kbuild: split out *.mod out of {single,multi}-used-m rules

Currently, *.mod is created as a side-effect of obj-m.

Split out *.mod as a dedicated build rule, which allows to unify
the %.c -> %.o rule, and remove the single-used-m rule.

This also makes the incremental build of allmodconfig faster because
it saves $(NM) invocation when there is no change in the module.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/Makefile.build

@@ -67,8 +67,10 @@ ifeq ($(CONFIG_MODULES)$(need-modorder),y1)
 modorder-target := $(obj)/modules.order
 endif
 
+mod-targets := $(patsubst %.o, %.mod, $(obj-m))
+
 __build: $(if $(KBUILD_BUILTIN),$(builtin-target) $(lib-target) $(extra-y)) \
-	 $(if $(KBUILD_MODULES),$(obj-m) $(modorder-target)) \
+	 $(if $(KBUILD_MODULES),$(obj-m) $(mod-targets) $(modorder-target)) \
 	 $(subdir-ym) $(always)
 	@:
 
@@ -261,7 +263,7 @@ endef
 
 # List module undefined symbols (or empty line if not enabled)
 ifdef CONFIG_TRIM_UNUSED_KSYMS
-cmd_undef_syms = $(NM) $@ | sed -n 's/^  *U //p' | xargs echo
+cmd_undef_syms = $(NM) $< | sed -n 's/^  *U //p' | xargs echo
 else
 cmd_undef_syms = echo
 endif
@@ -271,11 +273,15 @@ $(obj)/%.o: $(src)/%.c $(recordmcount_source) $(objtool_dep) FORCE
 	$(call cmd,force_checksrc)
 	$(call if_changed_rule,cc_o_c)
 
-$(single-used-m): $(obj)/%.o: $(src)/%.c $(recordmcount_source) $(objtool_dep) FORCE
-	$(call cmd,force_checksrc)
-	$(call if_changed_rule,cc_o_c)
-	@{ echo $@; \
-	   $(cmd_undef_syms); } > $(patsubst %.o,%.mod,$@)
+cmd_mod = { \
+	echo $(if $($*-objs)$($*-y)$($*-m), $(addprefix $(obj)/, $($*-objs) $($*-y) $($*-m)), $(@:.mod=.o)); \
+	$(cmd_undef_syms); \
+	} > $@
+
+$(obj)/%.mod: $(obj)/%.o FORCE
+	$(call if_changed,mod)
+
+targets += $(mod-targets)
 
 quiet_cmd_cc_lst_c = MKLST   $@
       cmd_cc_lst_c = $(CC) $(c_flags) -g -c -o $*.o $< && \
@@ -456,8 +462,6 @@ quiet_cmd_link_multi-m = LD [M]  $@
 
 $(multi-used-m): FORCE
 	$(call if_changed,link_multi-m)
-	@{ echo $(filter %.o,$^); \
-	   $(cmd_undef_syms); } > $(patsubst %.o,%.mod,$@)
 $(call multi_depend, $(multi-used-m), .o, -objs -y -m)
 
 targets += $(multi-used-m)