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From: Arnd Bergmann <>
To: Masahiro Yamada <>, Jonathan Corbet <>
Cc: Arnd Bergmann <>,
	Javier Martinez Canillas <>,
	Sakari Ailus <>,
	Nicolas Schier <>,
	Nathan Chancellor <>,
	Nick Desaulniers <>,,,
Subject: [PATCH] [v2] Documentation: kbuild: explain handling optional dependencies
Date: Sun, 17 Sep 2023 21:19:59 +0200	[thread overview]
Message-ID: <> (raw)

From: Arnd Bergmann <>

This problem frequently comes up in randconfig testing, with
drivers failing to link because of a dependency on an optional

The Kconfig language for this is very confusing, so try to
document it in "Kconfig hints" section.

Reviewed-by: Javier Martinez Canillas <>
Reviewed-by: Sakari Ailus <>
Reviewed-by: Nicolas Schier <>
Signed-off-by: Arnd Bergmann <>
v2: fix typos pointed out by Nicolas Schier
 Documentation/kbuild/kconfig-language.rst | 26 +++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/Documentation/kbuild/kconfig-language.rst b/Documentation/kbuild/kconfig-language.rst
index 858ed5d80defe..0135905c0aa35 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"
+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

             reply	other threads:[~2023-09-17 19:20 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-09-17 19:19 Arnd Bergmann [this message]
2023-09-25  7:06 ` [PATCH] [v2] Documentation: kbuild: explain handling optional dependencies Masahiro Yamada
2024-02-15 12:38   ` Nicolas Schier
2024-02-15 13:40     ` Masahiro Yamada
2024-02-15 13:51       ` Nicolas Schier

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