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>

1 files changed, 9 insertions, 0 deletions

diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index 934ee07809..57426d0302 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -162,6 +162,15 @@ dtc_cpp_flags  = -Wp,-MD,$(depfile).pre -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
+
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