add custom linker script (#103)

* add custom linker script
a custom linker script can be applied by adding the .ld filepath
as 2nd argument to mbed_set_post_build

* more detailed comment

* add if is_standalone for text cases

* replace COMMENT in add_custom_command by echo
COMMENT is not reliable printed

tools/cmake/mbed_set_linker_script.cmake

@@ -56,6 +56,8 @@ function(mbed_setup_linker_script mbed_os_target mbed_baremetal_target target_de
         OUTPUT
             ${LINKER_SCRIPT_PATH}
         PRE_LINK
+        COMMAND 
+            ${CMAKE_COMMAND} -E echo "Preprocess linker script: ${RAW_LINKER_SCRIPT_NAME} -> ${LINKER_SCRIPT_NAME}"
         COMMAND
             ${CMAKE_C_COMPILER} @${linker_defs_response_file}
             -E -x assembler-with-cpp
@@ -68,11 +70,10 @@ function(mbed_setup_linker_script mbed_os_target mbed_baremetal_target target_de
             ${target_defines_header}
         WORKING_DIRECTORY
             ${CMAKE_CURRENT_SOURCE_DIR}
-        COMMENT
-            "Preprocess linker script: ${RAW_LINKER_SCRIPT_NAME} -> ${LINKER_SCRIPT_NAME}"
         VERBATIM
     )
 
+    
     # The job to create the linker script gets attached to the mbed-linker-script target,
     # which is then added as a dependency of the MCU target.  This ensures the linker script will exist
     # by the time we need it.
@@ -80,11 +81,69 @@ function(mbed_setup_linker_script mbed_os_target mbed_baremetal_target target_de
     foreach(TARGET ${mbed_baremetal_target} ${mbed_os_target})
         add_dependencies(${TARGET} mbed-linker-script)
 
-        # Add linker flags to the MCU target to pick up the preprocessed linker script
+        # When building the Mbed internal tests, the tests get created before the mbed-os target does.  So, the normal logic 
-        target_link_options(${TARGET}
+        # in mbed_set_post_build() to set the linker script does not work.  So, we need to instead attach the linker script to 
+        # the mbed-os and mbed-baremetal libraries themselves, so it will get picked up automatically.
+        # This prevents a custom linker script from being used in STANDALONE mode, but we don't need to do that.
+        set_target_properties(${TARGET} PROPERTIES LINKER_SCRIPT_PATH  ${LINKER_SCRIPT_PATH})
+
+        #  add linker script only for tests
+        if(MBED_IS_STANDALONE)
+            target_link_options(${TARGET}
             INTERFACE
                 "-T" "${LINKER_SCRIPT_PATH}"
-        )
+            )
+        endif()
     endforeach()
 
 endfunction(mbed_setup_linker_script)
+
+
+#
+# Change the linker script to a custom supplied script instead of the built in.
+# this function is called by mbed_set_post_build(target linker_script)
+#
+# target: CMake target for Mbed OS
+# new_linker_script_path: raw linker script
+#
+function(mbed_set_custom_linker_script target new_linker_script_path)
+
+    set(RAW_LINKER_SCRIPT_PATHS  ${CMAKE_CURRENT_SOURCE_DIR}/${new_linker_script_path})
+    set(CUSTOM_LINKER_SCRIPT_PATH ${CMAKE_CURRENT_BINARY_DIR}/${target}.link_spript.ld)
+
+    # To avoid path limits on Windows, we create a "response file" and set the path to it as a
+    # global property. We need this solely to pass the compile definitions to GCC's preprocessor,
+    # so it can expand any macro definitions in the linker script.
+    get_property(linker_defs_response_file GLOBAL PROPERTY COMPILE_DEFS_RESPONSE_FILE)
+
+    get_filename_component(RAW_LINKER_SCRIPT_NAME ${RAW_LINKER_SCRIPT_PATHS} NAME)
+    get_filename_component(LINKER_SCRIPT_NAME ${CUSTOM_LINKER_SCRIPT_PATH} NAME)
+
+    add_custom_command(
+        TARGET
+            ${target}
+        PRE_LINK
+        COMMAND 
+            ${CMAKE_COMMAND} -E echo "Preprocess custom linker script: ${RAW_LINKER_SCRIPT_NAME} -> ${LINKER_SCRIPT_NAME}"
+        COMMAND
+            ${CMAKE_C_COMPILER} @${linker_defs_response_file}
+            -E -x assembler-with-cpp
+            -include ${CMAKE_BINARY_DIR}/mbed-os/mbed-target-config.h
+            -P ${RAW_LINKER_SCRIPT_PATHS}
+            -o ${CUSTOM_LINKER_SCRIPT_PATH}
+        DEPENDS
+            ${RAW_LINKER_SCRIPT_PATHS}
+            ${linker_defs_response_file}
+            ${target_defines_header}
+        WORKING_DIRECTORY
+            ${CMAKE_CURRENT_SOURCE_DIR}
+        VERBATIM
+    )
+
+    # Add linker flags to the target to pick up the preprocessed linker script
+    target_link_options(${target}
+        PRIVATE
+            "-T" "${CUSTOM_LINKER_SCRIPT_PATH}"
+    )
+
+endfunction(mbed_set_custom_linker_script)

tools/cmake/mbed_target_functions.cmake

@@ -126,7 +126,32 @@ endfunction()
 #
 # Set post build operations
 #
+# target: the affected target
+# 2nd arg: optional, linker script path 
+#
 function(mbed_set_post_build target)
+    # add linker script. Skip for greentea test code, there the linker script is set in mbed_setup_linker_script()
+    if (NOT MBED_IS_STANDALONE)
+        if("${ARGN}" STREQUAL "")
+            if(TARGET mbed-os)
+                get_target_property(LINKER_SCRIPT_PATH mbed-os LINKER_SCRIPT_PATH)
+                target_link_options(${target}
+                PRIVATE
+                    "-T" "${LINKER_SCRIPT_PATH}"
+            )
+            elseif(TARGET mbed-baremetal)
+                get_target_property(LINKER_SCRIPT_PATH mbed-baremetal LINKER_SCRIPT_PATH)
+                target_link_options(${target}
+                PRIVATE
+                    "-T" "${LINKER_SCRIPT_PATH}"
+            )
+            endif()
+        else()
+            message(STATUS "${target} uses custom linker script  ${ARGV2}")
+            mbed_set_custom_linker_script(${target} ${ARGV2})
+        endif()
+    endif()
+
     # The mapfile name includes the top-level target name and the
     # executable suffix for all toolchains as CMake hardcodes the name of the
     # diagnostic output file for some toolchains.