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Documentation: kbuild: explain handling optional dependencies

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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>
This commit is contained in:
Arnd Bergmann 2023-09-17 21:19:59 +02:00 committed by Masahiro Yamada
parent fbf5892df2
commit 28d49e1716
1 changed files with 26 additions and 0 deletions
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Documentation/kbuild/kconfig-language.rst
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@ -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
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~