Fixing Sysbuild MCUboot Python Interpreter Issue

by Alex Johnson 49 views

When working with embedded systems, integrating bootloaders like MCUboot with build systems such as Sysbuild is a common practice. However, encountering issues during the build process can be frustrating. This article addresses a specific problem where Sysbuild, when used with MCUboot, selects an incorrect Python interpreter, leading to a ModuleNotFoundError for the pykwalify module. Understanding the root cause and implementing the appropriate solutions can streamline your development workflow.

Understanding the Problem

When you're using Sysbuild to generate the MCUboot bootloader for a Zephyr project, you might encounter a situation where the build process fails. This often happens with an error message indicating that the pykwalify module cannot be found. The core issue lies in the Python interpreter that MCUboot's sub-build is using.

When building with the --sysbuild option, the main project appropriately uses the Python interpreter from your workspace's virtual environment (.venv). However, the MCUboot sub-build, which is a separate CMake invocation, sometimes mistakenly selects a different Python interpreter bundled within the Zephyr Workbench. This bundled Python interpreter typically resides in a directory like C:/Users/<user>/.zinstaller/tools/python/python/python.exe.

The problem arises because this bundled Python interpreter lacks the necessary pykwalify module. As a result, the MCUboot configuration step fails during the execution of list_boards.py, which depends on pykwalify for its operation. To illustrate, consider the following scenario:

  • Top-level Sysbuild: Correctly identifies and uses the Python interpreter from the virtual environment (Found Python3: C:/Users/.../.venv/Scripts/python.exe).
  • MCUboot Sub-build: Incorrectly selects the bundled Python interpreter (Found Python3: C:/Users/.../.zinstaller/tools/python/python/python.exe).

This discrepancy occurs because Sysbuild spawns a second CMake project specifically for MCUboot, located at deps/bootloader/mcuboot/boot/zephyr. This sub-project doesn't inherit the Python environment from the main project, leading to the interpreter mix-up.

Decoding the Error Message

To accurately diagnose the issue, understanding the error message is crucial. When the build fails, you'll likely see an error similar to the following:

CMake Error at C:/Users/nicol/radaon-fan-alarm-firmware/deps/zephyr/cmake/modules/boards.cmake:195 (message):
  Error finding board: esp32c6_devkitc

  Error message: Traceback (most recent call last):
    File "C:\Users\nicol\radaon-fan-alarm-firmware\deps\zephyr\scripts\list_boards.py", line 13, in <module>
      import list_hardware
    File "C:\Users\nicol\radaon-fan-alarm-firmware\deps\zephyr\scripts\list_hardware.py", line 12, in <module>
      import pykwalify.core
  ModuleNotFoundError: No module named 'pykwalify'

This error stack clearly indicates that the pykwalify module is missing when the list_boards.py script is executed. The script, part of the Zephyr build system, requires pykwalify to process board-specific configurations. The ModuleNotFoundError signals that the Python interpreter being used doesn't have access to this essential module. This message is a key indicator that the MCUboot sub-build is using the wrong Python interpreter.

Additionally, the error occurs within the context of CMake, specifically in the boards.cmake module. This file is responsible for identifying and configuring the target board for your project. The failure to find the board (esp32c6_devkitc in the example) is a direct consequence of the missing pykwalify module, as the board identification process relies on it.

Identifying the Incorrect Python Interpreter

To confirm that the MCUboot sub-build is indeed using the wrong Python interpreter, you can examine the build logs more closely. During the CMake configuration phase, the build system outputs messages indicating which Python interpreter it has found. Compare the Python interpreter paths for the top-level Sysbuild and the MCUboot sub-build. If they differ, and the MCUboot sub-build points to the bundled Python interpreter within Zephyr Workbench, you've likely identified the root cause of the problem.

For instance, in the provided example, the top-level Sysbuild correctly uses the Python interpreter from the virtual environment:

Found Python3: C:/Users/nicol/.zinstaller/.venv/Scripts/python.exe

However, the MCUboot sub-build selects the bundled Python interpreter:

Found Python3: C:/Users/nicol/.zinstaller/tools/python/python/python.exe

This discrepancy confirms that the MCUboot sub-build is not using the intended Python environment, which leads to the ModuleNotFoundError.

