kbuild: add generic support for built-in boot DTBs

Some architectures embed boot DTBs in vmlinux. A potential issue for
these architectures is a race condition during parallel builds because
Kbuild descends into arch/*/boot/dts/ twice.

One build thread is initiated by the 'dtbs' target, which is a
prerequisite of the 'all' target in the top-level Makefile:

  ifdef CONFIG_OF_EARLY_FLATTREE
  all: dtbs
  endif

For architectures that support the built-in boot dtb, arch/*/boot/dts/
is visited also during the ordinary directory traversal in order to
build obj-y objects that wrap DTBs.

Since these build threads are unaware of each other, they can run
simultaneously during parallel builds.

This commit introduces a generic build rule to scripts/Makefile.vmlinux
to support embedded boot DTBs in a race-free way. Architectures that
want to use this rule need to select CONFIG_GENERIC_BUILTIN_DTB.

After the migration, Makefiles under arch/*/boot/dts/ will be visited
only once to build only *.dtb files.

This change also aims to unify the CONFIG options used for built-in DTBs
support. Currently, different architectures use different CONFIG options
for the same purposes.

With this commit, the CONFIG options will be unified as follows:

 - CONFIG_GENERIC_BUILTIN_DTB

   This enables the generic rule for built-in boot DTBs. This will be
   renamed to CONFIG_BUILTIN_DTB after all architectures migrate to the
   generic rule.

 - CONFIG_BUILTIN_DTB_NAME

   This specifies the path to the embedded DTB.
   (relative to arch/*/boot/dts/)

 - CONFIG_BUILTIN_DTB_ALL

   If this is enabled, all DTB files compiled under arch/*/boot/dts/ are
   embedded into vmlinux. Only used by MIPS.

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

Makefile

@@ -1433,6 +1433,10 @@ ifdef CONFIG_OF_EARLY_FLATTREE
 all: dtbs
 endif
 
+ifdef CONFIG_GENERIC_BUILTIN_DTB
+vmlinux: dtbs
+endif
+
 endif
 
 PHONY += scripts_dtc
@@ -1500,7 +1504,8 @@ CLEAN_FILES += vmlinux.symvers modules-only.symvers \
 	       modules.builtin modules.builtin.modinfo modules.nsdeps \
 	       modules.builtin.ranges vmlinux.o.map \
 	       compile_commands.json rust/test \
-	       rust-project.json .vmlinux.objs .vmlinux.export.c
+	       rust-project.json .vmlinux.objs .vmlinux.export.c \
+               .builtin-dtbs-list .builtin-dtb.S
 
 # Directories & files removed with 'make mrproper'
 MRPROPER_FILES += include/config include/generated          \

drivers/of/Kconfig

@@ -2,6 +2,12 @@
 config DTC
 	bool
 
+config GENERIC_BUILTIN_DTB
+	bool
+
+config BUILTIN_DTB_ALL
+	bool
+
 menuconfig OF
 	bool "Device Tree and Open Firmware support"
 	help

scripts/Makefile.vmlinux

@@ -17,6 +17,50 @@ quiet_cmd_cc_o_c = CC      $@
 %.o: %.c FORCE
 	$(call if_changed_dep,cc_o_c)
 
+quiet_cmd_as_o_S = AS      $@
+      cmd_as_o_S = $(CC) $(a_flags) -c -o $@ $<
+
+%.o: %.S FORCE
+	$(call if_changed_dep,as_o_S)
+
+# Built-in dtb
+# ---------------------------------------------------------------------------
+
+quiet_cmd_wrap_dtbs = WRAP    $@
+      cmd_wrap_dtbs = {							\
+	echo '\#include <asm-generic/vmlinux.lds.h>';			\
+	echo '.section .dtb.init.rodata,"a"';				\
+	while read dtb; do						\
+		symbase=__dtb_$$(basename -s .dtb "$${dtb}" | tr - _);	\
+		echo '.balign STRUCT_ALIGNMENT';			\
+		echo ".global $${symbase}_begin";			\
+		echo "$${symbase}_begin:";				\
+		echo '.incbin "'$$dtb'" ';				\
+		echo ".global $${symbase}_end";				\
+		echo "$${symbase}_end:";				\
+	done < $<;							\
+	} > $@
+
+.builtin-dtbs.S: .builtin-dtbs-list FORCE
+	$(call if_changed,wrap_dtbs)
+
+quiet_cmd_gen_dtbs_list = GEN     $@
+      cmd_gen_dtbs_list = \
+	$(if $(CONFIG_BUILTIN_DTB_NAME), echo "arch/$(SRCARCH)/boot/dts/$(CONFIG_BUILTIN_DTB_NAME).dtb",:) > $@
+
+.builtin-dtbs-list: arch/$(SRCARCH)/boot/dts/dtbs-list FORCE
+	$(call if_changed,$(if $(CONFIG_BUILTIN_DTB_ALL),copy,gen_dtbs_list))
+
+targets += .builtin-dtbs-list
+
+ifdef CONFIG_GENERIC_BUILTIN_DTB
+targets += .builtin-dtbs.S .builtin-dtbs.o
+vmlinux: .builtin-dtbs.o
+endif
+
+# vmlinux
+# ---------------------------------------------------------------------------
+
 ifdef CONFIG_MODULES
 targets += .vmlinux.export.o
 vmlinux: .vmlinux.export.o

scripts/link-vmlinux.sh

@@ -68,6 +68,10 @@ vmlinux_link()
 		libs="${KBUILD_VMLINUX_LIBS}"
 	fi
 
+	if is_enabled CONFIG_GENERIC_BUILTIN_DTB; then
+		objs="${objs} .builtin-dtbs.o"
+	fi
+
 	if is_enabled CONFIG_MODULES; then
 		objs="${objs} .vmlinux.export.o"
 	fi