linux-kernel - [PATCH next 02/14] kbuild: Add W=c for additional compile time checks

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Message-Id: <20260121145731.3623-3-david.laight.linux@gmail.com>
Date: Wed, 21 Jan 2026 14:57:19 +0000
From: david.laight.linux@...il.com
To: Nathan Chancellor <nathan@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Arnd Bergmann <arnd@...db.de>,
	linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Yury Norov <yury.norov@...il.com>,
	Lucas De Marchi <lucas.demarchi@...el.com>,
	Jani Nikula <jani.nikula@...el.com>,
	Vincent Mailhol <mailhol.vincent@...adoo.fr>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Kees Cook <keescook@...omium.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc: David Laight <david.laight.linux@...il.com>
Subject: [PATCH next 02/14] kbuild: Add W=c for additional compile time checks

From: David Laight <david.laight.linux@...il.com>

Some compile time checks significantly bloat the pre-processor output
(particularly when the get nested).
Since the checks aren't really needed on every compilation enable with
W=c (adds -DKBUILD_EXTRA_WARNc) so the checks can be enabled per-build.
Make W=1 imply W=c so the build-bot includes the checks.

As well as reducing the bloat from existing checks (like those in
GENMASK() and FIELD_PREP()) it lets additional checks be added
while there are still 'false positives' without breaking normal builds.

Signed-off-by: David Laight <david.laight.linux@...il.com>
---
 scripts/Makefile.warn | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/scripts/Makefile.warn b/scripts/Makefile.warn
index 68e6fafcb80c..e8a799850973 100644
--- a/scripts/Makefile.warn
+++ b/scripts/Makefile.warn
@@ -2,8 +2,9 @@
 # ==========================================================================
 # make W=... settings
 #
-# There are four warning groups enabled by W=1, W=2, W=3, and W=e
-# They are independent, and can be combined like W=12 or W=123e.
+# There are five warning groups enabled by W=c, W=1, W=2, W=3, and W=e
+# They are independent, and can be combined like W=12 or W=123e,
+# except that W=1 implies W=c.
 # ==========================================================================
 
 # Default set of warnings, always enabled
@@ -109,6 +110,13 @@ KBUILD_CFLAGS += $(call cc-option,-Wenum-conversion)
 
 KBUILD_CFLAGS += -Wunused
 
+#
+# W=c - Expensive compile-time checks, implied by W=1
+#
+ifneq ($(findstring c, $(KBUILD_EXTRA_WARN))$(findstring 1, $(KBUILD_EXTRA_WARN)),)
+KBUILD_CPPFLAGS += -DKBUILD_EXTRA_WARNc
+endif
+
 #
 # W=1 - warnings which may be relevant and do not occur too often
 #
-- 
2.39.5