cmake: only write devicetree files when there are changes.

As part of #40167 is was discovered that devicetree headers are always
generated when CMake re-runs.

This causes all source files that directly or indirectly through
included headers to recompile when CMake re-runs, even if there are no
changes to devicetree.

This commits introduces `zephyr_file_copy(...)` similar to
`file(COPY_FILE ...)` from CMake 3.21.
However, as CMake 3.20 is supported by Zephyr we need a zephyr variant
of this function to allow usage with CMake 3.20.

This ensures that only when there are changes to devicetree headers,
then source files will recompile.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>

CMakeLists.txt

@@ -471,7 +471,7 @@ set_ifndef(DTS_CAT_OF_FIXUP_FILES ${ZEPHYR_BINARY_DIR}/include/generated/devicet
 
 # Concatenate the fixups into a single header file for easy
 # #include'ing
-file(WRITE ${DTS_CAT_OF_FIXUP_FILES} "/* May only be included by devicetree.h */\n\n")
+file(WRITE ${DTS_CAT_OF_FIXUP_FILES}.new "/* May only be included by devicetree.h */\n\n")
 set(DISCOVERED_FIXUP_FILES)
 foreach(fixup_file
     ${DTS_BOARD_FIXUP_FILE}
@@ -481,10 +481,12 @@ foreach(fixup_file
     )
   if(EXISTS ${fixup_file})
     file(READ ${fixup_file} contents)
-    file(APPEND ${DTS_CAT_OF_FIXUP_FILES} "${contents}")
+    file(APPEND ${DTS_CAT_OF_FIXUP_FILES}.new "${contents}")
     string(APPEND DISCOVERED_FIXUP_FILES "- ${fixup_file}\n")
   endif()
 endforeach()
+zephyr_file_copy(${DTS_CAT_OF_FIXUP_FILES}.new ${DTS_CAT_OF_FIXUP_FILES} ONLY_IF_DIFFERENT)
+file(REMOVE ${DTS_CAT_OF_FIXUP_FILES}.new)
 
 if (DISCOVERED_FIXUP_FILES)
   message(WARNING "One or more dts_fixup.h files detected:\n${DISCOVERED_FIXUP_FILES}Use of these files is deprecated; use the devicetree.h API instead.")

CODEOWNERS

@@ -613,6 +613,7 @@
 /subsys/portability/                      @nashif
 /lib/libc/                                @nashif
 /lib/libc/arcmwdt/                        @abrodkin @ruuddw @evgeniy-paltsev
+/misc/                                    @tejlmand
 /modules/                                 @nashif
 /modules/canopennode/                     @henrikbrixandersen
 /modules/mbedtls/                         @ceolin @d3zd3z

cmake/dts.cmake

@@ -189,9 +189,9 @@ if(SUPPORTS_DTS)
   --dts ${DTS_POST_CPP}
   --dtc-flags '${EXTRA_DTC_FLAGS_RAW}'
   --bindings-dirs ${DTS_ROOT_BINDINGS}
-  --header-out ${DEVICETREE_UNFIXED_H}
-  --device-header-out ${DEVICE_EXTERN_H}
-  --dts-out ${ZEPHYR_DTS} # for debugging and dtc
+  --header-out ${DEVICETREE_UNFIXED_H}.new
+  --device-header-out ${DEVICE_EXTERN_H}.new
+  --dts-out ${ZEPHYR_DTS}.new # for debugging and dtc
   --edt-pickle-out ${EDT_PICKLE}
   ${EXTRA_GEN_DEFINES_ARGS}
   )
@@ -204,6 +204,10 @@ if(SUPPORTS_DTS)
   if(NOT "${ret}" STREQUAL "0")
     message(FATAL_ERROR "gen_defines.py failed with return code: ${ret}")
   else()
+    zephyr_file_copy(${ZEPHYR_DTS}.new ${ZEPHYR_DTS} ONLY_IF_DIFFERENT)
+    zephyr_file_copy(${DEVICETREE_UNFIXED_H}.new ${DEVICETREE_UNFIXED_H} ONLY_IF_DIFFERENT)
+    zephyr_file_copy(${DEVICE_EXTERN_H}.new ${DEVICE_EXTERN_H})
+    file(REMOVE ${ZEPHYR_DTS}.new ${DEVICETREE_UNFIXED_H}.new ${DEVICE_EXTERN_H}.new)
     message(STATUS "Generated zephyr.dts: ${ZEPHYR_DTS}")
     message(STATUS "Generated devicetree_unfixed.h: ${DEVICETREE_UNFIXED_H}")
     message(STATUS "Generated device_extern.h: ${DEVICE_EXTERN_H}")
@@ -267,6 +271,7 @@ if(SUPPORTS_DTS)
   endif()
   endif(DTC)
 else()
-  file(WRITE ${DEVICETREE_UNFIXED_H} "/* WARNING. THIS FILE IS AUTO-GENERATED. DO NOT MODIFY! */")
-  file(WRITE ${DEVICE_EXTERN_H} "/* WARNING. THIS FILE IS AUTO-GENERATED. DO NOT MODIFY! */")
+  set(header_template ${ZEPHYR_BASE}/misc/generated/generated_header.template)
+  zephyr_file_copy(${header_template} ${DEVICETREE_UNFIXED_H} ONLY_IF_DIFFERENT)
+  zephyr_file_copy(${header_template} ${DEVICE_EXTERN_H} ONLY_IF_DIFFERENT)
 endif(SUPPORTS_DTS)

cmake/extensions.cmake

@@ -2282,6 +2282,42 @@ Relative paths are only allowed with `-D${ARGV1}=<path>`")
   endif()
 endfunction()
 
+# Usage:
+#   zephyr_file_copy(<oldname> <newname> [ONLY_IF_DIFFERENT])
+#
+# Zephyr file copy extension.
+# This function is similar to CMake function
+# 'file(COPY_FILE <oldname> <newname> [ONLY_IF_DIFFERENT])'
+# introduced with CMake 3.21.
+#
+# Because the minimal required CMake version with Zephyr is 3.20, this function
+# is not guaranteed to be available.
+#
+# When using CMake version 3.21 or newer 'zephyr_file_copy()' simply calls
+# 'file(COPY_FILE...)' directly.
+# When using CMake version 3.20, the implementation will execute using CMake
+# for running command line tool in a subprocess for identical functionality.
+function(zephyr_file_copy oldname newname)
+  set(options ONLY_IF_DIFFERENT)
+  cmake_parse_arguments(ZEPHYR_FILE_COPY "${options}" "" "" ${ARGN})
+
+  if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.21.0)
+    if(ZEPHYR_FILE_COPY_ONLY_IF_DIFFERENT)
+      set(copy_file_options ONLY_IF_DIFFERENT)
+    endif()
+    file(COPY_FILE ${oldname} ${newname} ${copy_file_options})
+  else()
+    if(ZEPHYR_FILE_COPY_ONLY_IF_DIFFERENT)
+      set(copy_file_command copy_if_different)
+    else()
+      set(copy_file_command copy)
+    endif()
+    execute_process(
+      COMMAND ${CMAKE_COMMAND} -E ${copy_file_command} ${oldname} ${newname}
+    )
+  endif()
+endfunction()
+
 # Usage:
 #   zephyr_string(<mode> <out-var> <input> ...)
 #

misc/generated/generated_header.template

@@ -0,0 +1 @@
+/* WARNING. THIS FILE IS AUTO-GENERATED. DO NOT MODIFY! */