Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.
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" <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 The default value deliberately defaults to 'n' in order to avoid bloating the 81 build. With few exceptions, new config options should not change this. The 82 intent is for "make oldconfig" to add as little as possible to the config from 83 release to release. 84 85 Note: 86 Things that merit "default y/m" include: 87 88 a) A new Kconfig option for something that used to always be built 89 should be "default y". 90 91 b) A new gatekeeping Kconfig option that hides/shows other Kconfig 92 options (but does not generate any code of its own), should be 93 "default y" so people will see those other options. 94 95 c) Sub-driver behavior or similar options for a driver that is 96 "default n". This allows you to provide sane defaults. 97 98 d) Hardware or infrastructure that everybody expects, such as CONFIG_NET 99 or CONFIG_BLOCK. These are rare exceptions. 100 101 - type definition + default value: 102 "def_bool"/"def_tristate" <expr> ["if" <expr>] 103 This is a shorthand notation for a type definition plus a value. 104 Optionally dependencies for this default value can be added with "if". 105 106 - dependencies: "depends on" <expr> 107 This defines a dependency for this menu entry. If multiple 108 dependencies are defined, they are connected with '&&'. Dependencies 109 are applied to all other options within this menu entry (which also 110 accept an "if" expression), so these two examples are equivalent: 111 112 bool "foo" if BAR 113 default y if BAR 114 and 115 depends on BAR 116 bool "foo" 117 default y 118 119 - reverse dependencies: "select" <symbol> ["if" <expr>] 120 While normal dependencies reduce the upper limit of a symbol (see 121 below), reverse dependencies can be used to force a lower limit of 122 another symbol. The value of the current menu symbol is used as the 123 minimal value <symbol> can be set to. If <symbol> is selected multiple 124 times, the limit is set to the largest selection. 125 Reverse dependencies can only be used with boolean or tristate 126 symbols. 127 Note: 128 select should be used with care. select will force 129 a symbol to a value without visiting the dependencies. 130 By abusing select you are able to select a symbol FOO even 131 if FOO depends on BAR that is not set. 132 In general use select only for non-visible symbols 133 (no prompts anywhere) and for symbols with no dependencies. 134 That will limit the usefulness but on the other hand avoid 135 the illegal configurations all over. 136 137 - weak reverse dependencies: "imply" <symbol> ["if" <expr>] 138 This is similar to "select" as it enforces a lower limit on another 139 symbol except that the "implied" symbol's value may still be set to n 140 from a direct dependency or with a visible prompt. 141 142 Given the following example: 143 144 config FOO 145 tristate 146 imply BAZ 147 148 config BAZ 149 tristate 150 depends on BAR 151 152 The following values are possible: 153 154 FOO BAR BAZ's default choice for BAZ 155 --- --- ------------- -------------- 156 n y n N/m/y 157 m y m M/y/n 158 y y y Y/n 159 y n * N 160 161 This is useful e.g. with multiple drivers that want to indicate their 162 ability to hook into a secondary subsystem while allowing the user to 163 configure that subsystem out without also having to unset these drivers. 164 165 - limiting menu display: "visible if" <expr> 166 This attribute is only applicable to menu blocks, if the condition is 167 false, the menu block is not displayed to the user (the symbols 168 contained there can still be selected by other symbols, though). It is 169 similar to a conditional "prompt" attribute for individual menu 170 entries. Default value of "visible" is true. 171 172 - numerical ranges: "range" <symbol> <symbol> ["if" <expr>] 173 This allows to limit the range of possible input values for int 174 and hex symbols. The user can only input a value which is larger than 175 or equal to the first symbol and smaller than or equal to the second 176 symbol. 177 178 - help text: "help" or "---help---" 179 This defines a help text. The end of the help text is determined by 180 the indentation level, this means it ends at the first line which has 181 a smaller indentation than the first line of the help text. 182 "---help---" and "help" do not differ in behaviour, "---help---" is 183 used to help visually separate configuration logic from help within 184 the file as an aid to developers. 