Fixing `CONFIG_DCACHE=y` Issue With `spi_nxp_lpspi_dma.c` On RT1060_evk

by Chloe Fitzgerald 72 views

Introduction

Hey guys! Today, we're diving deep into a tricky issue encountered while using the Zephyr RTOS on the RT1060_evk board. Specifically, we're looking at a bug where enabling the Data Cache (CONFIG_DCACHE=y) causes the spi_nxp_lpspi_dma.c driver to malfunction, leading to garbage data being transferred over SPI. This is a critical issue, especially when relying on SPI communication for essential functionalities. We'll explore the symptoms, reproduction steps, impact, and potential solutions to this problem. So, buckle up and let’s get started!

Understanding the Bug: Symptoms and Scope

The core issue revolves around the interaction between the Data Cache (DCACHE), Direct Memory Access (DMA), and the Low Power Serial Peripheral Interface (LPSPI) on the NXP RT1060_evk board. When CONFIG_DCACHE=y is enabled, and DMA is used for SPI communication via the spi_nxp_lpspi_dma.c driver, the data transmitted becomes corrupted. This means the data received at the other end is not what was originally sent, rendering the SPI communication useless. This problem specifically manifests when using lpspi1 for SPI communication.

Reproducing the Issue

To reproduce this bug, you'll need an RT1060_evk board and a Zephyr RTOS environment configured to use the mimxrt1060_evk/mimxrt1062/qspi target. Follow these steps:

  1. Enable LPSPI1: Ensure that lpspi1 is enabled in your device tree source (.dts) file. This usually involves setting the status to "okay". The default device tree files in the evaluation kit often have EDMA enabled for LPSPI1.
&lpspi1 {
    status = "okay";
};
  1. Enable DCACHE: Set CONFIG_DCACHE=y in your Zephyr project's configuration file (prj.conf).
CONFIG_DCACHE=y

  1. Build and Flash: Build your Zephyr application and flash it onto the RT1060_evk board.

  2. Run SPI Communication: Implement a simple SPI communication routine using the lpspi1 interface. Transmit a known set of data.

  3. Observe the Output: Check the received data at the receiving end. If the data is different from what was transmitted, you've successfully reproduced the bug.

Isolating the Problem: Disabling DMA and Cache

To confirm the interaction between DCACHE and DMA, you can try disabling each separately:

Disabling EDMA Only

To disable EDMA (Enhanced DMA), modify your device tree overlay file to remove the dmas and dma-names properties from the lpspi1 node.

&lpspi1 {
    /delete-property/ dmas;
    /delete-property/ dma-names;
    status = "okay";
};

With DMA disabled, SPI communication should work correctly, indicating that DMA is a contributing factor to the issue.

Disabling Cache Only

To disable the data cache, set CONFIG_DCACHE=n in your prj.conf file.

CONFIG_DCACHE=n

With DCACHE disabled, SPI communication should also work correctly, highlighting the role of the data cache in this bug.

The Impact: Functional Limitations

This bug can lead to significant functional limitations. If your application relies on SPI communication for critical tasks such as reading sensor data, communicating with peripherals, or storing data in external memory, the corruption of SPI data can lead to unpredictable behavior and system malfunction. The system remains usable in other aspects, but the SPI functionality is severely hampered.

Affected Environments

This issue has been observed across multiple Zephyr versions, including 4.1, 4.2, and the main branch, indicating that it's a persistent problem that needs to be addressed at a lower level, possibly within the driver or hardware abstraction layer.

Diving Deeper: Why is this Happening?

To truly understand this bug, we need to delve into the interaction between DCACHE, DMA, and the SPI driver.

The Role of DCACHE

The Data Cache (DCACHE) is a small, fast memory that stores frequently accessed data. When the CPU needs to read data from memory, it first checks the cache. If the data is present in the cache (a cache hit), it can be accessed quickly. If the data is not in the cache (a cache miss), the CPU retrieves it from the main memory and stores a copy in the cache for future use. This significantly improves performance by reducing the need to access slower main memory.

