Commit 700bd25b authored by Krzysztof Kozlowski's avatar Krzysztof Kozlowski Committed by Masahiro Yamada
Browse files

docs: kconfig: Mention IS_REACHABLE as way for optional dependency 



Several drivers express optional Kconfig dependency with FOO || !FOO,
but for many choices this is not suitable: lack of stubs for !FOO
like in HWMON.  Describe the second, less favorable way of optional
dependency with IS_REACHABLE by moving the code from "imply" chapter to
"Optional dependencies".

Signed-off-by: default avatarKrzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: default avatarArnd Bergmann <arnd@arndb.de>
Signed-off-by: default avatarMasahiro Yamada <masahiroy@kernel.org>
parent d0beb73d

Documentation/kbuild/kconfig-language.rst

+18 −11
Original line number Diff line number Diff line
@@ -194,16 +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 BAZ=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::
@@ -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

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~