kbuild: make single target builds even faster

Commit 2dffd23f ("kbuild: make single target builds much faster")
made the situation much better.

To improve it even more, apply the similar idea to the top Makefile.
Trim unrelated directories from build-dirs.

The single build code must be moved above the 'descend' target.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: Jens Axboe <axboe@kernel.dk>

Makefile

@@ -1634,6 +1634,50 @@ help:
 PHONY += prepare
 endif # KBUILD_EXTMOD
 
+# Single targets
+# ---------------------------------------------------------------------------
+# To build individual files in subdirectories, you can do like this:
+#
+#   make foo/bar/baz.s
+#
+# The supported suffixes for single-target are listed in 'single-targets'
+#
+# To build only under specific subdirectories, you can do like this:
+#
+#   make foo/bar/baz/
+
+ifdef single-build
+
+# .ko is special because modpost is needed
+single-ko := $(sort $(filter %.ko, $(MAKECMDGOALS)))
+single-no-ko := $(sort $(patsubst %.ko,%.mod, $(MAKECMDGOALS)))
+
+$(single-ko): single_modpost
+	@:
+$(single-no-ko): descend
+	@:
+
+ifeq ($(KBUILD_EXTMOD),)
+# For the single build of in-tree modules, use a temporary file to avoid
+# the situation of modules_install installing an invalid modules.order.
+MODORDER := .modules.tmp
+endif
+
+PHONY += single_modpost
+single_modpost: $(single-no-ko)
+	$(Q){ $(foreach m, $(single-ko), echo $(extmod-prefix)$m;) } > $(MODORDER)
+	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.modpost
+
+KBUILD_MODULES := 1
+
+export KBUILD_SINGLE_TARGETS := $(addprefix $(extmod-prefix), $(single-no-ko))
+
+# trim unrelated directories
+build-dirs := $(foreach d, $(build-dirs), \
+			$(if $(filter $(d)/%, $(KBUILD_SINGLE_TARGETS)), $(d)))
+
+endif
+
 # Handle descending into subdirectories listed in $(build-dirs)
 # Preset locale variables to speed up the build process. Limit locale
 # tweaks to this spot to avoid wrong language settings when running
@@ -1642,7 +1686,9 @@ endif # KBUILD_EXTMOD
 PHONY += descend $(build-dirs)
 descend: $(build-dirs)
 $(build-dirs): prepare
-	$(Q)$(MAKE) $(build)=$@ single-build=$(single-build) need-builtin=1 need-modorder=1
+	$(Q)$(MAKE) $(build)=$@ \
+	single-build=$(if $(filter-out $@/, $(single-no-ko)),1) \
+	need-builtin=1 need-modorder=1
 
 clean-dirs := $(addprefix _clean_, $(clean-dirs))
 PHONY += $(clean-dirs) clean
@@ -1745,48 +1791,6 @@ tools/%: FORCE
 	$(Q)mkdir -p $(objtree)/tools
 	$(Q)$(MAKE) LDFLAGS= MAKEFLAGS="$(tools_silent) $(filter --j% -j,$(MAKEFLAGS))" O=$(abspath $(objtree)) subdir=tools -C $(srctree)/tools/ $*
 
-# Single targets
-# ---------------------------------------------------------------------------
-# To build individual files in subdirectories, you can do like this:
-#
-#   make foo/bar/baz.s
-#
-# The supported suffixes for single-target are listed in 'single-targets'
-#
-# To build only under specific subdirectories, you can do like this:
-#
-#   make foo/bar/baz/
-
-ifdef single-build
-
-# .ko is special because modpost is needed
-single-ko := $(sort $(filter %.ko, $(MAKECMDGOALS)))
-single-no-ko := $(sort $(patsubst %.ko,%.mod, $(MAKECMDGOALS)))
-
-$(single-ko): single_modpost
-	@:
-$(single-no-ko): descend
-	@:
-
-ifeq ($(KBUILD_EXTMOD),)
-# For the single build of in-tree modules, use a temporary file to avoid
-# the situation of modules_install installing an invalid modules.order.
-MODORDER := .modules.tmp
-endif
-
-PHONY += single_modpost
-single_modpost: $(single-no-ko)
-	$(Q){ $(foreach m, $(single-ko), echo $(extmod-prefix)$m;) } > $(MODORDER)
-	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.modpost
-
-KBUILD_MODULES := 1
-
-export KBUILD_SINGLE_TARGETS := $(addprefix $(extmod-prefix), $(single-no-ko))
-
-single-build = $(if $(filter-out $@/, $(single-no-ko)),1)
-
-endif
-
 # FIXME Should go into a make.lib or something
 # ===========================================================================