kbuild: enable 'make CFLAGS=...' to add additional options to CC

The variable CFLAGS is a wellknown variable and the usage by
kbuild may result in unexpected behaviour.
On top of that several people over time has asked for a way to
pass in additional flags to gcc.

This patch replace use of CFLAGS with KBUILD_CFLAGS all over the
tree and enabling one to use:
make CFLAGS=...
to specify additional gcc commandline options.

One usecase is when trying to find gcc bugs but other
use cases has been requested too.

Patch was tested on following architectures:
alpha, arm, i386, x86_64, mips, sparc, sparc64, ia64, m68k

Test was simple to do a defconfig build, apply the patch and check
that nothing got rebuild.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

1 files changed, 9 insertions, 6 deletions

diff --git a/Documentation/kbuild/makefiles.txt b/Documentation/kbuild/makefiles.txt
index 54bbfd37f15..1ca535bcc8c 100644
--- a/Documentation/kbuild/makefiles.txt
+++ b/Documentation/kbuild/makefiles.txt
@@ -294,7 +294,7 @@ more details, with real examples.
 
 
         This variable is necessary because the top Makefile owns the
-        variable $(CFLAGS) and uses it for compilation flags for the
+        variable $(KBUILD_CFLAGS) and uses it for compilation flags for the
         entire tree.
 
         $(EXTRA_AFLAGS) is a similar string for per-directory options
@@ -437,6 +437,7 @@ more details, with real examples.
         -march=pentium-mmx if supported by $(CC), otherwise -march=i586.
         The second argument to cc-option is optional, and if omitted,
         cflags-y will be assigned no value if first option is not supported.
+        Note: cc-option uses KBUILD_CFLAGS for $(CC) options
 
    cc-option-yn
         cc-option-yn is used to check if gcc supports a given option
@@ -452,6 +453,7 @@ more details, with real examples.
         option. When $(biarch) equals 'y', the expanded variables $(aflags-y)
         and $(cflags-y) will be assigned the values -a32 and -m32,
         respectively.
+        Note: cc-option-yn uses KBUILD_CFLAGS for $(CC) options
 
     cc-option-align
         gcc versions >= 3.0 changed the type of options used to specify
@@ -463,10 +465,11 @@ more details, with real examples.
                 cc-option-align = -falign
 
         Example:
-                CFLAGS += $(cc-option-align)-functions=4
+                KBUILD_CFLAGS += $(cc-option-align)-functions=4
 
         In the above example, the option -falign-functions=4 is used for
         gcc >= 3.00. For gcc < 3.00, -malign-functions=4 is used.
+        Note: cc-option-align uses KBUILD_CFLAGS for $(CC) options
 
     cc-version
         cc-version returns a numerical version of the $(CC) compiler version.
@@ -825,17 +828,17 @@ When kbuild executes, the following steps are followed (roughly):
                 #arch/sparc64/Makefile
                 AFLAGS += -m64 -mcpu=ultrasparc
 
-    CFLAGS              $(CC) compiler flags
+    KBUILD_CFLAGS               $(CC) compiler flags
 
         Default value - see top level Makefile
         Append or modify as required per architecture.
 
-        Often, the CFLAGS variable depends on the configuration.
+        Often, the KBUILD_CFLAGS variable depends on the configuration.
 
         Example:
                 #arch/i386/Makefile
                 cflags-$(CONFIG_M386) += -march=i386
-                CFLAGS += $(cflags-y)
+                KBUILD_CFLAGS += $(cflags-y)
 
         Many arch Makefiles dynamically run the target C compiler to
         probe supported options:
@@ -847,7 +850,7 @@ When kbuild executes, the following steps are followed (roughly):
                                                 -march=pentium2,-march=i686)
                 ...
                 # Disable unit-at-a-time mode ...
-                CFLAGS += $(call cc-option,-fno-unit-at-a-time)
+                KBUILD_CFLAGS += $(call cc-option,-fno-unit-at-a-time)
                 ...