185 186 - misc options: "option" <symbol>[=<value>] 187 Various less common options can be defined via this option syntax, 188 which can modify the behaviour of the menu entry and its config 189 symbol. These options are currently possible: 190 191 - "defconfig_list" 192 This declares a list of default entries which can be used when 193 looking for the default configuration (which is used when the main 194 .config doesn't exists yet.) 195 196 - "modules" 197 This declares the symbol to be used as the MODULES symbol, which 198 enables the third modular state for all config symbols. 199 At most one symbol may have the "modules" option set. 200 201 - "env"=<value> 202 This imports the environment variable into Kconfig. It behaves like 203 a default, except that the value comes from the environment, this 204 also means that the behaviour when mixing it with normal defaults is 205 undefined at this point. The symbol is currently not exported back 206 to the build environment (if this is desired, it can be done via 207 another symbol). 208 209 - "allnoconfig_y" 210 This declares the symbol as one that should have the value y when 211 using "allnoconfig". Used for symbols that hide other symbols. 212 213 Menu dependencies 214 ----------------- 215 216 Dependencies define the visibility of a menu entry and can also reduce 217 the input range of tristate symbols. The tristate logic used in the 218 expressions uses one more state than normal boolean logic to express the 219 module state. Dependency expressions have the following syntax: 220 221 <expr> ::= <symbol> (1) 222 <symbol> '=' <symbol> (2) 223 <symbol> '!=' <symbol> (3) 224 <symbol1> '<' <symbol2> (4) 225 <symbol1> '>' <symbol2> (4) 226 <symbol1> '<=' <symbol2> (4) 227 <symbol1> '>=' <symbol2> (4) 228 '(' <expr> ')' (5) 229 '!' <expr> (6) 230 <expr> '&&' <expr> (7) 231 <expr> '||' <expr> (8) 232 233 Expressions are listed in decreasing order of precedence. 234 235 (1) Convert the symbol into an expression. Boolean and tristate symbols 236 are simply converted into the respective expression values. All 237 other symbol types result in 'n'. 238 (2) If the values of both symbols are equal, it returns 'y', 239 otherwise 'n'. 240 (3) If the values of both symbols are equal, it returns 'n', 241 otherwise 'y'. 242 (4) If value of <symbol1> is respectively lower, greater, lower-or-equal, 243 or greater-or-equal than value of <symbol2>, it returns 'y', 244 otherwise 'n'. 245 (5) Returns the value of the expression. Used to override precedence. 246 (6) Returns the result of (2-/expr/). 247 (7) Returns the result of min(/expr/, /expr/). 248 (8) Returns the result of max(/expr/, /expr/). 249 250 An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2 251 respectively for calculations). A menu entry becomes visible when its 252 expression evaluates to 'm' or 'y'. 253 254 There are two types of symbols: constant and non-constant symbols. 255 Non-constant symbols are the most common ones and are defined with the 256 'config' statement. Non-constant symbols consist entirely of alphanumeric 257 characters or underscores. 258 Constant symbols are only part of expressions. Constant symbols are 259 always surrounded by single or double quotes. Within the quote, any 260 other character is allowed and the quotes can be escaped using '\'. 261 262 Menu structure 263 -------------- 264 265 The position of a menu entry in the tree is determined in two ways. First 266 it can be specified explicitly: 267 268 menu "Network device support" 269 depends on NET 270 271 config NETDEVICES 272 ... 273 274 endmenu 275 276 All entries within the "menu" ... "endmenu" block become a submenu of 277 "Network device support". All subentries inherit the dependencies from 278 the menu entry, e.g. this means the dependency "NET" is added to the 279 dependency list of the config option NETDEVICES. 280 281 The other way to generate the menu structure is done by analyzing the 282 dependencies. If a menu entry somehow depends on the previous entry, it 283 can be made a submenu of it. First, the previous (parent) symbol must 284 be part of the dependency list and then one of these two conditions 285 must be true: 286 - the child entry must become invisible, if the parent is set to 'n' 287 - the child entry must only be visible, if the parent is visible 288 289 config MODULES 290 bool "Enable loadable module support" 291 292 config MODVERSIONS 293 bool "Set version information on all module symbols" 294 depends on MODULES 295 296 comment "module support disabled" 297 depends on !MODULES 298 299 MODVERSIONS directly depends on MODULES, this means it's only visible if 300 MODULES is different from 'n'. The comment on the other hand is only 301 visible when MODULES is set to 'n'. 