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Message-ID: <20160927190802.13636.10320.stgit@bhelgaas-glaptop2.roam.corp.google.com>
Date: Tue, 27 Sep 2016 14:08:02 -0500
From: Bjorn Helgaas <bhelgaas@...gle.com>
To: Jonathan Corbet <corbet@....net>
Cc: linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org
Subject: [PATCH] CodingStyle: Expand IS_ENABLED() documentation
CodingStyle recommends IS_ENABLED(CONFIG_FOO) over #ifdef. Add an example
of the #ifdef, since it's not completely obvious that in many cases the
#ifdef needs to test both CONFIG_FOO and CONFIG_FOO_MODULE.
Signed-off-by: Bjorn Helgaas <bhelgaas@...gle.com>
---
Documentation/CodingStyle | 15 +++++++++++++--
include/linux/kconfig.h | 3 ++-
2 files changed, 15 insertions(+), 3 deletions(-)
diff --git a/Documentation/CodingStyle b/Documentation/CodingStyle
index a096836..b3e9743 100644
--- a/Documentation/CodingStyle
+++ b/Documentation/CodingStyle
@@ -911,8 +911,19 @@ The compiler will constant-fold the conditional away, and include or exclude
the block of code just as with an #ifdef, so this will not add any runtime
overhead. However, this approach still allows the C compiler to see the code
inside the block, and check it for correctness (syntax, types, symbol
-references, etc). Thus, you still have to use an #ifdef if the code inside the
-block references symbols that will not exist if the condition is not met.
+references, etc).
+
+Because the compiler processes the block, you have to use an #ifdef instead
+of IS_ENABLED() when code inside the block references symbols that will not
+exist if the condition is not met. Different CONFIG_FOO autoconf.h symbols
+are generated for modular Kconfig options than for builtin ones, so you
+need "#if defined(CONFIG_FOO) || defined(CONFIG_FOO_MODULE)" if FOO can be
+a module:
+
+ .config include/generated/autoconf.h
+ ------------ ----------------------------
+ CONFIG_FOO=y #define CONFIG_FOO 1
+ CONFIG_FOO=m #define CONFIG_FOO_MODULE 1
At the end of any non-trivial #if or #ifdef block (more than a few lines),
place a comment after the #endif on the same line, noting the conditional
diff --git a/include/linux/kconfig.h b/include/linux/kconfig.h
index 15ec117..51a5f66 100644
--- a/include/linux/kconfig.h
+++ b/include/linux/kconfig.h
@@ -60,7 +60,8 @@
/*
* IS_ENABLED(CONFIG_FOO) evaluates to 1 if CONFIG_FOO is set to 'y' or 'm',
- * 0 otherwise.
+ * 0 otherwise. Note that CONFIG_FOO=y results in "#define CONFIG_FOO 1"
+ * in autoconf.h, while CONFIG_FOO=m results in "#define CONFIG_FOO_MODULE 1"
*/
#define IS_ENABLED(option) __or(IS_BUILTIN(option), IS_MODULE(option))
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