Fix: Mcumgr SMP Server Fails On Zephyr Serial Interface

Dec 4, 2025 by Alex Johnson 56 views

mcumgr SMP Server Fails with Serial Interface on Zephyr RTOS

Introduction

This article addresses a critical issue encountered while using the mcumgr SMP server with a serial interface on the Zephyr Real-Time Operating System (RTOS). Specifically, the smp_svr sample fails to function correctly when built for serial communication, preventing both console output and mcumgr client connectivity. This problem affects multiple boards and stems from a conflict between shell and log settings within the Zephyr environment. Understanding and resolving this issue is crucial for developers relying on mcumgr for device management and firmware updates over serial connections.

The mcumgr SMP (Simple Management Protocol) server is an essential component in many embedded systems, providing a standardized way to manage and update devices remotely. When this server fails to initialize or operate correctly, it can halt development and deployment processes. This article provides a detailed analysis of the problem, including reproduction steps, potential workarounds, and insights into the underlying causes. By addressing these issues, developers can ensure the reliable operation of their Zephyr-based systems and maintain efficient device management capabilities.

Properly configuring the serial interface and resolving conflicts between system components are key to ensuring the smooth operation of Zephyr RTOS in embedded systems. The symptoms of this issue include the absence of console output, which makes debugging and monitoring difficult, and the inability to connect with the mcumgr client, which prevents remote management and updates. This article walks through the intricacies of these problems, offering practical solutions and configuration adjustments to restore functionality and maintain a stable development environment. The goal is to equip developers with the knowledge needed to overcome these challenges and leverage the full potential of Zephyr RTOS in their projects.

Problem Description

The smp_svr sample, as detailed in the Zephyr documentation, is intended to provide a straightforward implementation of the mcumgr server over a serial interface. However, when built using the recommended commands and configuration files, the application fails to initialize correctly. The primary symptoms include:

No output to the debug console after MCUboot completes.
Inability of the mcumgr client to connect over serial, resulting in a timeout error.

This issue has been observed on multiple hardware platforms, indicating a systemic problem within the configuration or code. The conflict between shell and log settings appears to be the root cause, preventing the proper initialization of both the console output and the serial communication channels required by mcumgr. Addressing this conflict is essential to restoring the intended functionality of the smp_svr sample and enabling remote device management.

Reproduction Steps

To reproduce the bug, follow these steps:

Build the smp_svr sample using the following command:

west build -b frdm_rw612 samples/subsys/mgmt/mcumgr/smp_svr --sysbuild --pristine -- -DEXTRA_CONF_FILE="serial.conf;fs.conf;shell-mgmt.conf"

Flash the resulting image to one of the affected boards (e.g., frdm_rw612, frdm_mcxn947, or mimxrt1060_evk@C).
Observe the debug console output. You should see the MCUboot messages, but no subsequent output from the application.
Attempt to connect to the device using the mcumgr client:
```
mcumgr -c COMPORT image list
```
You should receive an Error: NMP timeout error, indicating that the mcumgr client cannot communicate with the device.

These steps confirm that the issue is reproducible and affects the basic functionality of the mcumgr server over serial.

Impact

The functional impact of this bug is significant. The inability to communicate with the mcumgr server over serial prevents essential device management tasks, such as firmware updates, configuration changes, and monitoring. This limitation hinders development, testing, and deployment processes, as developers cannot rely on mcumgr for remote device management.

While the system may remain partially usable, the lack of mcumgr functionality restricts its capabilities and increases the complexity of managing and maintaining the device. This issue can be particularly problematic in scenarios where physical access to the device is limited, making remote management essential.

Workarounds

Several workarounds can mitigate the issue, although each has its drawbacks:

Disabling the Shell

Disabling the Zephyr shell by setting CONFIG_SHELL=n in shell-mgmt.conf allows the application to log to the console and enables mcumgr communication. However, this approach sacrifices the shell's functionality, which is valuable for debugging and interactive control.

Modifying Log Settings

Changing the log settings to the following can also resolve the issue:

CONFIG_LOG_PRINTK=n
CONFIG_LOG_MODE_IMMEDIATE=y

This configuration alters the way logs are handled, potentially avoiding the conflict with the shell. However, it may also affect the performance or behavior of the logging system.

Proposed Solutions

Given the limitations of the workarounds, a more comprehensive solution is needed. The best approach would involve identifying and resolving the underlying conflict between the shell and log settings. This could include:

Reviewing the initialization sequence of the shell and logging subsystems to identify potential race conditions or resource conflicts.
Adjusting the configuration options to ensure that the shell and logging systems can coexist without interfering with each other.
Providing a clear and documented configuration profile for serial communication that avoids these conflicts.