302 303 304 Kconfig syntax 305 -------------- 306 307 The configuration file describes a series of menu entries, where every 308 line starts with a keyword (except help texts). The following keywords 309 end a menu entry: 310 - config 311 - menuconfig 312 - choice/endchoice 313 - comment 314 - menu/endmenu 315 - if/endif 316 - source 317 The first five also start the definition of a menu entry. 318 319 config: 320 321 "config" <symbol> 322 <config options> 323 324 This defines a config symbol <symbol> and accepts any of above 325 attributes as options. 326 327 menuconfig: 328 "menuconfig" <symbol> 329 <config options> 330 331 This is similar to the simple config entry above, but it also gives a 332 hint to front ends, that all suboptions should be displayed as a 333 separate list of options. To make sure all the suboptions will really 334 show up under the menuconfig entry and not outside of it, every item 335 from the <config options> list must depend on the menuconfig symbol. 336 In practice, this is achieved by using one of the next two constructs: 337 338 (1): 339 menuconfig M 340 if M 341 config C1 342 config C2 343 endif 344 345 (2): 346 menuconfig M 347 config C1 348 depends on M 349 config C2 350 depends on M 351 352 In the following examples (3) and (4), C1 and C2 still have the M 353 dependency, but will not appear under menuconfig M anymore, because 354 of C0, which doesn't depend on M: 355 356 (3): 357 menuconfig M 358 config C0 359 if M 360 config C1 361 config C2 362 endif 363 364 (4): 365 menuconfig M 366 config C0 367 config C1 368 depends on M 369 config C2 370 depends on M 371 372 choices: 373 374 "choice" [symbol] 375 <choice options> 376 <choice block> 377 "endchoice" 378 379 This defines a choice group and accepts any of the above attributes as 380 options. A choice can only be of type bool or tristate. If no type is 381 specified for a choice, it's type will be determined by the type of 382 the first choice element in the group or remain unknown if none of the 383 choice elements have a type specified, as well. 384 385 While a boolean choice only allows a single config entry to be 386 selected, a tristate choice also allows any number of config entries 387 to be set to 'm'. This can be used if multiple drivers for a single 388 hardware exists and only a single driver can be compiled/loaded into 389 the kernel, but all drivers can be compiled as modules. 390 391 A choice accepts another option "optional", which allows to set the 392 choice to 'n' and no entry needs to be selected. 393 If no [symbol] is associated with a choice, then you can not have multiple 394 definitions of that choice. If a [symbol] is associated to the choice, 395 then you may define the same choice (ie. with the same entries) in another 396 place. 397 398 comment: 399 400 "comment" <prompt> 401 <comment options> 402 403 This defines a comment which is displayed to the user during the 404 configuration process and is also echoed to the output files. The only 405 possible options are dependencies. 406 407 menu: 408 409 "menu" <prompt> 410 <menu options> 411 <menu block> 412 "endmenu" 413 414 This defines a menu block, see "Menu structure" above for more 415 information. The only possible options are dependencies and "visible" 416 attributes. 417 418 if: 419 420 "if" <expr> 421 <if block> 422 "endif" 423 424 This defines an if block. The dependency expression <expr> is appended 425 to all enclosed menu entries. 426 427 source: 428 429 "source" <prompt> 430 431 This reads the specified configuration file. This file is always parsed. 432 433 mainmenu: 434 435 "mainmenu" <prompt> 436 437 This sets the config program's title bar if the config program chooses 438 to use it. It should be placed at the top of the configuration, before any 439 other statement. 440 441 442 Kconfig hints 443 ------------- 444 This is a collection of Kconfig tips, most of which aren't obvious at 445 first glance and most of which have become idioms in several Kconfig 446 files. 447 448 Adding common features and make the usage configurable 449 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 450 It is a common idiom to implement a feature/functionality that are 451 relevant for some architectures but not all. 452 The recommended way to do so is to use a config variable named HAVE_* 453 that is defined in a common Kconfig file and selected by the relevant 454 architectures. 455 An example is the generic IOMAP functionality. 456 457 We would in lib/Kconfig see: 458 459 # Generic IOMAP is used to ... 