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

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.

Acked-by: Simon Glass <sjg@chromium.org>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Michal Marek <mmarek@suse.cz>
Acked-by: Srinivas Kandagatla <srinivas.kandagatla@st.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

1 files changed, 23 insertions, 0 deletions

diff --git a/Documentation/kbuild/makefiles.txt b/Documentation/kbuild/makefiles.txt
index 14c3f4f1b617..5198b742fde1 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