kbuild: list sub-directories in ./Kbuild

Use the ordinary obj-y syntax to list subdirectories.

Note1:
Previously, the link order of lib-y depended on CONFIG_MODULES; lib-y
was linked before drivers-y when CONFIG_MODULES=y, otherwise after
drivers-y. This was a bug of commit 7273ad2b ("kbuild: link lib-y
objects to vmlinux forcibly when CONFIG_MODULES=y"), but it was not a
big deal after all. Now, all objects listed in lib-y are linked last,
irrespective of CONFIG_MODULES.

Note2:
Finally, the single target build in arch/*/lib/ works correctly. There was
a bug report about this. [1]

  $ make ARCH=arm arch/arm/lib/findbit.o
    CALL    scripts/checksyscalls.sh
    AS      arch/arm/lib/findbit.o

[1]: https://lore.kernel.org/linux-kbuild/YvUQOwL6lD4%2F5%2FU6@shell.armlinux.org.uk/Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>

Kbuild

@@ -72,3 +72,27 @@ $(atomic-checks): $(obj)/.checked-%: include/linux/atomic/%  FORCE
 PHONY += prepare
 prepare: $(offsets-file) missing-syscalls $(atomic-checks)
 	@:
+
+# Ordinary directory descending
+# ---------------------------------------------------------------------------
+
+obj-y			+= init/
+obj-y			+= usr/
+obj-y			+= arch/$(SRCARCH)/
+obj-y			+= $(ARCH_CORE)
+obj-y			+= kernel/
+obj-y			+= certs/
+obj-y			+= mm/
+obj-y			+= fs/
+obj-y			+= ipc/
+obj-y			+= security/
+obj-y			+= crypto/
+obj-$(CONFIG_BLOCK)	+= block/
+obj-$(CONFIG_IO_URING)	+= io_uring/
+obj-y			+= $(ARCH_LIB)
+obj-y			+= drivers/
+obj-y			+= sound/
+obj-$(CONFIG_SAMPLES)	+= samples/
+obj-$(CONFIG_NET)	+= net/
+obj-y			+= virt/
+obj-y			+= $(ARCH_DRIVERS)

Makefile

@@ -676,11 +676,8 @@ endif
 
 ifeq ($(KBUILD_EXTMOD),)
 # Objects we will link into vmlinux / subdirs we need to visit
-core-y		:= init/ usr/ arch/$(SRCARCH)/
-drivers-y	:= drivers/ sound/
-drivers-$(CONFIG_SAMPLES) += samples/
-drivers-$(CONFIG_NET) += net/
-drivers-y	+= virt/
+core-y		:=
+drivers-y	:=
 libs-y		:= lib/
 endif # KBUILD_EXTMOD
 
@@ -1099,31 +1096,24 @@ export MODORDER := $(extmod_prefix)modules.order
 export MODULES_NSDEPS := $(extmod_prefix)modules.nsdeps
 
 ifeq ($(KBUILD_EXTMOD),)
-core-y			+= kernel/ certs/ mm/ fs/ ipc/ security/ crypto/
-core-$(CONFIG_BLOCK)	+= block/
-core-$(CONFIG_IO_URING)	+= io_uring/
 
-vmlinux-dirs	:= $(patsubst %/,%,$(filter %/, \
-		     $(core-y) $(core-m) $(drivers-y) $(drivers-m) \
-		     $(libs-y) $(libs-m)))
-
-build-dirs	:= $(vmlinux-dirs)
-clean-dirs	:= $(sort $(vmlinux-dirs) Documentation . \
+build-dir	:= .
+clean-dirs	:= $(sort . Documentation \
 		     $(patsubst %/,%,$(filter %/, $(core-) \
 			$(drivers-) $(libs-))))
 
-subdir-modorder := $(addsuffix /modules.order, $(build-dirs))
-
+export ARCH_CORE	:= $(core-y)
+export ARCH_LIB		:= $(filter %/, $(libs-y))
+export ARCH_DRIVERS	:= $(drivers-y) $(drivers-m)
 # Externally visible symbols (used by link-vmlinux.sh)
-KBUILD_VMLINUX_OBJS := $(head-y) $(patsubst %/,%/built-in.a, $(core-y))
-KBUILD_VMLINUX_OBJS += $(addsuffix built-in.a, $(filter %/, $(libs-y)))
+
+KBUILD_VMLINUX_OBJS := $(head-y) ./built-in.a
 ifdef CONFIG_MODULES
 KBUILD_VMLINUX_OBJS += $(patsubst %/, %/lib.a, $(filter %/, $(libs-y)))
 KBUILD_VMLINUX_LIBS := $(filter-out %/, $(libs-y))
 else
 KBUILD_VMLINUX_LIBS := $(patsubst %/,%/lib.a, $(libs-y))
 endif
-KBUILD_VMLINUX_OBJS += $(patsubst %/,%/built-in.a, $(drivers-y))
 
 export KBUILD_VMLINUX_OBJS KBUILD_VMLINUX_LIBS
 export KBUILD_LDS          := arch/$(SRCARCH)/kernel/vmlinux.lds
@@ -1138,7 +1128,7 @@ ifdef CONFIG_TRIM_UNUSED_KSYMS
 # (this can be evaluated only once include/config/auto.conf has been included)
 KBUILD_MODULES := 1
 