460 config HAVE_GENERIC_IOMAP 461 462 config GENERIC_IOMAP 463 depends on HAVE_GENERIC_IOMAP && FOO 464 465 And in lib/Makefile we would see: 466 obj-$(CONFIG_GENERIC_IOMAP) += iomap.o 467 468 For each architecture using the generic IOMAP functionality we would see: 469 470 config X86 471 select ... 472 select HAVE_GENERIC_IOMAP 473 select ... 474 475 Note: we use the existing config option and avoid creating a new 476 config variable to select HAVE_GENERIC_IOMAP. 477 478 Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is 479 introduced to overcome the limitation of select which will force a 480 config option to 'y' no matter the dependencies. 481 The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the 482 situation where select forces a symbol equals to 'y'. 483 484 Build as module only 485 ~~~~~~~~~~~~~~~~~~~~ 486 To restrict a component build to module-only, qualify its config symbol 487 with "depends on m". E.g.: 488 489 config FOO 490 depends on BAR && m 491 492 limits FOO to module (=m) or disabled (=n). 493 494 Kconfig recursive dependency limitations 495 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 496 497 If you've hit the Kconfig error: "recursive dependency detected" you've run 498 into a recursive dependency issue with Kconfig, a recursive dependency can be 499 summarized as a circular dependency. The kconfig tools need to ensure that 500 Kconfig files comply with specified configuration requirements. In order to do 501 that kconfig must determine the values that are possible for all Kconfig 502 symbols, this is currently not possible if there is a circular relation 503 between two or more Kconfig symbols. For more details refer to the "Simple 504 Kconfig recursive issue" subsection below. Kconfig does not do recursive 505 dependency resolution; this has a few implications for Kconfig file writers. 506 We'll first explain why this issues exists and then provide an example 507 technical limitation which this brings upon Kconfig developers. Eager 508 developers wishing to try to address this limitation should read the next 509 subsections. 510 511 Simple Kconfig recursive issue 512 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 513 514 Read: Documentation/kbuild/Kconfig.recursion-issue-01 515 516 Test with: 517 518 make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-01 allnoconfig 519 520 Cumulative Kconfig recursive issue 521 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 522 523 Read: Documentation/kbuild/Kconfig.recursion-issue-02 524 525 Test with: 526 527 make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-02 allnoconfig 528 529 Practical solutions to kconfig recursive issue 530 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 531 532 Developers who run into the recursive Kconfig issue have three options 533 at their disposal. We document them below and also provide a list of 534 historical issues resolved through these different solutions. 535 536 a) Remove any superfluous "select FOO" or "depends on FOO" 537 b) Match dependency semantics: 538 b1) Swap all "select FOO" to "depends on FOO" or, 539 b2) Swap all "depends on FOO" to "select FOO" 540 c) Consider the use of "imply" instead of "select" 541 542 The resolution to a) can be tested with the sample Kconfig file 543 Documentation/kbuild/Kconfig.recursion-issue-01 through the removal 544 of the "select CORE" from CORE_BELL_A_ADVANCED as that is implicit already 545 since CORE_BELL_A depends on CORE. At times it may not be possible to remove 546 some dependency criteria, for such cases you can work with solution b). 547 548 The two different resolutions for b) can be tested in the sample Kconfig file 549 Documentation/kbuild/Kconfig.recursion-issue-02. 550 551 Below is a list of examples of prior fixes for these types of recursive issues; 552 all errors appear to involve one or more select's and one or more "depends on". 553 554 commit fix 555 ====== === 556 06b718c01208 select A -> depends on A 557 c22eacfe82f9 depends on A -> depends on B 558 6a91e854442c select A -> depends on A 559 118c565a8f2e select A -> select B 560 f004e5594705 select A -> depends on A 561 c7861f37b4c6 depends on A -> (null) 562 80c69915e5fb select A -> (null) (1) 563 c2218e26c0d0 select A -> depends on A (1) 564 d6ae99d04e1c select A -> depends on A 565 95ca19cf8cbf select A -> depends on A 566 8f057d7bca54 depends on A -> (null) 567 8f057d7bca54 depends on A -> select A 568 a0701f04846e select A -> depends on A 569 0c8b92f7f259 depends on A -> (null) 570 e4e9e0540928 select A -> depends on A (2) 571 7453ea886e87 depends on A > (null) (1) 572 7b1fff7e4fdf select A -> depends on A 573 86c747d2a4f0 select A -> depends on A 574 d9f9ab51e55e select A -> depends on A 575 0c51a4d8abd6 depends on A -> select A (3) 576 e98062ed6dc4 select A -> depends on A (3) 577 91e5d284a7f1 select A -> (null) 578 579 (1) Partial (or no) quote of error. 