hwmv2: Build with ARC MWDT became broken after HMWv2 was implemented.

[kokas-a]

Describe the bug Build with ARC MWDT became broken after HWMv2 was implemented.

ARC MWDT toolchain doesn't contain it's own DTC-preprocessor. It uses preprocessor, which is a part of Zephyr-SDK. In current implementailon ${ZEPHYR_SDK_INSTALL_DIR}/cmake/zephyr/target.cmake file should be included on very early build stages to obtain the preprocessor via CROSS_COMPILE variable (it defined inside pointed .cmake file). Modules call sequence: zephyr_default->dtc->FindHostTools->toolchain/arcmwdt/generic.cmake->${ZEPHYR_SDK_INSTALL_DIR}/cmake/zephyr/target.cmake

The point is target.cmake requires ARCH variable to be defined. New HWMv2 defines ARCH variable much more later in arch_v2 module via CONFIG_ARCH.

In case of MWDT such construction as set(ARCH ${CONFIG_ARCH}) is not applicable as Kconifg module was not yes initialized. And it at the same time requires dtc, and we get circullar dependence.

So, here I'd like to ask for a advice how to fix the issue in the correct way.

To Reproduce

1. export ZEPHYR_TOOLCHAIN_VARIANT=arcmwdt
2. west build -b nsim/nsim_em samples/hello_world/
3. 
-- Board: nsim, identifier: nsim_em
-- ARCMWDT_TOOLCHAIN_PATH is not set, use default toolchain from METAWARE_ROOT: '/home/user/ARC_202312/MetaWare'
-- Found host-tools: zephyr 0.16.3 (/home/user/sandbox/zephyr-sdk-0.16.3)
-- Found dts preprocessor: CMAKE_DTS_PREPROCESSOR-NOTFOUND (Zephyr SDK 0.16.3)

Expected behavior Build not fails:

...
-- Found host-tools: zephyr 0.16.3 (/home/user/sandbox/zephyr-sdk-0.16.3)

CMake Debug Log at /home/user/sandbox/zephyr-sdk-0.16.3/cmake/zephyr/target.cmake:32 (if):
  Variable "ARCH" was accessed using READ_ACCESS with value "arc".
Call Stack (most recent call first):
  /home/user/sandbox/zephyrproject/zephyr/cmake/toolchain/arcmwdt/generic.cmake:26 (include)
  /home/user/sandbox/zephyrproject/zephyr/cmake/modules/FindHostTools.cmake:103 (include)
  /home/user/sandbox/zephyrproject/zephyr/cmake/modules/dts.cmake:9 (find_package)
  /home/user/sandbox/zephyrproject/zephyr/cmake/modules/zephyr_default.cmake:129 (include)
  /home/user/sandbox/zephyrproject/zephyr/share/zephyr-package/cmake/ZephyrConfig.cmake:66 (include)
  /home/user/sandbox/zephyrproject/zephyr/share/zephyr-package/cmake/ZephyrConfig.cmake:97 (include_boilerplate)
  CMakeLists.txt:5 (find_package)

-- Found dts preprocessor: /home/user/sandbox/zephyr-sdk-0.16.3/arc-zephyr-elf/bin/arc-zephyr-elf-gcc (Zephyr SDK 0.16.3)
...

Impact Showstopper

Logs and console output

-- Application: /home/user/sandbox/zephyrproject/zephyr/samples/hello_world
-- CMake version: 3.25.1
-- Found Python3: /home/user/sandbox/zephyrproject/.venv/bin/python3 (found suitable version "3.11.2", minimum required is "3.8") found components: Interpreter 
-- Cache files will be written to: /home/user/.cache/zephyr
-- Zephyr version: 3.6.99 (/home/user/sandbox/zephyrproject/zephyr)
-- Found west (found suitable version "1.1.0", minimum required is "0.14.0")
-- Board: nsim, identifier: nsim_em
-- ARCMWDT_TOOLCHAIN_PATH is not set, use default toolchain from METAWARE_ROOT: '/home/user/ARC_202312/MetaWare'
-- Found host-tools: zephyr 0.16.3 (/home/user/sandbox/zephyr-sdk-0.16.3)
CMake Debug Log at /home/user/sandbox/zephyr-sdk-0.16.3/cmake/zephyr/target.cmake:32 (if):
  Variable "ARCH" was accessed using UNKNOWN_READ_ACCESS with value "".
Call Stack (most recent call first):
  /home/user/sandbox/zephyrproject/zephyr/cmake/toolchain/arcmwdt/generic.cmake:26 (include)
  /home/user/sandbox/zephyrproject/zephyr/cmake/modules/FindHostTools.cmake:104 (include)
  /home/user/sandbox/zephyrproject/zephyr/cmake/modules/dts.cmake:9 (find_package)
  /home/user/sandbox/zephyrproject/zephyr/cmake/modules/zephyr_default.cmake:132 (include)
  /home/user/sandbox/zephyrproject/zephyr/share/zephyr-package/cmake/ZephyrConfig.cmake:66 (include)
  /home/user/sandbox/zephyrproject/zephyr/share/zephyr-package/cmake/ZephyrConfig.cmake:92 (include_boilerplate)
  CMakeLists.txt:5 (find_package)

-- Found dts preprocessor: CMAKE_DTS_PREPROCESSOR-NOTFOUND (Zephyr SDK 0.16.3)
-- Found Dtc: /home/user/sandbox/zephyr-sdk-0.16.3/sysroots/x86_64-pokysdk-linux/usr/bin/dtc (found suitable version "1.6.0", minimum required is "1.4.6") 
-- Found BOARD.dts: /home/user/sandbox/zephyrproject/zephyr/boards/synopsys/nsim/nsim_nsim_em.dts
CMake Error at /home/user/sandbox/zephyrproject/zephyr/cmake/modules/extensions.cmake:4205 (message):
  failed to preprocess devicetree files (error code No such file or
  directory)

Environment (please complete the following information):

• OS: Linux
• Toolchain: arcmwdt
• Commit SHA or Version used: 1a55caf8263e11bc22f93b88e2ccf99eb30f690f

Additional context

[fabiobaltieri]

Bumped into this issue as well (reported in https://github.com/zephyrproject-rtos/zephyr/issues/68999), in our case I ended up removing the depenency of ARCH from our cmake file, not sure if it can be done in your case but I'm also not sure whether it's possible to get ARCH at that stage in the current architecture.

[aescolar]

@tejlmand or @nordicjm , @57300 can you please take a look at this?

[aescolar]

@kokas-a After https://github.com/zephyrproject-rtos/zephyr/pull/71163 was merged, can this be closed?

[kokas-a]

Fixed in https://github.com/zephyrproject-rtos/zephyr/pull/71163