Скачать или смотреть Understanding Cache Behavior with DMA in ARM Cortex M7: A Deep Dive into Memory Management

Explore how cache operations can affect changes made by DMA devices on an ARM Cortex M7, and learn best practices for handling concurrent memory access.
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Understanding Cache Behavior with DMA in ARM Cortex M7: A Deep Dive into Memory Management

When developing drivers for hardware devices, especially with complex microcontrollers like the STM32H743, it’s crucial to understand how various components interact with each other. A frequent issue that arises is the interaction between Direct Memory Access (DMA) devices and the cache memory managed by the CPU. In this post, we’ll break down a specific scenario involving the ARM Cortex M7 architecture and discuss what happens when a cache clean operation occurs after a DMA writes to a memory location.

The Dilemma: Can Cache Clean Overwrite DMA Changes?

Imagine you are dealing with two memory locations, x and y, that share the same cache line in a normal, write-back cacheable memory configuration. You start by defining initial values for these memory locations, followed by a series of operations performed by both the CPU and a DMA device. The sequence of operations can be illustrated as:

Start with x = x1 and y = y1, where the cache line is invalid.

The CPU reads y.

The DMA device updates x to x2 in memory.

The CPU then sets y to y2.

Finally, the CPU cleans the cache line.

The question arises: when the cache clean operation is executed, what value does the DMA device see for x? Will it reflect the change made by the DMA device, or will it be overwritten?

Dissecting the Problem

To address this question, we need to analyze the sequence of operations step by step:

Initial State

Before any operations, both variables are stored in memory and have initial values:

Memory: x = x1, y = y1

Cache: Invalid

Step-by-Step Breakdown

CPU reads y:

At this point, the CPU pulls both x and y into the cache, marking them as valid.

Memory: x = x2, y = y1 (in memory, as the DMA has not yet acted)

Cache: x = x1, y = y1 (the initial cached values)

DMA updates x to x2:

The DMA updates x in memory, resulting in:

Memory: x = x2, y = y1

Cache remains unchanged: x = x1, y = y1

CPU sets y to y2:

When the CPU updates y, it marks its cache as dirty because it is now out of sync with memory:

Memory: x = x2, y = y2

Cache: x = x1, y = y2 (now the cache has stale data for x)

CPU cleans the cache line:

Lastly, when the CPU cleans the cache line, it writes back the contents of the cache line to memory. Since both x and y belong to the same cache line, the values in the cache will overwrite what is in memory.

Memory ends up with:

x = x1 (overwritten value due to cache clean)

y = y2

Conclusion: DMA’s View of x

From the above steps, the DMA would see x equal to x1 due to the cache’s behavior during the clean operation. Here’s why:

The cache flush writes back the data in the cache to memory.

Since x and y are stored in the same cache line, the updates made by the DMA are effectively lost (overwritten) by the CPU's cached values upon cache cleaning.

What Happens If x and y Are in Different Cache Lines?

If the memory locations x and y do not share the same cache line, only the relevant cache line for y would be affected during the clean operation. In this case, the DMA device would see the updated value of x, and this scenario highlights the importance of understanding cache line organization.

Final Thoughts

In conclusion, understanding how cache operations interact with DMA and memory is crucial for efficient driver development on platforms like the ARM Cortex M7. The integrity of data management relies on awareness of these interactions, and developers should consider both cache line configurations and the timing of DMA operations relative to CPU cache cleaning. This knowledge not only helps in troubleshooting issues but also enables more robust and reliable system