580 (2) That seems to be the gist of that fix. 581 (3) Same error. 582 583 Future kconfig work 584 ~~~~~~~~~~~~~~~~~~~ 585 586 Work on kconfig is welcomed on both areas of clarifying semantics and on 587 evaluating the use of a full SAT solver for it. A full SAT solver can be 588 desirable to enable more complex dependency mappings and / or queries, 589 for instance on possible use case for a SAT solver could be that of handling 590 the current known recursive dependency issues. It is not known if this would 591 address such issues but such evaluation is desirable. If support for a full SAT 592 solver proves too complex or that it cannot address recursive dependency issues 593 Kconfig should have at least clear and well defined semantics which also 594 addresses and documents limitations or requirements such as the ones dealing 595 with recursive dependencies. 596 597 Further work on both of these areas is welcomed on Kconfig. We elaborate 598 on both of these in the next two subsections. 599 600 Semantics of Kconfig 601 ~~~~~~~~~~~~~~~~~~~~ 602 603 The use of Kconfig is broad, Linux is now only one of Kconfig's users: 604 one study has completed a broad analysis of Kconfig use in 12 projects [0]. 605 Despite its widespread use, and although this document does a reasonable job 606 in documenting basic Kconfig syntax a more precise definition of Kconfig 607 semantics is welcomed. One project deduced Kconfig semantics through 608 the use of the xconfig configurator [1]. Work should be done to confirm if 609 the deduced semantics matches our intended Kconfig design goals. 610 611 Having well defined semantics can be useful for tools for practical 612 evaluation of depenencies, for instance one such use known case was work to 613 express in boolean abstraction of the inferred semantics of Kconfig to 614 translate Kconfig logic into boolean formulas and run a SAT solver on this to 615 find dead code / features (always inactive), 114 dead features were found in 616 Linux using this methodology [1] (Section 8: Threats to validity). 617 618 Confirming this could prove useful as Kconfig stands as one of the the leading 619 industrial variability modeling languages [1] [2]. Its study would help 620 evaluate practical uses of such languages, their use was only theoretical 621 and real world requirements were not well understood. As it stands though 622 only reverse engineering techniques have been used to deduce semantics from 623 variability modeling languages such as Kconfig [3]. 624 625 [0] http://www.eng.uwaterloo.ca/~shshe/kconfig_semantics.pdf 626 [1] http://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf 627 [2] http://gsd.uwaterloo.ca/sites/default/files/ase241-berger_0.pdf 628 [3] http://gsd.uwaterloo.ca/sites/default/files/icse2011.pdf 629 630 Full SAT solver for Kconfig 631 ~~~~~~~~~~~~~~~~~~~~~~~~~~~ 632 633 Although SAT solvers [0] haven't yet been used by Kconfig directly, as noted in 634 the previous subsection, work has been done however to express in boolean 635 abstraction the inferred semantics of Kconfig to translate Kconfig logic into 636 boolean formulas and run a SAT solver on it [1]. Another known related project 637 is CADOS [2] (former VAMOS [3]) and the tools, mainly undertaker [4], which has 638 been introduced first with [5]. The basic concept of undertaker is to exract 639 variability models from Kconfig, and put them together with a propositional 640 formula extracted from CPP #ifdefs and build-rules into a SAT solver in order 641 to find dead code, dead files, and dead symbols. If using a SAT solver is 642 desirable on Kconfig one approach would be to evaluate repurposing such efforts 643 somehow on Kconfig. There is enough interest from mentors of existing projects 644 to not only help advise how to integrate this work upstream but also help 645 maintain it long term. Interested developers should visit: 646 647 http://kernelnewbies.org/KernelProjects/kconfig-sat 648 649 [0] http://www.cs.cornell.edu/~sabhar/chapters/SATSolvers-KR-Handbook.pdf 650 [1] http://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf 651 [2] https://cados.cs.fau.de 652 [3] https://vamos.cs.fau.de 653 [4] https://undertaker.cs.fau.de 654 [5] https://www4.cs.fau.de/Publications/2011/tartler_11_eurosys.pdf