kbuild: make single targets work more correctly

Currently, the single target build directly descends into the directory
of the target. For example,

  $ make foo/bar/baz.o

... directly descends into foo/bar/.

On the other hand, the normal build usually descends one directory at
a time, i.e. descends into foo/, and then foo/bar/.

This difference causes some problems.

[1] miss subdir-asflags-y, subdir-ccflags-y in upper Makefiles

    The options in subdir-{as,cc}flags-y take effect in the current
    and its sub-directories. In other words, they are inherited
    downward. In the example above, the single target will miss
    subdir-{as,cc}flags-y if they are defined in foo/Makefile.

[2] could be built in a different directory

    As Documentation/kbuild/modules.rst section 4.3 says, Kbuild can
    handle files that are spread over several sub-directories.

    The build rule of foo/bar/baz.o may not necessarily be specified in
    foo/bar/Makefile. It might be specifies in foo/Makefile as follows:

    [foo/Makefile]
    obj-y := bar/baz.o

    This often happens when a module is so big that its source files
    are divided into sub-directories.

    In this case, there is no Makefile in the foo/bar/ directory, yet
    the single target descends into foo/bar/, then fails due to the
    missing Makefile. You can still do 'make foo/bar/' for partial
    building, but cannot do 'make foo/bar/baz.s'. I believe the single
    target '%.s' is a useful feature for inspecting the compiler output.

    Some modules work around this issue by putting an empty Makefile
    in every sub-directory.

This commit fixes those problems by making the single target build
descend in the same way as the normal build does.

Another change is the single target build will observe the CONFIG
options. Previously, it allowed users to build the foo.o even when
the corresponding CONFIG_FOO is disabled:

   obj-$(CONFIG_FOO) += foo.o

In the new behavior, the single target build will just fail and show
"No rule to make target ..." (or "Nothing to be done for ..." if the
stale object already exists, but cannot be updated).

The disadvantage of this commit is the build speed. Now that the
single target build visits every directory and parses lots of
Makefiles, it is slower than before. (But, I hope it will not be
too slow.)
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Makefile

@@ -230,6 +230,8 @@ endif
 
 export KBUILD_CHECKSRC KBUILD_EXTMOD
 
+extmod-prefix = $(if $(KBUILD_EXTMOD),$(KBUILD_EXTMOD)/)
+
 ifeq ($(abs_srctree),$(abs_objtree))
         # building in the source tree
         srctree := .
@@ -271,11 +273,13 @@ no-dot-config-targets := $(clean-targets) \
 			 %asm-generic kernelversion %src-pkg
 no-sync-config-targets := $(no-dot-config-targets) install %install \
 			   kernelrelease
+single-targets := %.a %.i %.ko %.lds %.ll %.lst %.mod %.o %.s %.symtypes %/
 
 config-build	:=
 mixed-build	:=
 need-config	:= 1
 may-sync-config	:= 1
+single-build	:=
 
 ifneq ($(filter $(no-dot-config-targets), $(MAKECMDGOALS)),)
 	ifeq ($(filter-out $(no-dot-config-targets), $(MAKECMDGOALS)),)
@@ -302,6 +306,14 @@ ifeq ($(KBUILD_EXTMOD),)
         endif
 endif
 
+# We cannot build single targets and the others at the same time
+ifneq ($(filter $(single-targets), $(MAKECMDGOALS)),)
+	single-build := 1
+	ifneq ($(filter-out $(single-targets), $(MAKECMDGOALS)),)
+		mixed-build := 1
+	endif
+endif
+
 # For "make -j clean all", "make -j mrproper defconfig all", etc.
 ifneq ($(filter $(clean-targets),$(MAKECMDGOALS)),)
         ifneq ($(filter-out $(clean-targets),$(MAKECMDGOALS)),)
@@ -1003,7 +1015,7 @@ endif
 
 PHONY += prepare0
 
-export MODORDER := $(if $(KBUILD_EXTMOD),$(KBUILD_EXTMOD)/)modules.order
+export MODORDER := $(extmod-prefix)modules.order
 
 ifeq ($(KBUILD_EXTMOD),)
 core-y		+= kernel/ certs/ mm/ fs/ ipc/ security/ crypto/ block/
@@ -1655,7 +1667,7 @@ endif # KBUILD_EXTMOD
 PHONY += descend $(build-dirs)
 descend: $(build-dirs)
 $(build-dirs): prepare
-	$(Q)$(MAKE) $(build)=$@ need-builtin=1 need-modorder=1
+	$(Q)$(MAKE) $(build)=$@ single-build=$(single-build) need-builtin=1 need-modorder=1
 
 clean-dirs := $(addprefix _clean_, $(clean-dirs))
 PHONY += $(clean-dirs) clean
@@ -1752,40 +1764,47 @@ tools/%: FORCE
 
