cmake: create a root.cmake CMake module

The root.cmake CMake module remove boilerplate code and place
it inside a dedicated root.cmake CMake module.

This is part of a general CMake overhaul to allow better modularization
and reuse of the Zephyr build system.

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

cmake/app/boilerplate.cmake

@@ -71,13 +71,7 @@ include(${ZEPHYR_BASE}/cmake/python.cmake)
 include(${ZEPHYR_BASE}/cmake/west.cmake)
 include(${ZEPHYR_BASE}/cmake/ccache.cmake)
 
-# 'MODULE_EXT_ROOT' is a prioritized list of directories where module glue code
-# may be found. It always includes ${ZEPHYR_BASE} at the lowest priority.
-# For module roots, later entries may overrule module settings already defined
-# by processed module roots, hence first in list means lowest priority.
-zephyr_file(APPLICATION_ROOT MODULE_EXT_ROOT)
-list(INSERT MODULE_EXT_ROOT 0 ${ZEPHYR_BASE})
-
+include(${ZEPHYR_BASE}/cmake/root.cmake)
 #
 # Find Zephyr modules.
 # Those may contain additional DTS, BOARD, SOC, ARCH ROOTs.
@@ -85,12 +79,6 @@ list(INSERT MODULE_EXT_ROOT 0 ${ZEPHYR_BASE})
 #
 include(${ZEPHYR_BASE}/cmake/zephyr_module.cmake)
 
-zephyr_file(APPLICATION_ROOT BOARD_ROOT)
-
-zephyr_file(APPLICATION_ROOT SOC_ROOT)
-
-zephyr_file(APPLICATION_ROOT ARCH_ROOT)
-
 include(${ZEPHYR_BASE}/cmake/boards.cmake)
 include(${ZEPHYR_BASE}/cmake/shields.cmake)
 include(${ZEPHYR_BASE}/cmake/arch.cmake)

cmake/root.cmake

@@ -0,0 +1,30 @@
+# SPDX-License-Identifier: Apache-2.0
+#
+# Copyright (c) 2021, Nordic Semiconductor ASA
+
+# Convert Zephyr roots to absolute paths.
+#
+# This CMake module will convert all relative paths in existing ROOT lists to
+# absolute path relative from APPLICATION_SOURCE_DIR.
+#
+# Optional variables:
+# - ARCH_ROOT:       CMake list of arch roots containing arch implementations
+# - SOC_ROOT:        CMake list of SoC roots containing SoC implementations
+# - BOARD_ROOT:      CMake list of board roots containing board and shield implementations
+# - MODULE_EXT_ROOT: CMake list of module external roots containing module glue code
+#
+# If a root is defined it will check the list of paths in the root and convert
+# any relative path to absolute path and update the root list.
+# If a root is undefined it will still be undefined when this module has loaded.
+
+# Convert paths to absolute, relative from APPLICATION_SOURCE_DIR
+zephyr_file(APPLICATION_ROOT MODULE_EXT_ROOT)
+
+# Convert paths to absolute, relative from APPLICATION_SOURCE_DIR
+zephyr_file(APPLICATION_ROOT BOARD_ROOT)
+
+# Convert paths to absolute, relative from APPLICATION_SOURCE_DIR
+zephyr_file(APPLICATION_ROOT SOC_ROOT)
+
+# Convert paths to absolute, relative from APPLICATION_SOURCE_DIR
+zephyr_file(APPLICATION_ROOT ARCH_ROOT)

cmake/zephyr_module.cmake

@@ -3,17 +3,17 @@
 # This cmake file provides functionality to import CMakeLists.txt and Kconfig
 # files for Zephyr modules into Zephyr build system.
 #
-# CMakeLists.txt and Kconfig files can reside directly in the module or in a
-# MODULE_EXT_ROOT.
+# CMakeLists.txt and Kconfig files can reside directly in the Zephyr module or
+# in a MODULE_EXT_ROOT.
 # The `<module>/zephyr/module.yml` file specifies whether the build files are
-# located in the module or in a MODULE_EXT_ROOT.
+# located in the Zephyr module or in a MODULE_EXT_ROOT.
 #
 # A list of Zephyr modules can be provided to the build system using:
 #   -DZEPHYR_MODULES=<module-path>[;<additional-module(s)-path>]
 #
 # It looks for: <module>/zephyr/module.yml or
 #               <module>/zephyr/CMakeLists.txt
-# to load the module into Zephyr build system.
+# to load the Zephyr module into Zephyr build system.
 # If west is available, it uses `west list` to obtain a list of projects to
 # search for zephyr/module.yml
 #
@@ -81,17 +81,13 @@ if(WEST OR ZEPHYR_MODULES)
       # lazy regexes (it supports greedy only).
       string(REGEX REPLACE "\"(.*)\":\".*\"" "\\1" key ${setting})
       string(REGEX REPLACE "\".*\":\"(.*)\"" "\\1" value ${setting})
-      # MODULE_EXT_ROOT is process order which means module roots processed
-      # later wins. To ensure ZEPHYR_BASE stays first, and command line settings
-      # are processed last, we insert at position 1.
-      if ("${key}" STREQUAL "MODULE_EXT_ROOT")
-        list(INSERT ${key} 1 ${value})
-      else()
-        list(APPEND ${key} ${value})
-      endif()
+      list(APPEND ${key} ${value})
     endforeach()
   endif()
 
+  # Append ZEPHYR_BASE as a default ext root at lowest priority
+  list(APPEND MODULE_EXT_ROOT ${ZEPHYR_BASE})
+
   if(EXISTS ${CMAKE_BINARY_DIR}/zephyr_modules.txt)
     file(STRINGS ${CMAKE_BINARY_DIR}/zephyr_modules.txt ZEPHYR_MODULES_TXT
          ENCODING UTF-8)
@@ -106,6 +102,9 @@ if(WEST OR ZEPHYR_MODULES)
     endforeach()
   endif()
 
+  # MODULE_EXT_ROOT is process order which means Zephyr module roots processed
+  # later wins. therefore we reverse the list before processing.
+  list(REVERSE MODULE_EXT_ROOT)
   foreach(root ${MODULE_EXT_ROOT})
     if(NOT EXISTS ${root})
       message(FATAL_ERROR "No `modules.cmake` found in module root `${root}`.")
@@ -117,8 +116,8 @@ if(WEST OR ZEPHYR_MODULES)
   if(DEFINED ZEPHYR_MODULES_TXT)
     foreach(module ${ZEPHYR_MODULES_TXT})
       # Match "<name>":"<path>" for each line of file, each corresponding to
-      # one module. The use of quotes is required due to CMake not supporting
-      # lazy regexes (it supports greedy only).
+      # one Zephyr module. The use of quotes is required due to CMake not
+      # supporting lazy regexes (it supports greedy only).
       string(CONFIGURE ${module} module)
       string(REGEX REPLACE "\"(.*)\":\".*\":\".*\"" "\\1" module_name ${module})
       string(REGEX REPLACE "\".*\":\"(.*)\":\".*\"" "\\1" module_path ${module})
@@ -138,7 +137,7 @@ ${MODULE_NAME_UPPER} is a restricted name for Zephyr modules as it is used for \
 else()
 
   file(WRITE ${KCONFIG_MODULES_FILE}
-    "# No west and no modules\n"
+    "# No west and no Zephyr modules\n"
     )
 
 endif()