Solutions to Resolve the Issue

There are several approaches to address this Python interpreter discrepancy. Let's explore each in detail:

1. Ensure pykwalify is Installed in the Bundled Python Environment

One straightforward solution is to manually install the pykwalify module into the bundled Python environment. This ensures that the interpreter used by the MCUboot sub-build has the necessary dependencies. Here's how you can do it:

  1. Locate the Bundled Python Executable: Identify the exact path to the bundled Python executable. This is typically found within the Zephyr Workbench installation directory, similar to C:/Users/<user>/.zinstaller/tools/python/python/python.exe.

  2. Open a Command Prompt or Terminal: Launch a command prompt or terminal window.

  3. Navigate to the Bundled Python's Scripts Directory: Use the cd command to navigate to the Scripts directory within the bundled Python installation. For example:

    cd C:/Users/<user>/.zinstaller/tools/python/python/Scripts

  4. Use pip to Install pykwalify: Execute the pip install pykwalify command. This will install the pykwalify module and its dependencies into the bundled Python environment.

    pip install pykwalify

After completing these steps, the bundled Python interpreter should have access to pykwalify, and the MCUboot sub-build should no longer encounter the ModuleNotFoundError. However, this solution requires manual intervention and may need to be repeated if the Zephyr Workbench installation is updated or reinstalled.

2. Modify the CMake Configuration to Force the Correct Python Interpreter

A more robust solution involves modifying the CMake configuration to explicitly specify the Python interpreter to be used for the MCUboot sub-build. This ensures that the correct interpreter is always selected, regardless of the environment.

  1. Identify the Correct Python Interpreter: Determine the path to the Python interpreter within your virtual environment. This is typically located in the .venv directory within your project.
  2. Set the PYTHON CMake Variable: You can set the PYTHON CMake variable to the correct Python interpreter path. This can be done in several ways:

    • CMake Command-Line Argument: Pass the -DPYTHON=<path_to_python> argument when invoking CMake. For example:

      west build -b <board_name> -- -DPYTHON=<path_to_python>

    • CMake Cache Variable: Set the PYTHON variable in the CMake cache. You can do this using the cmake-gui tool or by directly editing the CMakeCache.txt file.

    • Environment Variable: Set the PYTHON environment variable before running CMake. For example:

      export PYTHON=<path_to_python>
west build -b <board_name>

By setting the PYTHON CMake variable, you explicitly tell CMake which Python interpreter to use. This ensures that the MCUboot sub-build uses the same interpreter as the main project, resolving the pykwalify issue.

3. Update Zephyr and Related Tools

In some cases, the issue might stem from outdated versions of Zephyr, West, or other related tools. Updating to the latest versions can often resolve compatibility issues and bugs that might be causing the incorrect Python interpreter selection. Use the following commands to update your environment:

  • Update Zephyr: Follow the instructions in the Zephyr documentation to update your Zephyr installation. This typically involves using the west update command.
  • Update West: Use the pip install -U west command to update West to the latest version.
  • Update Other Tools: Ensure that other relevant tools, such as CMake and Python packages, are also up-to-date.

Keeping your development environment current is a best practice that can prevent many common issues and ensure you're benefiting from the latest features and bug fixes.

Step-by-Step Troubleshooting Guide

To effectively troubleshoot this issue, follow these steps:

  1. Review the Build Output: Carefully examine the build output for error messages related to ModuleNotFoundError or issues with finding the board configuration.
  2. Identify Python Interpreter Paths: Compare the Python interpreter paths used by the top-level Sysbuild and the MCUboot sub-build. Look for discrepancies.
  3. Verify pykwalify Installation: Check if pykwalify is installed in both the virtual environment and the bundled Python environment.
  4. Implement a Solution: Choose one of the solutions described above (installing pykwalify, modifying CMake configuration, or updating tools) and apply it.
  5. Rebuild the Project: After implementing a solution, rebuild your project to see if the issue is resolved.
  6. Repeat if Necessary: If the issue persists, try a different solution or seek further assistance.

Conclusion

The ModuleNotFoundError for pykwalify when using Sysbuild with MCUboot can be a tricky issue to diagnose. By understanding the root cause—the incorrect Python interpreter selection for the MCUboot sub-build—you can effectively implement the appropriate solutions. Whether you choose to install pykwalify in the bundled Python environment, modify the CMake configuration, or update your tools, addressing this issue will ensure a smoother development experience with Zephyr and MCUboot.

For more information on Zephyr, MCUboot, and embedded systems development, check out trusted resources like the Zephyr Project Documentation. By following these troubleshooting steps and understanding the underlying causes, you can overcome this hurdle and get back to building your embedded projects.