kbuild: update modules.order only when contained modules are updated

Make modules.order depend on $(obj-m), and use if_changed to build it.
This will avoid unneeded update of modules.order, which will be useful
to optimize the modpost stage.

Currently, the second pass of modpost is always invoked. By checking the
timestamp of modules.order, we can avoid the unneeded modpost.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Makefile

@@ -1066,6 +1066,10 @@ vmlinux-alldirs	:= $(sort $(vmlinux-dirs) Documentation \
 		     $(patsubst %/,%,$(filter %/, $(core-) \
 			$(drivers-) $(libs-))))
 
+subdir-modorder := $(addsuffix modules.order,$(filter %/, \
+			$(core-y) $(core-m) $(libs-y) $(libs-m) \
+			$(drivers-y) $(drivers-m)))
+
 build-dirs	:= $(vmlinux-dirs)
 clean-dirs	:= $(vmlinux-alldirs)
 
@@ -1124,7 +1128,7 @@ targets := vmlinux
 
 # The actual objects are generated when descending,
 # make sure no implicit rule kicks in
-$(sort $(vmlinux-deps)): descend ;
+$(sort $(vmlinux-deps) $(subdir-modorder)): descend ;
 
 filechk_kernel.release = \
 	echo "$(KERNELVERSION)$$($(CONFIG_SHELL) $(srctree)/scripts/setlocalversion $(srctree))"
@@ -1345,8 +1349,12 @@ PHONY += modules_check
 modules_check: modules.order
 	$(Q)$(CONFIG_SHELL) $(srctree)/scripts/modules-check.sh $<
 
-modules.order: descend
-	$(Q)$(AWK) '!x[$$0]++' $(addsuffix /$@, $(build-dirs)) > $@
+cmd_modules_order = $(AWK) '!x[$$0]++' $(real-prereqs) > $@
+
+modules.order: $(subdir-modorder) FORCE
+	$(call if_changed,modules_order)
+
+targets += modules.order
 
 # Target to prepare building external modules
 PHONY += modules_prepare

scripts/Makefile.build

@@ -71,7 +71,7 @@ endif
 
 # subdir-builtin and subdir-modorder may contain duplications. Use $(sort ...)
 subdir-builtin := $(sort $(filter %/built-in.a, $(real-obj-y)))
-subdir-modorder := $(sort $(filter %/modules.order, $(modorder)))
+subdir-modorder := $(sort $(filter %/modules.order, $(obj-m)))
 
 targets-for-builtin := $(extra-y)
 
@@ -83,8 +83,7 @@ ifdef need-builtin
 targets-for-builtin += $(obj)/built-in.a
 endif
 
-targets-for-modules := $(obj-m)
-targets-for-modules += $(patsubst %.o, %.mod, $(obj-m))
+targets-for-modules := $(patsubst %.o, %.mod, $(filter %.o, $(obj-m)))
 
 ifdef need-modorder
 targets-for-modules += $(obj)/modules.order
@@ -361,8 +360,9 @@ endif
 $(obj)/%.o: $(src)/%.S $(objtool_dep) FORCE
 	$(call if_changed_rule,as_o_S)
 
-targets += $(filter-out $(subdir-builtin), $(real-obj-y)) $(real-obj-m) $(lib-y)
-targets += $(always-y) $(MAKECMDGOALS)
+targets += $(filter-out $(subdir-builtin), $(real-obj-y))
+targets += $(filter-out $(subdir-modorder), $(real-obj-m))
+targets += $(lib-y) $(always-y) $(MAKECMDGOALS)
 
 # Linker scripts preprocessor (.lds.S -> .lds)
 # ---------------------------------------------------------------------------
@@ -404,11 +404,16 @@ $(obj)/built-in.a: $(real-obj-y) FORCE
 #
 # Create commands to either record .ko file or cat modules.order from
 # a subdirectory
-$(obj)/modules.order: $(subdir-modorder) FORCE
-	$(Q){ $(foreach m, $(modorder), \
-	$(if $(filter $^, $m), cat $m, echo $m);) :; } \
+# Add $(obj-m) as the prerequisite to avoid updating the timestamp of
+# modules.order unless contained modules are updated.
+
+cmd_modules_order = { $(foreach m, $(real-prereqs), \
+	$(if $(filter %/modules.order, $m), cat $m, echo $(patsubst %.o,%.ko,$m));) :; } \
 	| $(AWK) '!x[$$0]++' - > $@
 
+$(obj)/modules.order: $(obj-m) FORCE
+	$(call if_changed,modules_order)
+
 #
 # Rule to compile a set of .o files into one .a file (with symbol table)
 #

scripts/Makefile.lib

@@ -32,27 +32,29 @@ obj-m := $(filter-out $(obj-y),$(obj-m))
 # Filter out objects already built-in
 lib-y := $(filter-out $(obj-y), $(sort $(lib-y) $(lib-m)))
 
-# Determine modorder.
-# Unfortunately, we don't have information about ordering between -y
-# and -m subdirs.  Just put -y's first.
-ifdef need-modorder
-modorder	:= $(patsubst %/,%/modules.order, $(filter %/, $(obj-y)) $(obj-m:.o=.ko))
-endif
-
 # Subdirectories we need to descend into
 subdir-ym := $(sort $(subdir-y) $(subdir-m) \
 			$(patsubst %/,%, $(filter %/, $(obj-y) $(obj-m))))
 
-# Handle objects in subdirs
-# ---------------------------------------------------------------------------
-# o if we encounter foo/ in $(obj-y), replace it by foo/built-in.a
-# o if we encounter foo/ in $(obj-m), remove it from $(obj-m)
+# Handle objects in subdirs:
+# - If we encounter foo/ in $(obj-y), replace it by foo/built-in.a and
+#   foo/modules.order
+# - If we encounter foo/ in $(obj-m), replace it by foo/modules.order
+#
+# Generate modules.order to determine modorder. Unfortunately, we don't have
+# information about ordering between -y and -m subdirs. Just put -y's first.
+
+ifdef need-modorder
+obj-m := $(patsubst %/,%/modules.order, $(filter %/, $(obj-y)) $(obj-m))
+else
+obj-m := $(filter-out %/, $(obj-m))
+endif
+
 ifdef need-builtin
 obj-y		:= $(patsubst %/, %/built-in.a, $(obj-y))
 else
 obj-y		:= $(filter-out %/, $(obj-y))
 endif
-obj-m		:= $(filter-out %/, $(obj-m))
 
 # If $(foo-objs), $(foo-y), $(foo-m), or $(foo-) exists, foo.o is a composite object
 multi-used-y := $(sort $(foreach m,$(obj-y), $(if $(strip $($(m:.o=-objs)) $($(m:.o=-y)) $($(m:.o=-))), $(m))))
@@ -81,7 +83,6 @@ endif
 extra-y		:= $(addprefix $(obj)/,$(extra-y))
 always-y	:= $(addprefix $(obj)/,$(always-y))
 targets		:= $(addprefix $(obj)/,$(targets))
-modorder	:= $(addprefix $(obj)/,$(modorder))
 obj-m		:= $(addprefix $(obj)/,$(obj-m))
 lib-y		:= $(addprefix $(obj)/,$(lib-y))
 real-obj-y	:= $(addprefix $(obj)/,$(real-obj-y))