kbuild: handle multi-objs dependency appropriately

The comment in scripts/Makefile.build says as follows:

  We would rather have a list of rules like
        foo.o: $(foo-objs)
  but that's not so easy, so we rather make all composite objects depend
  on the set of all their parts

This commit makes it possible!

For example, assume a Makefile like this

  obj-m = foo.o bar.o
  foo-objs := foo1.o foo2.o
  bar-objs := bar1.o bar2.o

Without this patch, foo.o depends on all of
foo1.o foo2.o bar1.o bar2.o.
It looks funny that foo.o is regenerated when bar1.c is updated.

Now we can handle the dependency of foo.o and bar.o separately.
Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>

scripts/Makefile.build

@@ -382,16 +382,14 @@ cmd_link_multi-y = $(LD) $(ld_flags) -r -o $@ $(link_multi_deps) $(cmd_secanalys
 quiet_cmd_link_multi-m = LD [M]  $@
 cmd_link_multi-m = $(cmd_link_multi-y)
 
-# We would rather have a list of rules like
-# 	foo.o: $(foo-objs)
-# but that's not so easy, so we rather make all composite objects depend
-# on the set of all their parts
-$(multi-used-y) : %.o: $(multi-objs-y) FORCE
+$(multi-used-y): FORCE
 	$(call if_changed,link_multi-y)
+$(call multi_depend, $(multi-used-y), .o, -objs -y)
 
-$(multi-used-m) : %.o: $(multi-objs-m) FORCE
+$(multi-used-m): FORCE
 	$(call if_changed,link_multi-m)
 	@{ echo $(@:.o=.ko); echo $(link_multi_deps); } > $(MODVERDIR)/$(@F:.o=.mod)
+$(call multi_depend, $(multi-used-m), .o, -objs -y)
 
 targets += $(multi-used-y) $(multi-used-m)
 

scripts/Makefile.lib

@@ -159,6 +159,15 @@ dtc_cpp_flags  = -Wp,-MD,$(depfile).pre.tmp -nostdinc                    \
 modname-multi = $(sort $(foreach m,$(multi-used),\
 		$(if $(filter $(subst $(obj)/,,$*.o), $($(m:.o=-objs)) $($(m:.o=-y))),$(m:.o=))))
 
+# Useful for describing the dependency of composite objects
+# Usage:
+#   $(call multi_depend, multi_used_targets, suffix_to_remove, suffix_to_add)
+define multi_depend
+$(foreach m, $(notdir $1), \
+	$(eval $(obj)/$m: \
+	$(addprefix $(obj)/, $(foreach s, $3, $($(m:%$(strip $2)=%$(s)))))))
+endef
+
 ifdef REGENERATE_PARSERS
 
 # GPERF