[PATCH] kbuild: allow modules to link *.a archives

[Masahiro Yamada <masahiroy@kernel.org>]

Since commit 69ea912fda74 ("kbuild: remove unneeded link_multi_deps"),
modules cannot link *.a archives.

I do not see such a usecase in the upstream code, but multiple people
reported this issue, so it seems to be a desired feature for external
modules.

For example, libfoo.a is not linked in the following test case:

    obj-m := foo.o
    foo-objs := foo1.o libfoo.a

Instead of reverting the offending commit, I wrote slightly cleaner code.
I added multi-m-prereqs to avoid code duplication.

Reported-by: Fumiya Shigemitsu <shfy1014@gmail.com>
Reported-by: Yongxin Liu <yongxin.liu@windriver.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
---

 scripts/Makefile.build | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index b734ac8a654e..a8481b765ea8 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -265,8 +265,10 @@ $(obj)/%.o: $(src)/%.c $(recordmcount_source) $(objtool_dep) FORCE
 	$(call cmd,force_checksrc)
 	$(call if_changed_rule,cc_o_c)
 
+multi-m-prereqs = $(addprefix $(obj)/, $($*-objs) $($*-y) $($*-m))
+
 cmd_mod = { \
-	echo $(if $($*-objs)$($*-y)$($*-m), $(addprefix $(obj)/, $($*-objs) $($*-y) $($*-m)), $(@:.mod=.o)); \
+	echo $(if $(multi-m-prereqs), $(multi-m-prereqs), $(@:.mod=.o)); \
 	$(cmd_undef_syms); \
 	} > $@
 
@@ -427,13 +429,13 @@ targets += $(obj)/lib-ksyms.o
 endif
 
 # NOTE:
-# Do not replace $(filter %.o,^) with $(real-prereqs). When a single object
+# Do not replace $(multi-m-prereqs) with $(real-prereqs). When a single object
 # module is turned into a multi object module, $^ will contain header file
 # dependencies recorded in the .*.cmd file.
 quiet_cmd_link_multi-m = LD [M]  $@
-      cmd_link_multi-m = $(LD) $(ld_flags) -r -o $@ $(filter %.o,$^)
+      cmd_link_multi-m = $(LD) $(ld_flags) -r -o $@ $(multi-m-prereqs)
 
-$(multi-used-m): FORCE
+$(multi-used-m): $(obj)/%.o: FORCE
 	$(call if_changed,link_multi-m)
 $(call multi_depend, $(multi-used-m), .o, -objs -y -m)
 
-- 
2.17.1