[PATCH v3] docs: kconfig: Mention IS_REACHABLE as way for optional dependency
From: Krzysztof Kozlowski
Date: Mon Feb 17 2025 - 06:32:37 EST
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".
Cc: Masahiro Yamada <masahiroy@xxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@xxxxxxxxxx>
---
Changes in v3:
1. Significant rewrite to mark IS_REACHABLE as less favorable.
Changes in v2:
1. Replace FOO->BAR
2. Instead of referencing earlier "imply", move the code here and add
more text (Masahiro)
---
Documentation/kbuild/kconfig-language.rst | 29 ++++++++++++++---------
1 file changed, 18 insertions(+), 11 deletions(-)
diff --git a/Documentation/kbuild/kconfig-language.rst b/Documentation/kbuild/kconfig-language.rst
index 2619fdf56e68..c8c20a34525e 100644
--- a/Documentation/kbuild/kconfig-language.rst
+++ b/Documentation/kbuild/kconfig-language.rst
@@ -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
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
--
2.43.0