where is the rule to build a $(obj)/subdir target in linux kernel makefile? - makefile

Question

In linux kernel Makefile.build:
`
# To build objects in subdirs, we need to descend into the directories
$(subdir-builtin): $(obj)/%/built-in.a: $(obj)/% ;
$(subdir-modorder): $(obj)/%/modules.order: $(obj)/% ;`
the $(obj)/subdir/built-in.a depends on $(obj)/subdir prereq, but where is the rule to build $(obj)/subdir?
I assume the following rule is only for $(obj)/ directory, and can't apply for the above subdir.
`
# Build
# ---------------------------------------------------------------------------
$(obj)/: $(if $(KBUILD_BUILTIN), $(targets-for-builtin)) \
$(if $(KBUILD_MODULES), $(targets-for-modules)) \
$(subdir-ym) $(always-y)
#:`
Thanks!
I have looked up the makefile, but have not found any clue.

Answer 1

I already got the answer from a Linux kernel maintainer, as below:
See around line 500.
$(subdir-ym):
$(Q)$(MAKE) $(build)=$# \
need-builtin=$(if $(filter $#/built-in.a, $(subdir-builtin)),1) \
need-modorder=$(if $(filter $#/modules.order, $(subdir-modorder)),1) \
$(filter $#/%, $(single-subdir-goals))