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>

1 files changed, 6 insertions, 1 deletions

diff --git a/Makefile b/Makefile
index 9b30c42ce3b3..855913b71e7a 100644
--- a/Makefile
+++ b/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          \