-autoksyms_recursive: descend modules.order
+autoksyms_recursive: $(build-dir) modules.order
 	$(Q)$(CONFIG_SHELL) $(srctree)/scripts/adjust_autoksyms.sh \
 	  "$(MAKE) -f $(srctree)/Makefile autoksyms_recursive"
 endif
@@ -1166,7 +1156,7 @@ targets := vmlinux
 
 # The actual objects are generated when descending,
 # make sure no implicit rule kicks in
-$(sort $(vmlinux-deps) $(subdir-modorder)): descend ;
+$(sort $(vmlinux-deps)): . ;
 
 filechk_kernel.release = \
 	echo "$(KERNELVERSION)$$($(CONFIG_SHELL) $(srctree)/scripts/setlocalversion $(srctree))"
@@ -1437,13 +1427,6 @@ endif
 
 modules: $(if $(KBUILD_BUILTIN),vmlinux) modules_prepare
 
-cmd_modules_order = cat $(real-prereqs) > $@
-
-modules.order: $(subdir-modorder) FORCE
-	$(call if_changed,modules_order)
-
-targets += modules.order
-
 # Target to prepare building external modules
 modules_prepare: prepare
 	$(Q)$(MAKE) $(build)=scripts scripts/module.lds
@@ -1714,9 +1697,7 @@ else # KBUILD_EXTMOD
 KBUILD_BUILTIN :=
 KBUILD_MODULES := 1
 
-build-dirs := $(KBUILD_EXTMOD)
-$(MODORDER): descend
-	@:
+build-dir := $(KBUILD_EXTMOD)
 
 compile_commands.json: $(extmod_prefix)compile_commands.json
 PHONY += compile_commands.json
@@ -1754,6 +1735,9 @@ PHONY += modules modules_install modules_prepare
 
 ifdef CONFIG_MODULES
 
+$(MODORDER): $(build-dir)
+	@:
+
 modules: modules_check
 	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.modpost
 
@@ -1807,7 +1791,7 @@ single-no-ko := $(filter-out $(single-ko), $(MAKECMDGOALS)) \
 
 $(single-ko): single_modpost
 	@:
-$(single-no-ko): descend
+$(single-no-ko): $(build-dir)
 	@:
 
 # Remove MODORDER when done because it is not the real one.
@@ -1817,24 +1801,17 @@ single_modpost: $(single-no-ko) modules_prepare
 	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.modpost
 	$(Q)rm -f $(MODORDER)
 
-single-goals := $(addprefix $(extmod_prefix), $(single-no-ko))
-
-# trim unrelated directories
-build-dirs := $(foreach d, $(build-dirs), \
-			$(if $(filter $d/%, $(single-goals)), $d))
+single-goals := $(addprefix $(build-dir)/, $(single-no-ko))
 
 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
 # make menuconfig etc.
 # Error messages still appears in the original language
-PHONY += descend $(build-dirs)
-descend: $(build-dirs)
-$(build-dirs): prepare
-	$(Q)$(MAKE) $(build)=$@ need-builtin=1 need-modorder=1 \
-	$(filter $@/%, $(single-goals))
+PHONY += $(build-dir)
+$(build-dir): prepare
+	$(Q)$(MAKE) $(build)=$@ need-builtin=1 need-modorder=1 $(single-goals)
 
 clean-dirs := $(addprefix _clean_, $(clean-dirs))
 PHONY += $(clean-dirs) clean

scripts/Makefile.lib

@@ -89,6 +89,7 @@ always-y			+= $(dtb-y)
 
 # Add subdir path
 
+ifneq ($(obj),.)
 extra-y		:= $(addprefix $(obj)/,$(extra-y))
 always-y	:= $(addprefix $(obj)/,$(always-y))
 targets		:= $(addprefix $(obj)/,$(targets))
@@ -100,6 +101,7 @@ multi-obj-m	:= $(addprefix $(obj)/, $(multi-obj-m))
 multi-dtb-y	:= $(addprefix $(obj)/, $(multi-dtb-y))
 real-dtb-y	:= $(addprefix $(obj)/, $(real-dtb-y))
 subdir-ym	:= $(addprefix $(obj)/,$(subdir-ym))
+endif
 
 # Finds the multi-part object the current object will be linked into.
 # If the object belongs to two or more multi-part objects, list them all.