[PATCH V4 REPOST 2/2] kbuild: run the pre-processor on *.dts files

[Stephen Warren <swarren-3lzwWm7+Weoh9ZMKESR00Q@public.gmane.org>]

From: Stephen Warren <swarren-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>

Modify cmd_dtc to run the C pre-processor on the input .dts file before
passing it to dtc for final compilation. This allows the use of #define
and #include within the .dts file.

Signed-off-by: Stephen Warren <swarren-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>
---
v4:
* Use -x assembler-with-cpp so pre-defined macros are set up so that
  #included header files know to only use cpp syntax, not C syntax.
* Define __DTS__ for similar reasons.
* use $(CPP) not $(CC) -E, and use $(cpp_flags).
* Save the pre-processed results so they can be easily inspected when
  debugging build issues.
* The use of -x assembler-with-cpp causes cpp to recognize directives in
  column 1 only. Hence, there's no need to escape property names that
  begin with #. Hence, there's no need for separate skeleton.dtsi and
  skeleton.dtsip. Maintain a separate file extension and build rule so that
  CPP-usage is opt-in. In particular, when using CPP, #include must be used
  rather than /include/ so that dependencies work.
v3: Pass "-x c" not "-xc" to cpp.
v2: Place make %.dtb: %.dtsp rule into Makefile.lib.
---
 scripts/Makefile.lib |    9 +++++++++
 1 files changed, 9 insertions(+), 0 deletions(-)

diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index 425578e..33432f4 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -269,6 +269,15 @@ cmd_dtc = $(objtree)/scripts/dtc/dtc -O dtb -o $@ -b 0 $(DTC_FLAGS) -d $(depfile
 $(obj)/%.dtb: $(src)/dts/%.dts FORCE
 	$(call if_changed_dep,dtc)
 
+dtc-tmp = $(subst $(comma),_,$(dot-target).dts)
+
+quiet_cmd_dtc_cpp = DTC+CPP $@
+cmd_dtc_cpp = $(CPP) $(cpp_flags) -D__DTS__ -x assembler-with-cpp -o $(dtc-tmp) $< ; \
+	$(objtree)/scripts/dtc/dtc -O dtb -o $@ -b 0 $(DTC_FLAGS) $(dtc-tmp)
+
+$(obj)/%.dtb: $(src)/dts/%.dtsp FORCE
+	$(call if_changed_dep,dtc_cpp)
+
 # Bzip2
 # ---------------------------------------------------------------------------
 
-- 
1.7.0.4