The Role of DMA

Direct Memory Access (DMA) allows peripherals to access system memory directly, without involving the CPU. This is crucial for high-speed data transfers, as it frees up the CPU to perform other tasks. In the case of SPI communication, DMA can be used to transfer data between the SPI controller and memory, improving the efficiency of the communication.

The Interaction and the Problem

The issue arises when DCACHE is enabled, and DMA is used for SPI communication. Here's a breakdown of what likely happens:

  1. Data in Memory: The data to be transmitted over SPI is prepared in system memory.

  2. DMA Transfer: The DMA controller is configured to transfer this data from memory to the SPI controller.

  3. Cache Incoherency: If the data in memory is also present in the DCACHE, the DMA controller might read stale data from memory if the cache hasn't been updated with the latest changes. Conversely, if the SPI controller receives data via DMA and writes it to memory, the DCACHE might not be updated, leading to the CPU reading stale data from the cache.

This cache incoherency is the root cause of the garbage data being transmitted over SPI. The data in the cache and the data in main memory are out of sync, leading to incorrect data being used for SPI communication.

Potential Solutions and Workarounds

Now that we understand the problem, let's explore some potential solutions and workarounds.

1. Disabling DCACHE (CONFIG_DCACHE=n)

As demonstrated earlier, disabling DCACHE resolves the issue. However, this comes at the cost of performance, as the CPU will need to access main memory more frequently. This might be an acceptable workaround for applications where SPI communication is critical, and performance is not the primary concern.

2. Disabling DMA for SPI

Another workaround is to disable DMA for SPI communication, as shown by deleting the dmas and dma-names properties in the device tree. This will force the CPU to handle the data transfer, which ensures cache coherency. However, this approach can significantly reduce the performance of SPI communication, especially for large data transfers.

3. Cache Management Operations

A more sophisticated solution involves explicitly managing the cache. This can be done by performing cache invalidate and cache flush operations before and after DMA transfers.

  • Cache Invalidate: This operation removes the data from the cache, forcing the CPU to read the latest data from main memory.

  • Cache Flush: This operation writes the data from the cache back to main memory, ensuring that main memory contains the most up-to-date data.

By carefully using these operations, we can maintain cache coherency and avoid the data corruption issue. The specific functions to use for cache management depend on the architecture and the Zephyr API. For example, on ARM architectures, you might use functions like sys_cache_invd_range() and sys_cache_flush_range().

Here’s a conceptual example of how you might use cache management operations in your SPI driver:

// Before DMA transfer
sys_cache_invd_range(data_buffer, data_length);

// Perform DMA transfer
spi_transceive_dma(data_buffer, data_length);

// After DMA transfer
sys_cache_flush_range(data_buffer, data_length);

4. Driver-Level Fix

The ideal solution is to fix the spi_nxp_lpspi_dma.c driver to handle cache coherency correctly. This might involve implementing cache management operations within the driver itself or using a hardware abstraction layer that automatically handles cache coherency. This would provide the best balance between performance and reliability.

5. Hardware Considerations

In some cases, hardware-level configurations can also impact cache coherency. For example, ensuring that the memory regions used for DMA transfers are marked as cacheable or non-cacheable can influence the behavior of the cache. Consulting the hardware documentation and reference manuals for the RT1060_evk and the LPSPI controller can provide valuable insights into these configurations.

Conclusion: Towards a Robust Solution

The issue of CONFIG_DCACHE=y breaking spi_nxp_lpspi_dma.c on the RT1060_evk is a significant challenge that highlights the complexities of embedded systems development. By understanding the interaction between DCACHE, DMA, and SPI, we can develop effective solutions and workarounds. While disabling DCACHE or DMA can provide immediate relief, they come with performance tradeoffs. The most promising approach involves careful cache management or a driver-level fix that ensures cache coherency during DMA transfers. As we continue to explore and refine these solutions, we can ensure the robust and reliable operation of SPI communication on the RT1060_evk and other similar platforms. Keep experimenting, keep learning, and let's build a more stable and efficient embedded world together! Remember to always validate your solutions thoroughly to ensure they meet your application's specific requirements and performance goals.