kbuild: introduce cc-cross-prefix

cc-cross-prefix is useful for the architecture that like
to provide a default CROSS_COMPILE value,
but may have several to select between.

Sample usage:

ifneq ($(SUBARCH),$(ARCH))
        ifeq ($(CROSS_COMPILE),)
               CROSS_COMPILE := $(call cc-cross-prefix, m68k-linux-gnu- m68k-linux-)
        endif
endif

Actual usage by the different archs will taken care of later.

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

2 files changed, 33 insertions, 0 deletions

diff --git a/Documentation/kbuild/makefiles.txt b/Documentation/kbuild/makefiles.txt
index f099b814d383..6166e2d7da76 100644
--- a/Documentation/kbuild/makefiles.txt
+++ b/Documentation/kbuild/makefiles.txt
@@ -518,6 +518,28 @@ more details, with real examples.
         In this example for a specific GCC version the build will error out explaining
         to the user why it stops.
 
+    cc-cross-prefix
+        cc-cross-prefix is used to check if there exist a $(CC) in path with
+        one of the listed prefixes. The first prefix where there exist a
+        prefix$(CC) in the PATH is returned - and if no prefix$(CC) is found
+        then nothing is returned.
+        Additional prefixes are separated by a single space in the
+        call of cc-cross-prefix.
+        This functionality is usefull for architecture Makefile that try
+        to set CROSS_COMPILE to well know values but may have several
+        values to select between.
+        It is recommended only to try to set CROSS_COMPILE is it is a cross
+        build (host arch is different from target arch). And is CROSS_COMPILE
+        is already set then leave it with the old value.
+
+        Example:
+                #arch/m68k/Makefile
+                ifneq ($(SUBARCH),$(ARCH))
+                        ifeq ($(CROSS_COMPILE),)
+                               CROSS_COMPILE := $(call cc-cross-prefix, m68k-linux-gnu-)
+                        endif
+                endif
+
 === 4 Host Program support
 
 Kbuild supports building executables on the host for use during the
diff --git a/scripts/Kbuild.include b/scripts/Kbuild.include
index de7bb284c611..b96ea8d6a5ed 100644
--- a/scripts/Kbuild.include
+++ b/scripts/Kbuild.include
@@ -56,6 +56,17 @@ endef
 # gcc support functions
 # See documentation in Documentation/kbuild/makefiles.txt
 
+# cc-cross-prefix
+# Usage: CROSS_COMPILE := $(call cc-cross-prefix, m68k-linux-gnu- m68k-linux-)
+# Return first prefix where a prefix$(CC) is found in PATH.
+# If no $(CC) found in PATH with listed prefixes return nothing
+cc-cross-prefix =  \
+        $(word 1, $(foreach c,$(1),                                   \
+                $(shell set -e;                                       \
+                if (which $(strip $(c))$(CC)) > /dev/null 2>&1 ; then \
+                        echo $(c);                                    \
+                fi)))
+
 # output directory for tests below
 TMPOUT := $(if $(KBUILD_EXTMOD),$(firstword $(KBUILD_EXTMOD))/)