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From:   Bjorn Helgaas <helgaas@kernel.org>
To:     linux-kernel@vger.kernel.org
Cc:     Masahiro Yamada <masahiroy@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Linus Walleij <linus.walleij@linaro.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Paul Cercueil <paul@crapouillou.net>,
        =?UTF-8?q?Krzysztof=20Wilczy=C5=84ski?= <kw@linux.com>,
        Bjorn Helgaas <bhelgaas@google.com>
Subject: [PATCH] kconfig.h: explain IS_MODULE(), IS_ENABLED()
Date:   Tue,  1 Jun 2021 16:31:43 -0500
Message-Id: <20210601213143.1973770-1-helgaas@kernel.org>
MIME-Version: 1.0
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From: Bjorn Helgaas <bhelgaas@google.com>

Extend IS_MODULE() and IS_ENABLED comments to explain why one might use
"#if IS_ENABLED(CONFIG_FOO)" instead of "#ifdef CONFIG_FOO".

To wit, "#ifdef CONFIG_FOO" is true only for CONFIG_FOO=y, while
"#if IS_ENABLED(CONFIG_FOO)" is true for both CONFIG_FOO=y and
CONFIG_FOO=m.

This is because "CONFIG_FOO=m" in .config does not result in "CONFIG_FOO"
being defined.  The actual definitions are in autoconf.h, where:

  CONFIG_FOO=y   results in   #define CONFIG_FOO 1
  CONFIG_FOO=m   results in   #define CONFIG_FOO_MODULE 1

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
---
 include/linux/kconfig.h | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/include/linux/kconfig.h b/include/linux/kconfig.h
index cc8fa109cfa3..20d1079e92b4 100644
--- a/include/linux/kconfig.h
+++ b/include/linux/kconfig.h
@@ -51,7 +51,8 @@
 
 /*
  * IS_MODULE(CONFIG_FOO) evaluates to 1 if CONFIG_FOO is set to 'm', 0
- * otherwise.
+ * otherwise.  CONFIG_FOO=m results in "#define CONFIG_FOO_MODULE 1" in
+ * autoconf.h.
  */
 #define IS_MODULE(option) __is_defined(option##_MODULE)
 
@@ -66,7 +67,8 @@
 
 /*
  * 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))
 
-- 
2.25.1