kbuild: refactor Makefile.asm-generic

- Use conventional $(MAKE) $(asm-generic)=<dir> style
   for directory descending

 - Remove unneeded FORCE since "all" is a phony target

 - Remove unneeded "_dummy :=" assignment

 - Skip $(shell mkdir ...) when headers exist in the directory

 - Misc cleanups

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>

Makefile

@@ -1119,13 +1119,13 @@ prepare0: archprepare
 prepare: prepare0 prepare-objtool
 
 # Support for using generic headers in asm-generic
+asm-generic := -f $(srctree)/scripts/Makefile.asm-generic obj
+
 PHONY += asm-generic uapi-asm-generic
 asm-generic: uapi-asm-generic
-	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.asm-generic \
-	            src=asm obj=arch/$(SRCARCH)/include/generated/asm
+	$(Q)$(MAKE) $(asm-generic)=arch/$(SRCARCH)/include/generated/asm
 uapi-asm-generic:
-	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.asm-generic \
-	            src=uapi/asm obj=arch/$(SRCARCH)/include/generated/uapi/asm
+	$(Q)$(MAKE) $(asm-generic)=arch/$(SRCARCH)/include/generated/uapi/asm
 
 PHONY += prepare-objtool
 prepare-objtool: $(objtool_target)

scripts/Makefile.asm-generic

@@ -2,41 +2,42 @@
 # include/asm-generic contains a lot of files that are used
 # verbatim by several architectures.
 #
-# This Makefile reads the file arch/$(SRCARCH)/include/$(src)/Kbuild
+# This Makefile reads the file arch/$(SRCARCH)/include/(uapi/)/asm/Kbuild
 # and for each file listed in this file with generic-y creates
-# a small wrapper file in $(obj) (arch/$(SRCARCH)/include/generated/$(src))
+# a small wrapper file in arch/$(SRCARCH)/include/generated/(uapi/)/asm.
 
 PHONY := all
 all:
 
-kbuild-file := $(srctree)/arch/$(SRCARCH)/include/$(src)/Kbuild
--include $(kbuild-file)
+src := $(subst /generated,,$(obj))
+-include $(src)/Kbuild
 
 include scripts/Kbuild.include
 
-# Create output directory if not already present
-_dummy := $(shell [ -d $(obj) ] || mkdir -p $(obj))
+generic-y   := $(addprefix $(obj)/, $(generic-y))
+generated-y := $(addprefix $(obj)/, $(generated-y))
 
-# Stale wrappers when the corresponding files are removed from generic-y
-# need removing.
-generated-y   := $(generic-y) $(generated-y)
-all-files     := $(patsubst %, $(obj)/%, $(generated-y))
-old-headers   := $(wildcard $(obj)/*.h)
-unwanted      := $(filter-out $(all-files),$(old-headers))
+# Remove stale wrappers when the corresponding files are removed from generic-y
+old-headers := $(wildcard $(obj)/*.h)
+unwanted    := $(filter-out $(generic-y) $(generated-y),$(old-headers))
 
 quiet_cmd_wrap = WRAP    $@
-cmd_wrap = echo "\#include <asm-generic/$*.h>" >$@
+      cmd_wrap = echo "\#include <asm-generic/$*.h>" > $@
 
 quiet_cmd_remove = REMOVE  $(unwanted)
-cmd_remove = rm -f $(unwanted)
+      cmd_remove = rm -f $(unwanted)
 
-all: $(patsubst %, $(obj)/%, $(generic-y)) FORCE
-	$(if $(unwanted),$(call cmd,remove),)
+all: $(generic-y)
+	$(if $(unwanted),$(call cmd,remove))
 	@:
 
 $(obj)/%.h:
 	$(call cmd,wrap)
 
-PHONY += FORCE
+# Create output directory. Skip it if at least one old header exists
+# since we know the output directory already exists.
+ifeq ($(old-headers),)
+$(shell mkdir -p $(obj))
+endif
+
 .PHONY: $(PHONY)
-FORCE: ;