[Feature Request] Flexible CMake configuration

[neuschwander-lmi]

Is your feature request related to a problem? Please describe.
We have multiple CMake targets in one project, which build the same controller with different configurations.
Currently, FreeRTOS builds as a single static library with fixed configuration for each CMake invocation. This would require separate CMake build directories for different configurations.

This refers both to:

• port selection via FREERTOS_PORT
• configuration variables via linking to freertos_config and including FreeRTOSConfig.h

Describe the solution you'd like
Switch from STATIC and OBJECT libraries to INTERFACE libraries.
Different application configurations explicitly link against different ports or different config-INTERFACE-libraries. FreeRTOS itself does not explicitly link to port or config libraries.

Advantages:

• Different application targets can use different freertos_config libraries
• Different application targets can use different ports (e.g. one NS and one NTZ configuration)

Disadvantages:

• As far as I understand, some headers are hidden from the application by not exposing their directories as target_include_dirs. This would not work when using the INTERFACE targets. However, this would remain effective when using the backwards-compatible mode.

Describe alternatives you've considered

• Keeping local/internal patches
• Writing own CMake code instead of using the provided one

How many devices will this feature impact?
I cannot disclose this information.

What are your project timelines?
I will implement a local/internal solution in the next few days. This issue is mostly an offer to integrate the local work upstream. Timeline for the PR would be ~1-4weeks.

Additional context
I am willing to provide a PR for this feature if the issue is accepted as a valid improvement.

Example usage:

// old:
if(CONFIG_SELECTION STREQUAL "NTZ")
        set(FREERTOS_PORT GCC_ARM_CM33_NTZ_NONSECURE)
        target_include_directories(freertos_config INTERFACE ${CMAKE_CURRENT_LIST_DIR}/config/ntz)
elseif(CONFIG_SELECTION STREQUAL "NS")
        set(FREERTOS_PORT GCC_ARM_CM33_NONSECURE)
        target_include_directories(freertos_config INTERFACE ${CMAKE_CURRENT_LIST_DIR}/config/ns)
endif()

target_link_libraries(myApp
        freertos_kernel
)

// cmake -DCONFIG_SELECTION=NTZ && cmake --build myApp
// cmake -DCONFIG_SELECTION=NS && cmake --build myApp

// new:
set(FREERTOS_PORTS GCC_ARM_CM33_NTZ_NONSECURE GCC_ARM_CM33_NONSECURE)
target_include_directories(freertos_config_ntz INTERFACE ${CMAKE_CURRENT_LIST_DIR}/config/ntz)
target_include_directories(freertos_config_ns INTERFACE ${CMAKE_CURRENT_LIST_DIR}/config/ns)

target_link_libraries(myApp_NTZ
        freertos_config_ntz
        freertos_kernel_port_GCC_ARM_CM33_NTZ_NONSECURE
)
target_link_libraries(myApp_NS
        freertos_config_ns
        freertos_kernel_port_GCC_ARM_CM33_NONSECURE
)

// cmake && cmake --build myApp_NTZ myApp_NS

Example port definition in portable/CMakeLists.txt

addPort(GCC_ARM_CM33_NTZ_NONSECURE
        WITH_MPU_WRAPPERS
        SOURCES
        GCC/ARM_CM33_NTZ/non_secure/port.c
        GCC/ARM_CM33_NTZ/non_secure/portasm.c
        GCC/ARM_CM33_NTZ/non_secure/mpu_wrappers_v2_asm.c
        INCLUDE_DIRS
        ${CMAKE_CURRENT_LIST_DIR}/GCC/ARM_CM33_NTZ/non_secure
)

CMake interface changes:

• Add a configuration variable FREERTOS_PORTS which is a list of all ports which should be available (not all ports can be used with the same compiler). Only the ports which are linked against will actually get built.
• For each entry in FREERTOS_PORTS, there will be a corresponding freertos_kernel_port_* INTERFACE target

Backwards compatibility:

• If FREERTOS_PORTS is not defined, it defaults to a list with one item equal to FREERTOS_PORT to make the use of multi-ports optional.
• add_library(freertos_kernel STATIC) links against freertos_config and freertos_kernel_port_${FREERTOS_PORT} if FREERTOS_PORT is defined to make the use of interface targets optional.