linux-kernel - [PATCH] docs: kconfig: Mention IS_REACHABLE as way for optional dependency

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-ID: <20250215114223.140688-1-krzysztof.kozlowski@linaro.org>
Date: Sat, 15 Feb 2025 12:42:23 +0100
From: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To: Masahiro Yamada <masahiroy@...nel.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Nicolas Schier <nicolas@...sle.eu>,
	Jonathan Corbet <corbet@....net>,
	linux-kbuild@...r.kernel.org,
	linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org
Cc: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
	Arnd Bergmann <arnd@...db.de>
Subject: [PATCH] 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 neither suitable (lack of stubs for !FOO
like in HWMON) nor really needed and driver can be built in even if FOO
is the module.  This is achieved with IS_REACHABLE, so provide cross
reference to it.

Cc: Masahiro Yamada <masahiroy@...nel.org>
Cc: Arnd Bergmann <arnd@...db.de>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
---
 Documentation/kbuild/kconfig-language.rst | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/Documentation/kbuild/kconfig-language.rst b/Documentation/kbuild/kconfig-language.rst
index 2619fdf56e68..66248294a552 100644
--- a/Documentation/kbuild/kconfig-language.rst
+++ b/Documentation/kbuild/kconfig-language.rst
@@ -194,6 +194,8 @@ 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.
 
+.. _is_reachable:
+
   Note: If the combination of FOO=y and BAZ=m causes a link error,
   you can guard the function call with IS_REACHABLE()::
 
@@ -580,10 +582,15 @@ 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::
+There are two ways to express this optional dependency:
 
-  config FOO
+1. If pre-processor can discard entire optional code or module FOO does not
+   provide !FOO stubs then in the C code :ref:`IS_REACHABLE<is_reachable>`
+
+2. Otherwise (and module FOO must provide all !FOO stubs) use the slightly
+   counterintuitive Kconfig syntax::
+
+     config FOO
 	tristate "Support for foo hardware"
 	depends on BAR || !BAR
 
-- 
2.43.0