kbuild: link $(real-obj-y) instead of $(obj-y) into built-in.a

In Kbuild, Makefiles can add the same object to obj-y multiple
times. So,

obj-y += foo.o
obj-y += foo.o

is fine.

However, this is not true when the same object is added multiple
times via composite objects. For example,

obj-y += foo.o bar.o
foo-objs := foo-bar-common.o foo-only.o
bar-objs := foo-bar-common.o bar-only.o

causes build error because two instances of foo-bar-common.o are
linked into the vmlinux.

Makefiles tend to invent ugly work-around, for example
- lib/zstd/Makefile
- drivers/net/ethernet/cavium/liquidio/Makefile

The technique used in Kbuild to avoid the multiple definition error
is to use $(filter $(obj-y), $^). Here, $^ lists the names of all
the prerequisites with duplicated names removed.

By replacing it with $(filter $(real-obj-y), $^) we can do likewise
for composite objects. For built-in objects, we do not need to keep
the composite object structure. We can simply expand them, and link
$(real-obj-y) to built-in.a.

Signed-off-by: Masahiro Yamada <[e-mail]>

scripts/Makefile.build

···
 
 ifneq ($(strip $(lib-y) $(lib-m) $(lib-)),)
 lib-target := $(obj)/lib.a
-obj-y += $(obj)/lib-ksyms.o
+real-obj-y += $(obj)/lib-ksyms.o
 endif
 
-ifneq ($(strip $(obj-y) $(need-builtin)),)
+ifneq ($(strip $(real-obj-y) $(need-builtin)),)
 builtin-target := $(obj)/built-in.a
 endif
 
···
 $(obj)/%.o: $(src)/%.S $(objtool_dep) FORCE
 	$(call if_changed_rule,as_o_S)
 
-targets += $(real-obj-y) $(real-obj-m) $(lib-y)
+targets += $(filter-out $(subdir-obj-y), $(real-obj-y)) $(real-obj-m) $(lib-y)
 targets += $(extra-y) $(MAKECMDGOALS) $(always)
 
 # Linker scripts preprocessor (.lds.S -> .lds)
···
 quiet_cmd_link_o_target = AR      $@
 
 # If the list of objects to link is empty, just create an empty built-in.a
-cmd_link_o_target = $(if $(strip $(obj-y)),\
-		      $(cmd_make_builtin) $@ $(filter $(obj-y), $^) \
+cmd_link_o_target = $(if $(strip $(real-obj-y)),\
+		      $(cmd_make_builtin) $@ $(filter $(real-obj-y), $^) \
 		      $(cmd_secanalysis),\
 		      $(cmd_make_empty_builtin) $@)
 
-$(builtin-target): $(obj-y) FORCE
+$(builtin-target): $(real-obj-y) FORCE
 	$(call if_changed,link_o_target)
 
 targets += $(builtin-target)
···
 
 cmd_link_multi-link = $(LD) $(ld_flags) -r -o $@ $(link_multi_deps) $(cmd_secanalysis)
 
-quiet_cmd_link_multi-y = AR      $@
-cmd_link_multi-y = rm -f $@; $(AR) rcSTP$(KBUILD_ARFLAGS) $@ $(link_multi_deps)
-
 quiet_cmd_link_multi-m = LD [M]  $@
 cmd_link_multi-m = $(cmd_link_multi-link)
-
-$(multi-used-y): FORCE
-	$(call if_changed,link_multi-y)
-$(call multi_depend, $(multi-used-y), .o, -objs -y)
 
 $(multi-used-m): FORCE
 	$(call if_changed,link_multi-m)
···
 	   $(cmd_undef_syms); } > $(MODVERDIR)/$(@F:.o=.mod)
 $(call multi_depend, $(multi-used-m), .o, -objs -y -m)
 
-targets += $(multi-used-y) $(multi-used-m)
+targets += $(multi-used-m)
 targets := $(filter-out $(PHONY), $(targets))
 
 # Descending

scripts/Makefile.lib

···
 # tell kbuild to descend
 subdir-obj-y := $(filter %/built-in.a, $(obj-y))
 
-# Replace multi-part objects by their individual parts, look at local dir only
-real-obj-y := $(foreach m, $(filter-out $(subdir-obj-y), $(obj-y)), $(if $(strip $($(m:.o=-objs)) $($(m:.o=-y))),$($(m:.o=-objs)) $($(m:.o=-y)),$(m)))
+# Replace multi-part objects by their individual parts,
+# including built-in.a from subdirectories
+real-obj-y := $(foreach m, $(obj-y), $(if $(strip $($(m:.o=-objs)) $($(m:.o=-y))),$($(m:.o=-objs)) $($(m:.o=-y)),$(m)))
 real-obj-m := $(foreach m, $(obj-m), $(if $(strip $($(m:.o=-objs)) $($(m:.o=-y)) $($(m:.o=-m))),$($(m:.o=-objs)) $($(m:.o=-y)) $($(m:.o=-m)),$(m)))
 
 # DTB
···
 always		:= $(addprefix $(obj)/,$(always))
 targets		:= $(addprefix $(obj)/,$(targets))
 modorder	:= $(addprefix $(obj)/,$(modorder))
-obj-y		:= $(addprefix $(obj)/,$(obj-y))
 obj-m		:= $(addprefix $(obj)/,$(obj-m))
 lib-y		:= $(addprefix $(obj)/,$(lib-y))
 subdir-obj-y	:= $(addprefix $(obj)/,$(subdir-obj-y))
 real-obj-y	:= $(addprefix $(obj)/,$(real-obj-y))
 real-obj-m	:= $(addprefix $(obj)/,$(real-obj-m))
 single-used-m	:= $(addprefix $(obj)/,$(single-used-m))
-multi-used-y	:= $(addprefix $(obj)/,$(multi-used-y))
 multi-used-m	:= $(addprefix $(obj)/,$(multi-used-m))
 subdir-ym	:= $(addprefix $(obj)/,$(subdir-ym))