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

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
http://gcc.gnu.org/PR28322, 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 <sfr@canb.auug.org.au>
Signed-off-by: Michal Marek <mmarek@suse.cz>

1 files changed, 12 insertions, 0 deletions

diff --git a/Documentation/kbuild/makefiles.txt b/Documentation/kbuild/makefiles.txt
index 835b64acf0b4..47435e56c5da 100644
--- a/Documentation/kbuild/makefiles.txt
+++ b/Documentation/kbuild/makefiles.txt
@@ -501,6 +501,18 @@ 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
         cc-version returns a numerical version of the $(CC) compiler version.
         The format is <major><minor> where both are two digits. So for example