[PATCH V7] kbuild: create a rule to run the pre-processor on *.dts files

[Stephen Warren]

From: Stephen Warren <swarren at nvidia.com>

Create cmd_dtc_cpp to run the C pre-processor on *.dts file before
passing them to dtc for final compilation. This allows the use of #define
and #include within the .dts file.

Signed-off-by: Stephen Warren <swarren at nvidia.com>
---
Grant, back in mid-November, you said you'd make a decision on this in
the next couple of days, but I think this got overlooked.

v7: Build *.dtb from *.dts not src/*.dts.
v6: No change.
v5:
* Update Documentation/kbuild for the new command and rule.
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.
---
 Documentation/kbuild/makefiles.txt |   23 +++++++++++++++++++++++
 scripts/Makefile.lib               |    9 +++++++++
 2 files changed, 32 insertions(+)

diff --git a/Documentation/kbuild/makefiles.txt b/Documentation/kbuild/makefiles.txt
index 14c3f4f..5198b74 100644
--- a/Documentation/kbuild/makefiles.txt
+++ b/Documentation/kbuild/makefiles.txt
@@ -1186,6 +1186,29 @@ When kbuild executes, the following steps are followed (roughly):
 		clean-files += *.dtb
 		DTC_FLAGS ?= -p 1024
 
+    dtc_cpp
+	This is just like dtc as describe above, except that the C pre-
+	processor is invoked upon the .dtsp file before compiling the result
+	with dtc.
+
+	In order for build dependencies to work, all files compiled using
+	dtc_cpp must use the C pre-processor's #include functionality and not
+	dtc's /include/ functionality.
+
+	Using the C pre-processor allows use of #define to create named
+	constants. In turn, the #defines will typically appear in a header
+	file, which may be shared with regular C code. Since the dtc language
+	represents a data structure rather than code in C syntax, similar
+	restrictions are placed on a header file included by a device tree
+	file as for a header file included by an assembly language file.
+	In particular, the C pre-processor is passed -x assembler-with-cpp,
+	which sets macro __ASSEMBLY__. __DTS__ is also set. These allow header
+	files to restrict their content to that compatible with device tree
+	source.
+
+	A central rule exists to create $(obj)/%.dtb from $(src)/%.dtsp;
+	architecture Makefiles do no need to explicitly write out that rule.
+
 --- 6.8 Custom kbuild commands
 
 	When kbuild is executing with KBUILD_VERBOSE=0, then only a shorthand
diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index bdf42fd..2c2a302 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 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)/%.dtsp FORCE
+	$(call if_changed_dep,dtc_cpp)
+
 # Bzip2
 # ---------------------------------------------------------------------------
 
-- 
1.7.10.4