kbuild: centralize .dts->.dtb rule

All architectures that use cmd_dtc do so in almost the same way. Create
a central build rule to avoid duplication. The one difference is that
most current uses of dtc build $(obj)/%.dtb from $(src)/dts/%.dts rather
than building the .dtb in the same directory as the .dts file. This
difference will be eliminated arch-by-arch in future patches.

MIPS is the exception here; it already uses the exact same rule as the
new common rule, so the duplicate is removed in this patch to avoid any
conflict. arch/mips changes courtesy of Ralf Baechle.

Update Documentation/kbuild to remove the explicit call to cmd_dtc from
the example, now that the rule exists in a centralized location.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: linux-arm-kernel@lists.infradead.org
Cc: Olof Johansson <olof@lixom.net>
Cc: Russell King <linux@arm.linux.org.uk>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Jonas Bonn <jonas@southpole.se>
Cc: linux@lists.openrisc.net
Cc: Aurelien Jacquiot <a-jacquiot@ti.com>
Cc: linux-c6x-dev@linux-c6x.org
Cc: Mark Salter <msalter@redhat.com>
Cc: Michal Simek <monstr@monstr.eu>
Cc: microblaze-uclinux@itee.uq.edu.au
Cc: Chris Zankel <chris@zankel.net>
Cc: linux-xtensa@linux-xtensa.org
Cc: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>

1 files changed, 8 insertions, 7 deletions

diff --git a/Documentation/kbuild/makefiles.txt b/Documentation/kbuild/makefiles.txt
index ec9ae6708691..14c3f4f1b617 100644
--- a/Documentation/kbuild/makefiles.txt
+++ b/Documentation/kbuild/makefiles.txt
@@ -1175,15 +1175,16 @@ When kbuild executes, the following steps are followed (roughly):
         in an init section in the image. Platform code *must* copy the
         blob to non-init memory prior to calling unflatten_device_tree().
 
-        Example:
-                #arch/x86/platform/ce4100/Makefile
-                clean-files := *dtb.S
+        To use this command, simply add *.dtb into obj-y or targets, or make
+        some other target depend on %.dtb
 
-                DTC_FLAGS := -p 1024
-                obj-y += foo.dtb.o
+        A central rule exists to create $(obj)/%.dtb from $(src)/%.dts;
+        architecture Makefiles do no need to explicitly write out that rule.
 
-                $(obj)/%.dtb: $(src)/%.dts
-                        $(call cmd,dtc)
+        Example:
+                targets += $(dtb-y)
+                clean-files += *.dtb
+                DTC_FLAGS ?= -p 1024
 
 --- 6.8 Custom kbuild commands