Detailed Analysis of Workarounds

Let's delve deeper into the available workarounds, assessing their advantages and disadvantages to provide a clearer picture of their suitability for different scenarios.

Disabling the Shell: A Closer Look

Disabling the shell by setting CONFIG_SHELL=n in the shell-mgmt.conf file is a straightforward solution that often resolves the conflict preventing proper serial communication. When the shell is disabled, the system's resources are freed up, and the logging system can function without interference. The primary advantage of this method is its simplicity and effectiveness in restoring mcumgr functionality.

However, this approach comes at the cost of losing the interactive shell, which is a powerful tool for debugging, testing, and configuring the device during development and in the field. The shell allows developers to execute commands, inspect system state, and diagnose issues in real-time. Disabling it can significantly hamper these activities, making it more difficult to troubleshoot problems and manage the device.

In scenarios where remote management via mcumgr is paramount and interactive debugging is less critical, disabling the shell may be an acceptable trade-off. However, for development and testing environments, the loss of the shell's capabilities can be a significant drawback. Therefore, it's important to carefully weigh the pros and cons before implementing this workaround.

Modifying Log Settings: A Detailed Examination

Altering the log settings by setting CONFIG_LOG_PRINTK=n and CONFIG_LOG_MODE_IMMEDIATE=y is another potential workaround. This configuration changes how the Zephyr RTOS handles log messages. By disabling CONFIG_LOG_PRINTK, the system stops using the printk function for logging, which can sometimes conflict with other serial communication processes. Enabling CONFIG_LOG_MODE_IMMEDIATE ensures that log messages are processed immediately, which can help avoid buffering issues that might interfere with mcumgr.

The advantage of this approach is that it allows the shell to remain enabled while still addressing the conflict. This means developers can retain the interactive debugging capabilities of the shell while also ensuring that mcumgr functions correctly. However, modifying log settings can have unintended consequences on system performance and behavior. Immediate logging can increase CPU usage and potentially impact real-time performance, which may be unacceptable in some applications.

Additionally, disabling printk may affect other parts of the system that rely on it for logging. It's crucial to thoroughly test the system after making these changes to ensure that all components continue to function as expected. Therefore, while this workaround offers a more balanced approach by preserving the shell, it requires careful consideration and testing to avoid introducing new issues.

Long-Term Solutions and Best Practices

While the workarounds mentioned above can provide immediate relief, they are not ideal long-term solutions. The best approach is to address the root cause of the conflict between the shell and log settings. This requires a deeper understanding of the Zephyr RTOS architecture and the interactions between different system components. Here are some potential avenues for investigation and resolution:

Identifying the Root Cause

The first step is to pinpoint the exact cause of the conflict. This may involve analyzing the initialization sequences of the shell and logging subsystems, examining the interrupt handling routines, and profiling the system's resource usage. Tools like debuggers, system analyzers, and logic analyzers can be invaluable in this process. By carefully monitoring the system's behavior, developers can identify the specific point at which the conflict occurs and understand the underlying mechanisms.

Optimizing Configuration Options

Once the root cause is identified, the next step is to optimize the configuration options to avoid the conflict. This may involve adjusting the priorities of different tasks, modifying the interrupt configurations, or changing the memory allocation strategies. The goal is to find a configuration that allows the shell and logging systems to coexist harmoniously without interfering with each other.

Providing Clear Documentation

Finally, it's essential to provide clear and comprehensive documentation for configuring the serial interface in Zephyr RTOS. This documentation should include best practices for avoiding conflicts between the shell and logging systems, as well as troubleshooting tips for resolving common issues. By providing developers with the knowledge and tools they need to configure their systems correctly, we can prevent these types of problems from occurring in the first place.

Conclusion

The issue of the mcumgr SMP server failing with the serial interface on Zephyr RTOS is a significant problem that can hinder development and deployment efforts. While workarounds such as disabling the shell or modifying log settings can provide temporary relief, they are not ideal long-term solutions. The best approach is to identify and address the root cause of the conflict between the shell and logging systems, optimize the configuration options, and provide clear documentation for developers. By taking these steps, we can ensure the reliable operation of mcumgr over serial and enable efficient device management in Zephyr-based systems.

For more information on Zephyr RTOS and its capabilities, please visit the Zephyr Project website. Understanding the intricacies of Zephyr's configuration and debugging tools will help in resolving such conflicts and optimizing system performance.