"obj-y += foo/" syntax requires Kbuild to visit the "foo" subdirectory
and link built-in.o from that directory. This means foo/Makefile is
responsible for creating built-in.o even if there is no object to
link (in this case, built-in.o is an empty archive).
We have had several fixups like commit 4b024242e8a4 ("kbuild: Fix
linking error built-in.o no such file or directory"), then ended up
with a complex condition as follows:
ifneq ($(strip $(obj-y) $(obj-m) $(obj-) $(subdir-m) $(lib-target)),)
builtin-target := $(obj)/built-in.o
endif
We still have more cases not covered by the above, so we need to add
obj- := dummy.o
in several places just for creating empty built-in.o.
A key point is, the parent Makefile knows whether built-in.o is needed
or not. If a subdirectory needs to create built-in.o, its parent can
tell the fact when Kbuild descends into it.
If non-empty $(need-builtin) flag is passed from the parent, built-in.o
should be created. $(obj-y) should be still checked to support the
single target "%/". All of ugly tricks will go away.
Signed-off-by: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
---
Makefile | 2 +-
scripts/Makefile.build | 4 ++--
2 files changed, 3 insertions(+), 3 deletions(-)
diff --git a/Makefile b/Makefile
index 008a4e5..cc0b618 100644
--- a/Makefile
+++ b/Makefile
@@ -1003,7 +1003,7 @@ $(sort $(vmlinux-deps)): $(vmlinux-dirs) ;
PHONY += $(vmlinux-dirs)
$(vmlinux-dirs): prepare scripts
- $(Q)$(MAKE) $(build)=$@
+ $(Q)$(MAKE) $(build)=$@ need-builtin=1
define filechk_kernel.release
echo "$(KERNELVERSION)$$($(CONFIG_SHELL) $(srctree)/scripts/setlocalversion $(srctree))"
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 061d0c3..e1c6efd 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -84,7 +84,7 @@ lib-target := $(obj)/lib.a
obj-y += $(obj)/lib-ksyms.o
endif
-ifneq ($(strip $(obj-y) $(obj-m) $(obj-) $(subdir-m) $(lib-target)),)
+ifneq ($(strip $(obj-y) $(need-builtin)),)
builtin-target := $(obj)/built-in.o
endif
@@ -569,7 +569,7 @@ targets += $(multi-used-y) $(multi-used-m)
PHONY += $(subdir-ym)
$(subdir-ym):
- $(Q)$(MAKE) $(build)=$@
+ $(Q)$(MAKE) $(build)=$@ need-builtin=$(if $(findstring $@,$(subdir-obj-y)),1)
# Add FORCE to the prequisites of a target to force it to be always rebuilt.
# ---------------------------------------------------------------------------
--
2.7.4
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