kbuild: fix and refactor single target build

The single target build has a subtle bug for the combination for
an individual file and a subdirectory.

[1] 'make kernel/fork.i' builds only kernel/fork.i

  $ make kernel/fork.i
    CALL    scripts/checksyscalls.sh
    DESCEND objtool
    CPP     kernel/fork.i

[2] 'make kernel/' builds only under the kernel/ directory.

  $ make kernel/
    CALL    scripts/checksyscalls.sh
    DESCEND objtool
    CC      kernel/fork.o
    CC      kernel/exec_domain.o
       [snip]
    CC      kernel/rseq.o
    AR      kernel/built-in.a

But, if you try to do [1] and [2] in a single command, you will get
only [1] with a weird log:

  $ make kernel/fork.i kernel/
    CALL    scripts/checksyscalls.sh
    DESCEND objtool
    CPP     kernel/fork.i
  make[2]: Nothing to be done for 'kernel/'.

With 'make kernel/fork.i kernel/', you should get both [1] and [2].

Rewrite the single target build.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Makefile

@@ -1819,11 +1819,11 @@ single_modpost: $(single-no-ko) modules_prepare
 	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.modpost
 	$(Q)rm -f $(MODORDER)
 
-export KBUILD_SINGLE_TARGETS := $(addprefix $(extmod_prefix), $(single-no-ko))
+single-goals := $(addprefix $(extmod_prefix), $(single-no-ko))
 
 # trim unrelated directories
 build-dirs := $(foreach d, $(build-dirs), \
-			$(if $(filter $(d)/%, $(KBUILD_SINGLE_TARGETS)), $(d)))
+			$(if $(filter $d/%, $(single-goals)), $d))
 
 endif
 
@@ -1835,9 +1835,8 @@ endif
 PHONY += descend $(build-dirs)
 descend: $(build-dirs)
 $(build-dirs): prepare
-	$(Q)$(MAKE) $(build)=$@ \
-	single-build=$(if $(filter-out $@/, $(filter $@/%, $(KBUILD_SINGLE_TARGETS))),1) \
-	need-builtin=1 need-modorder=1
+	$(Q)$(MAKE) $(build)=$@ need-builtin=1 need-modorder=1 \
+	$(filter $@/%, $(single-goals))
 
 clean-dirs := $(addprefix _clean_, $(clean-dirs))
 PHONY += $(clean-dirs) clean

scripts/Makefile.build

@@ -5,8 +5,8 @@
 
 src := $(obj)
 
-PHONY := __build
-__build:
+PHONY := $(obj)/
+$(obj)/:
 
 # Init all relevant variables used in kbuild files so
 # 1) they have correct type
@@ -323,7 +323,7 @@ $(obj)/%.o: $(src)/%.S FORCE
 
 targets += $(filter-out $(subdir-builtin), $(real-obj-y))
 targets += $(filter-out $(subdir-modorder), $(real-obj-m))
-targets += $(real-dtb-y) $(lib-y) $(always-y) $(MAKECMDGOALS)
+targets += $(real-dtb-y) $(lib-y) $(always-y)
 
 # Linker scripts preprocessor (.lds.S -> .lds)
 # ---------------------------------------------------------------------------
@@ -400,8 +400,6 @@ $(multi-obj-m): %.o: %.mod FORCE
 	$(call if_changed_rule,ld_multi_m)
 $(call multi_depend, $(multi-obj-m), .o, -objs -y -m)
 
-targets := $(filter-out $(PHONY), $(targets))
-
 # Add intermediate targets:
 # When building objects with specific suffix patterns, add intermediate
 # targets that the final targets are derived from.
@@ -420,52 +418,29 @@ targets += $(call intermediate_targets, .asn1.o, .asn1.c .asn1.h) \
 # Build
 # ---------------------------------------------------------------------------
 
-ifdef single-build
-
-KBUILD_SINGLE_TARGETS := $(filter $(obj)/%, $(KBUILD_SINGLE_TARGETS))
-
-curdir-single := $(sort $(foreach x, $(KBUILD_SINGLE_TARGETS), \
-			$(if $(filter $(x) $(basename $(x)).o, $(targets)), $(x))))
-
-# Handle single targets without any rule: show "Nothing to be done for ..." or
-# "No rule to make target ..." depending on whether the target exists.
-unknown-single := $(filter-out $(addsuffix /%, $(subdir-ym)), \
-			$(filter-out $(curdir-single), $(KBUILD_SINGLE_TARGETS)))
-
-single-subdirs := $(foreach d, $(subdir-ym), \
-			$(if $(filter $(d)/%, $(KBUILD_SINGLE_TARGETS)), $(d)))
-
-__build: $(curdir-single) $(single-subdirs)
-ifneq ($(unknown-single),)
-	$(Q)$(MAKE) -f /dev/null $(unknown-single)
-endif
+$(obj)/: $(if $(KBUILD_BUILTIN), $(targets-for-builtin)) \
+	 $(if $(KBUILD_MODULES), $(targets-for-modules)) \
+	 $(subdir-ym) $(always-y)
 	@:
 
-ifeq ($(curdir-single),)
-# Nothing to do in this directory. Do not include any .*.cmd file for speed-up
-targets :=
-else
-targets += $(curdir-single)
-endif
+# Single targets
+# ---------------------------------------------------------------------------
 
-else
+single-subdirs := $(foreach d, $(subdir-ym), $(if $(filter $d/%, $(MAKECMDGOALS)), $d))
+single-subdir-goals := $(filter $(addsuffix /%, $(single-subdirs)), $(MAKECMDGOALS))
 
-__build: $(if $(KBUILD_BUILTIN), $(targets-for-builtin)) \
-	 $(if $(KBUILD_MODULES), $(targets-for-modules)) \
-	 $(subdir-ym) $(always-y)
+$(single-subdir-goals): $(single-subdirs)
 	@:
 
-endif
-
 # Descending
 # ---------------------------------------------------------------------------
 
 PHONY += $(subdir-ym)
 $(subdir-ym):
 	$(Q)$(MAKE) $(build)=$@ \
-	$(if $(filter $@/, $(KBUILD_SINGLE_TARGETS)),single-build=) \
 	need-builtin=$(if $(filter $@/built-in.a, $(subdir-builtin)),1) \
-	need-modorder=$(if $(filter $@/modules.order, $(subdir-modorder)),1)
+	need-modorder=$(if $(filter $@/modules.order, $(subdir-modorder)),1) \
+	$(filter $@/%, $(single-subdir-goals))
 
 # Add FORCE to the prequisites of a target to force it to be always rebuilt.
 # ---------------------------------------------------------------------------
@@ -474,6 +449,9 @@ PHONY += FORCE
 
 FORCE:
 
+targets += $(filter-out $(single-subdir-goals), $(MAKECMDGOALS))
+targets := $(filter-out $(PHONY), $(targets))
+
 # Read all saved command lines and dependencies for the $(targets) we
 # may be building above, using $(if_changed{,_dep}). As an
 # optimization, we don't need to read them if the target does not