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author | Arnd Bergmann <arnd@arndb.de> | 2023-09-17 22:19:59 +0300 |
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committer | Masahiro Yamada <masahiroy@kernel.org> | 2023-09-25 10:01:05 +0300 |
commit | 28d49e171676afb7df7f47798391364af9abed7f (patch) | |
tree | 958b34bd540d22f2d8af5c2f64b6ca978fd1ba05 | |
parent | fbf5892df21a8ccfcb2fda0fd65bc3169c89ed28 (diff) | |
download | linux-28d49e171676afb7df7f47798391364af9abed7f.tar.xz |
Documentation: kbuild: explain handling optional dependencies
This problem frequently comes up in randconfig testing, with
drivers failing to link because of a dependency on an optional
feature.
The Kconfig language for this is very confusing, so try to
document it in "Kconfig hints" section.
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
-rw-r--r-- | Documentation/kbuild/kconfig-language.rst | 26 |
1 files changed, 26 insertions, 0 deletions
diff --git a/Documentation/kbuild/kconfig-language.rst b/Documentation/kbuild/kconfig-language.rst index 858ed5d80def..0135905c0aa3 100644 --- a/Documentation/kbuild/kconfig-language.rst +++ b/Documentation/kbuild/kconfig-language.rst @@ -573,6 +573,32 @@ above, leading to: bool "Support for foo hardware" depends on ARCH_FOO_VENDOR || COMPILE_TEST +Optional dependencies +~~~~~~~~~~~~~~~~~~~~~ + +Some drivers are able to optionally use a feature from another module +or build cleanly with that module disabled, but cause a link failure +when trying to use that loadable module from a built-in driver. + +The most common way to express this optional dependency in Kconfig logic +uses the slightly counterintuitive:: + + config FOO + tristate "Support for foo hardware" + 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. +For a more formalized approach if there are multiple drivers that have +the same dependency, a helper symbol can be used, like:: + + config FOO + tristate "Support for foo hardware" + depends on BAR_OPTIONAL + + config BAR_OPTIONAL + def_tristate BAR || !BAR + Kconfig recursive dependency limitations ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |