections. 561 562Simple Kconfig recursive issue 563~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 564 565Read: Documentation/kbuild/Kconfig.recursion-issue-01 566 567Test with:: 568 569 make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-01 allnoconfig 570 571Cumulative Kconfig recursive issue 572~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 573 574Read: Documentation/kbuild/Kconfig.recursion-issue-02 575 576Test with:: 577 578 make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-02 allnoconfig 579 580Practical solutions to kconfig recursive issue 581~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 582 583Developers who run into the recursive Kconfig issue have two options 584at their disposal. We document them below and also provide a list of 585historical issues resolved through these different solutions. 586 587 a) Remove any superfluous "select FOO" or "depends on FOO" 588 b) Match dependency semantics: 589 590 b1) Swap all "select FOO" to "depends on FOO" or, 591 592 b2) Swap all "depends on FOO" to "select FOO" 593 594The resolution to a) can be tested with the sample Kconfig file 595Documentation/kbuild/Kconfig.recursion-issue-01 through the removal 596of the "select CORE" from CORE_BELL_A_ADVANCED as that is implicit already 597since CORE_BELL_A depends on CORE. At times it may not be possible to remove 598some dependency criteria, for such cases you can work with solution b). 599 600The two different resolutions for b) can be tested in the sample Kconfig file 601Documentation/kbuild/Kconfig.recursion-issue-02. 602 603Below is a list of examples of prior fixes for these types of recursive issues; 604all errors appear to involve one or more "select" statements and one or more 605"depends on". 606 607============ =================================== 608commit fix 609============ =================================== 61006b718c01208 select A -> depends on A 611c22eacfe82f9 depends on A -> depends on B 6126a91e854442c select A -> depends on A 613118c565a8f2e select A -> select B 614f004e5594705 select A -> depends on A 615c7861f37b4c6 depends on A -> (null) 61680c69915e5fb select A -> (null) (1) 617c2218e26c0d0 select A -> depends on A (1) 618d6ae99d04e1c select A -> depends on A 61995ca19cf8cbf select A -> depends on A 6208f057d7bca54 depends on A -> (null) 6218f057d7bca54 depends on A -> select A 622a0701f04846e select A -> depends on A 6230c8b92f7f259 depends on A -> (null) 624e4e9e0540928 select A -> depends on A (2) 6257453ea886e87 depends on A > (null) (1) 6267b1fff7e4fdf select A -> depends on A 62786c747d2a4f0 select A -> depends on A 628d9f9ab51e55e select A -> depends on A 6290c51a4d8abd6 depends on A -> select A (3) 630e98062ed6dc4 select A -> depends on A (3) 63191e5d284a7f1 select A -> (null) 632============ =================================== 633 634(1) Partial (or no) quote of error. 635(2) That seems to be the gist of that fix. 636(3) Same error. 637 638Future kconfig work 639~~~~~~~~~~~~~~~~~~~ 640 641Work on kconfig is welcomed on both areas of clarifying semantics and on 642evaluating the use of a full SAT solver for it. A full SAT solver can be 643desirable to enable more complex dependency mappings and / or queries, 644for instance on possible use case for a SAT solver could be that of handling 645the current known recursive dependency issues. It is not known if this would 646address such issues but such evaluation is desirable. If support for a full SAT 647solver proves too complex or that it cannot address recursive dependency issues 648Kconfig should have at least clear and well defined semantics which also 649addresses and documents limitations or requirements such as the ones dealing 650with recursive dependencies. 651 652Further work on both of these areas is welcomed on Kconfig. We elaborate 653on both of these in the next two subsections. 654 655Semantics of Kconfig 656~~~~~~~~~~~~~~~~~~~~ 657 658The use of Kconfig is broad, Linux is now only one of Kconfig's users: 659one study has completed a broad analysis of Kconfig use in 12 projects [0]_. 660Despite its widespread use, and although this document does a reasonable job 661in documenting basic Kconfig syntax a more precise definition of Kconfig 662semantics is welcomed. One project deduced Kconfig semantics through 663the use of the xconfig configurator [1]_. Work should be done to confirm if 664the deduced semantics matches our intended Kconfig design goals. 665 666Having well defined semantics can be useful for tools for practical 667evaluation of dependencies, for instance one such case was work to 668express in boolean abstraction of the inferred semantics of Kconfig to 669translate Kconfig logic into boolean formulas and run a SAT solver on this to 670find dead code / features (always inactive), 114 dead features were found in 671Linux using this methodology [1]_ (Section 8: Threats to validity). 672 673Confirming this could prove useful as Kconfig stands as one of the the leading 674industrial variability modeling languages [1]_ [2]_. Its study would help 675evaluate practical uses of such languages, their use was only theoretical 676and real world requirements were not well understood. As it stands though 677only reverse engineering techniques have been used to deduce semantics from 678variability modeling languages such as Kconfig [3]_. 679 680.. [0] http://www.eng.uwaterloo.ca/~shshe/kconfig_semantics.pdf 681.. [1] http://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf 682.. [2] http://gsd.uwaterloo.ca/sites/default/files/ase241-berger_0.pdf 683.. [3] http://gsd.uwaterloo.ca/sites/default/files/icse2011.pdf 684 685Full SAT solver for Kconfig 686~~~~~~~~~~~~~~~~~~~~~~~~~~~ 687 688Although SAT solvers [4]_ haven't yet been used by Kconfig directly, as noted 689in the previous subsection, work has been done however to express in boolean 690abstraction the inferred semantics of Kconfig to translate Kconfig logic into 691boolean formulas and run a SAT solver on it [5]_. Another known related project 692is CADOS [6]_ (former VAMOS [7]_) and the tools, mainly undertaker [8]_, which 693has been introduced first with [9]_. The basic concept of undertaker is to 694extract variability models from Kconfig and put them together with a 695propositional formula extracted from CPP #ifdefs and build-rules into a SAT 696solver in order to find dead code, dead files, and dead symbols. If using a SAT 697solver is desirable on Kconfig one approach would be to evaluate repurposing 698such efforts somehow on Kconfig. There is enough interest from mentors of 699existing projects to not only help advise how to integrate this work upstream 700but also help maintain it long term. Interested developers should visit: 701 702http://kernelnewbies.org/KernelProjects/kconfig-sat 703 704.. [4] http://www.cs.cornell.edu/~sabhar/chapters/SATSolvers-KR-Handbook.pdf 705.. [5] http://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf 706.. [6] https://cados.cs.fau.de 707.. [7] https://vamos.cs.fau.de 708.. [8] https://undertaker.cs.fau.de 709.. [9] https://www4.cs.fau.de/Publications/2011/tartler_11_eurosys.pdf 710