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kbuild: Fix passing -Wno-* options to gcc 4.4+

[ upstream commit 8417da6 ]

Starting with 4.4, gcc will happily accept -Wno-<anything> in the
cc-option test and complain later when compiling a file that has some
other warning. This rather unexpected behavior is intentional as per, so work around it by testing for support of
the opposite option (without the no-). Introduce a new Makefile function
cc-disable-warning that does this and update two uses of cc-option in
the toplevel Makefile.

Reported-and-tested-by: Stephen Rothwell <>
Signed-off-by: Michal Marek <>
Signed-off-by: Andi Kleen <>
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commit a3695ef071b35cb99753fce7f6e87ff17401ac50 1 parent 8cafb8f
Burst Lam burstlam authored committed
12 Documentation/kbuild/makefiles.txt
@@ -502,6 +502,18 @@ more details, with real examples.
gcc >= 3.00. For gcc < 3.00, -malign-functions=4 is used.
Note: cc-option-align uses KBUILD_CFLAGS for $(CC) options
+ cc-disable-warning
+ cc-disable-warning checks if gcc supports a given warning and returns
+ the commandline switch to disable it. This special function is needed,
+ because gcc 4.4 and later accept any unknown -Wno-* option and only
+ warn about it if there is another warning in the source file.
+ Example:
+ KBUILD_CFLAGS += $(call cc-disable-warning, unused-but-set-variable)
+ In the above example, -Wno-unused-but-set-variable will be added to
+ KBUILD_CFLAGS only if gcc really accepts it.
cc-version returns a numerical version of the $(CC) compiler version.
The format is <major><minor> where both are two digits. So for example
5 Makefile
@@ -547,6 +547,9 @@ ifndef CONFIG_CC_STACKPROTECTOR
KBUILD_CFLAGS += $(call cc-option, -fno-stack-protector)
+# This warning generated too much noise in a regular build.
+KBUILD_CFLAGS += $(call cc-disable-warning, unused-but-set-variable)
KBUILD_CFLAGS += -fno-omit-frame-pointer -fno-optimize-sibling-calls
@@ -575,7 +578,7 @@ CHECKFLAGS += $(NOSTDINC_FLAGS)
KBUILD_CFLAGS += $(call cc-option,-Wdeclaration-after-statement,)
# disable pointer signed / unsigned warnings in gcc 4.0
-KBUILD_CFLAGS += $(call cc-option,-Wno-pointer-sign,)
+KBUILD_CFLAGS += $(call cc-disable-warning, pointer-sign)
# disable invalid "can't wrap" optimizations for signed / pointers
KBUILD_CFLAGS += $(call cc-option,-fno-strict-overflow)
5 scripts/Kbuild.include
@@ -118,6 +118,11 @@ cc-option-yn = $(call try-run,\
cc-option-align = $(subst -functions=0,,\
$(call cc-option,-falign-functions=0,-malign-functions=0))
+# cc-disable-warning
+# Usage: cflags-y += $(call cc-disable-warning,unused-but-set-variable)
+cc-disable-warning = $(call try-run,\
+ $(CC) $(KBUILD_CPPFLAGS) $(KBUILD_CFLAGS) -W$(strip $(1)) -c -xc /dev/null -o "$$TMP",-Wno-$(strip $(1)))
# cc-version
# Usage gcc-ver := $(call cc-version)
cc-version = $(shell $(CONFIG_SHELL) $(srctree)/scripts/ $(CC))
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