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Diffstat (limited to 'Documentation/kbuild/kconfig-language.rst')
| -rw-r--r-- | Documentation/kbuild/kconfig-language.rst | 69 |
1 files changed, 38 insertions, 31 deletions
diff --git a/Documentation/kbuild/kconfig-language.rst b/Documentation/kbuild/kconfig-language.rst index 0135905c0aa3..a91abb8f6840 100644 --- a/Documentation/kbuild/kconfig-language.rst +++ b/Documentation/kbuild/kconfig-language.rst @@ -70,7 +70,11 @@ applicable everywhere (see syntax). Every menu entry can have at most one prompt, which is used to display to the user. Optionally dependencies only for this prompt can be added - with "if". + with "if". If a prompt is not present, the config option is a non-visible + symbol, meaning its value cannot be directly changed by the user (such as + altering the value in ``.config``) and the option will not appear in any + config menus. Its value can only be set via "default" and "select" (see + below). - default value: "default" <expr> ["if" <expr>] @@ -150,6 +154,12 @@ applicable everywhere (see syntax). That will limit the usefulness but on the other hand avoid the illegal configurations all over. + If "select" <symbol> is followed by "if" <expr>, <symbol> will be + selected by the logical AND of the value of the current menu symbol + and <expr>. This means, the lower limit can be downgraded due to the + presence of "if" <expr>. This behavior may seem weird, but we rely on + it. (The future of this behavior is undecided.) + - weak reverse dependencies: "imply" <symbol> ["if" <expr>] This is similar to "select" as it enforces a lower limit on another @@ -184,16 +194,6 @@ applicable everywhere (see syntax). ability to hook into a secondary subsystem while allowing the user to configure that subsystem out without also having to unset these drivers. - Note: If the combination of FOO=y and BAR=m causes a link error, - you can guard the function call with IS_REACHABLE():: - - foo_init() - { - if (IS_REACHABLE(CONFIG_BAZ)) - baz_register(&foo); - ... - } - Note: If the feature provided by BAZ is highly desirable for FOO, FOO should imply not only BAZ, but also its dependency BAR:: @@ -202,6 +202,10 @@ applicable everywhere (see syntax). imply BAR imply BAZ + Note: If "imply" <symbol> is followed by "if" <expr>, the default of <symbol> + will be the logical AND of the value of the current menu symbol and <expr>. + (The future of this behavior is undecided.) + - limiting menu display: "visible if" <expr> This attribute is only applicable to menu blocks, if the condition is @@ -393,29 +397,15 @@ of C0, which doesn't depend on M:: choices:: - "choice" [symbol] + "choice" <choice options> <choice block> "endchoice" -This defines a choice group and accepts any of the above attributes as -options. A choice can only be of type bool or tristate. If no type is -specified for a choice, its type will be determined by the type of -the first choice element in the group or remain unknown if none of the -choice elements have a type specified, as well. - -While a boolean choice only allows a single config entry to be -selected, a tristate choice also allows any number of config entries -to be set to 'm'. This can be used if multiple drivers for a single -hardware exists and only a single driver can be compiled/loaded into -the kernel, but all drivers can be compiled as modules. - -A choice accepts another option "optional", which allows to set the -choice to 'n' and no entry needs to be selected. -If no [symbol] is associated with a choice, then you can not have multiple -definitions of that choice. If a [symbol] is associated to the choice, -then you may define the same choice (i.e. with the same entries) in another -place. +This defines a choice group and accepts "prompt", "default", "depends on", and +"help" attributes as options. + +A choice only allows a single config entry to be selected. comment:: @@ -588,7 +578,9 @@ uses the slightly counterintuitive:: depends on BAR || !BAR This means that there is either a dependency on BAR that disallows -the combination of FOO=y with BAR=m, or BAR is completely disabled. +the combination of FOO=y with BAR=m, or BAR is completely disabled. The BAR +module must provide all the stubs for !BAR case. + For a more formalized approach if there are multiple drivers that have the same dependency, a helper symbol can be used, like:: @@ -599,6 +591,21 @@ the same dependency, a helper symbol can be used, like:: config BAR_OPTIONAL def_tristate BAR || !BAR +Much less favorable way to express optional dependency is IS_REACHABLE() within +the module code, useful for example when the module BAR does not provide +!BAR stubs:: + + foo_init() + { + if (IS_REACHABLE(CONFIG_BAR)) + bar_register(&foo); + ... + } + +IS_REACHABLE() is generally discouraged, because the code will be silently +discarded, when CONFIG_BAR=m and this code is built-in. This is not what users +usually expect when enabling BAR as module. + Kconfig recursive dependency limitations ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |