Скачать или смотреть Resolving EAGAIN Error When Reading M4 Messages on STM32MP1

Learn how to fix the `EAGAIN` error when trying to read messages sent from M4 to A7 on the STM32MP1 board using VIRTUAL UART communication.
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This video is based on the question https://stackoverflow.com/q/74517849/ asked by the user 'Telmo' ( https://stackoverflow.com/u/20561773/ ) and on the answer https://stackoverflow.com/a/74520081/ provided by the user 'Mathieu' ( https://stackoverflow.com/u/1212012/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Resolving EAGAIN Error When Reading M4 Messages on STM32MP1

If you're using the STM32MP157-DK2 board and facing issues while trying to read messages from the M4 core to the A7 core, you're not alone. Many developers experience hiccups when setting up communication channels between these processors, particularly when working with VIRTUAL UART. This post will guide you through understanding why you're seeing the EAGAIN error and how to fix it.

Understanding the Problem

You are attempting to send a message from the M4 core to the A7 core using the VIRTUAL UART (ttyRPMSG0), but you're unable to read the message on the A7 side. Instead of getting the expected data, the read function returns a size of -1 along with an empty data buffer.

The Error Code Explained

The error you’re encountering is denoted by the code 11: EAGAIN. According to the Linux man pages, this error indicates that:

The file descriptor refers to a file other than a socket.

The file has been marked as non-blocking (with the O_NONBLOCK flag).

A read operation on the file descriptor would block because no data is ready to be read.

When you try to read from a non-blocking file descriptor and no data is available, the system signals this condition with the EAGAIN error.

Proposed Solutions

To resolve the EAGAIN error and successfully read messages sent from M4, consider the following approaches:

1. Modify the Open Call

The simplest solution may be to remove the non-blocking option from your open call:

[[See Video to Reveal this Text or Code Snippet]]

By omitting the O_NONBLOCK | O_NDELAY flags, your read operation will now block until data becomes available, allowing you to retrieve messages successfully.

2. Implement a Waiting Mechanism

If you want to retain the non-blocking nature of the open call but still successfully read messages, you can implement a mechanism to wait until data is available before attempting to read. This could be done using polling or select mechanisms. For example, you could periodically check if data is ready to be read using:

[[See Video to Reveal this Text or Code Snippet]]

3. Check Your Communication Logic

Ensure that the message from M4 is sent correctly and that you are checking for the right conditions on both sides. Confirm that the M4 is indeed sending the message before the A7 attempts to read it.

Conclusion

Encountering the EAGAIN error when trying to read from the VIRTUAL UART can be frustrating, but by understanding the implications of non-blocking IO and adjusting your code, you can facilitate effective communication between the A7 and M4 processors. By either removing the non-blocking flag or implementing a robust waiting mechanism, you can successfully manage inter-process communication on your STM32MP157-DK2 board.

Feel free to reach out if you have any questions or need further clarification on any aspect of this process!