 # Single targets
 # ---------------------------------------------------------------------------
-# Single targets are compatible with:
-# - build with mixed source and output
-# - build with separate output dir 'make O=...'
-# - external modules
+# To build individual files in subdirectories, you can do like this:
+#
+#   make foo/bar/baz.s
 #
-#  target-dir => where to store outputfile
-#  build-dir  => directory in kernel source tree to use
-
-build-target = $(if $(KBUILD_EXTMOD), $(KBUILD_EXTMOD)/)$@
-build-dir = $(patsubst %/,%,$(dir $(build-target)))
-
-%.i: prepare FORCE
-	$(Q)$(MAKE) $(build)=$(build-dir) $(build-target)
-%.ll: prepare FORCE
-	$(Q)$(MAKE) $(build)=$(build-dir) $(build-target)
-%.lst: prepare FORCE
-	$(Q)$(MAKE) $(build)=$(build-dir) $(build-target)
-%.o: prepare FORCE
-	$(Q)$(MAKE) $(build)=$(build-dir) $(build-target)
-%.s: prepare FORCE
-	$(Q)$(MAKE) $(build)=$(build-dir) $(build-target)
-%.symtypes: prepare FORCE
-	$(Q)$(MAKE) $(build)=$(build-dir) $(build-target)
+# 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
+
+single-all := $(filter $(single-targets), $(MAKECMDGOALS))
+
+# .ko is special because modpost is needed
+single-ko := $(sort $(filter %.ko, $(single-all)))
+single-no-ko := $(sort $(patsubst %.ko,%.mod, $(single-all)))
+
+$(single-ko): single_modpost
+	@:
+$(single-no-ko): descend
+	@:
+
 ifeq ($(KBUILD_EXTMOD),)
-# For the single build of an in-tree module, use a temporary file to avoid
+# For the single build of in-tree modules, use a temporary file to avoid
 # the situation of modules_install installing an invalid modules.order.
-%.ko: MODORDER := .modules.tmp
+MODORDER := .modules.tmp
 endif
-%.ko: prepare FORCE
-	$(Q)$(MAKE) $(build)=$(build-dir) $(build-target:.ko=.mod)
-	$(Q)echo $(build-target) > $(MODORDER)
+
+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
-%/: prepare FORCE
-	$(Q)$(MAKE) KBUILD_MODULES=1 $(build)=$(build-dir) need-modorder=1
+
+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
 # ===========================================================================

scripts/Makefile.build

@@ -52,7 +52,7 @@ ifndef obj
 $(warning kbuild: Makefile.build is included improperly)
 endif
 
-ifeq ($(MAKECMDGOALS)$(need-modorder),)
+ifeq ($(need-modorder),)
 ifneq ($(obj-m),)
 $(warning $(patsubst %.o,'%.ko',$(obj-m)) will not be built even though obj-m is specified.)
 $(warning You cannot use subdir-y/m to visit a module Makefile. Use obj-y/m instead.)
@@ -76,11 +76,6 @@ endif
 
 mod-targets := $(patsubst %.o, %.mod, $(obj-m))
 
-__build: $(if $(KBUILD_BUILTIN),$(builtin-target) $(lib-target) $(extra-y)) \
-	 $(if $(KBUILD_MODULES),$(obj-m) $(mod-targets) $(modorder-target)) \
-	 $(subdir-ym) $(always)
-	@:
-
 # Linus' kernel sanity checking tool
 ifeq ($(KBUILD_CHECKSRC),1)
   quiet_cmd_checksrc       = CHECK   $<
@@ -487,12 +482,50 @@ targets += $(call intermediate_targets, .asn1.o, .asn1.c .asn1.h) \
 	   $(call intermediate_targets, .lex.o, .lex.c) \
 	   $(call intermediate_targets, .tab.o, .tab.c .tab.h)
 
+# Build
+# ---------------------------------------------------------------------------
+
+ifdef single-build
+
+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 $(obj)/%, \
+				$(filter-out $(curdir-single), \
+					$(KBUILD_SINGLE_TARGETS))))
+
+__build: $(curdir-single) $(subdir-ym)
+ifneq ($(unknown-single),)
+	$(Q)$(MAKE) -f /dev/null $(unknown-single)
+endif
+	@:
+
+ifeq ($(curdir-single),)
+# Nothing to do in this directory. Do not include any .*.cmd file for speed-up
+targets :=
+else
+targets += $(curdir-single)
+endif
+
+else
+
+__build: $(if $(KBUILD_BUILTIN),$(builtin-target) $(lib-target) $(extra-y)) \
+	 $(if $(KBUILD_MODULES),$(obj-m) $(mod-targets) $(modorder-target)) \
+	 $(subdir-ym) $(always)
+	@:
+
+endif
+
 # Descending
 # ---------------------------------------------------------------------------
 
 PHONY += $(subdir-ym)
 $(subdir-ym):
 	$(Q)$(MAKE) $(build)=$@ \
+	$(if $(filter $@/, $(KBUILD_SINGLE_TARGETS)),single-build=) \
 	need-builtin=$(if $(filter $@/built-in.a, $(subdir-obj-y)),1) \
 	need-modorder=$(if $(need-modorder),$(if $(filter $@/modules.order, $(modorder)),1))