ARM Cortex-M4 MPU Configuration and Write Buffer Performance Analysis

Understanding MPU Bufferable Attributes and Their Impact on Cortex-M4 Performance

The ARM Cortex-M4 processor, widely used in embedded systems, provides a Memory Protection Unit (MPU) to enforce memory access rules and attributes. One of the key attributes configurable via the MPU is the "bufferable" attribute, which influences how write operations are handled by the processor’s write buffer. This attribute is particularly relevant when dealing with peripherals and internal SRAM, as it can affect both performance and error handling. However, the interaction between the MPU, write buffer, and memory attributes is nuanced and often misunderstood. This guide delves into the technical details of bufferable attributes, their implications, and how to configure them effectively for optimal performance and reliability.

Bufferable Attribute Behavior in Peripheral and SRAM Regions

The bufferable attribute, when enabled, allows write operations to be buffered, meaning the processor can continue executing subsequent instructions without waiting for the write to complete. This can significantly improve performance, especially when writing to slower peripherals. However, the behavior of the bufferable attribute differs depending on whether the memory region is mapped to peripherals or SRAM.

For peripheral regions (e.g., addresses 0x40000000 – 0x5FFFFFFF), enabling the bufferable attribute can lead to faster write operations. This is because the processor does not need to wait for the write to complete before moving on to the next instruction. However, this also introduces the possibility of imprecise bus faults. If a write operation to an invalid peripheral address is buffered, the resulting bus fault may be reported asynchronously, making it harder to pinpoint the exact instruction that caused the fault. In contrast, non-bufferable writes generate precise bus faults, where the fault is reported immediately, allowing for easier debugging.

For internal SRAM, the impact of the bufferable attribute is less pronounced. While enabling the bufferable attribute may still provide a slight performance improvement, the difference in clock cycles is typically minimal. This is because SRAM accesses are inherently faster than peripheral accesses, and the write buffer’s effect is less noticeable. Additionally, SRAM accesses do not generate bus faults, so the error-handling implications of the bufferable attribute do not apply.

The cacheable attribute, often used in conjunction with the bufferable attribute, also plays a role in memory performance. When the cacheable attribute is enabled, the processor may use its internal write buffer for write-through or write-back caching strategies. However, the Cortex-M4 does not have a cache, so the cacheable attribute primarily affects how the write buffer is utilized. For example, setting the cacheable attribute without enabling the bufferable attribute (C=1, B=0) configures the memory as write-through cacheable, where writes are immediately propagated to memory but may still benefit from the write buffer. On the other hand, enabling both the cacheable and bufferable attributes (C=1, B=1) configures the memory as write-back cacheable, where writes are initially held in the buffer and later written to memory, potentially improving performance.

Performance Implications of MPU and Write Buffer Configuration

The configuration of the MPU and write buffer can have a measurable impact on system performance, particularly in applications with frequent memory accesses. To illustrate this, consider the following test scenarios, each with different MPU and write buffer configurations:

1. Write Buffer Disabled (DISDEFWBUF set) and MPU Not Used: In this configuration, the write buffer is disabled, and the MPU is not active. This serves as a baseline for performance comparison. The measured CPU clock cycles for 500 program cycles were 1,696,833.

2. Write Buffer Enabled and MPU Not Used: Enabling the write buffer without MPU configuration resulted in a slight performance improvement, with the measured clock cycles dropping to 1,676,663. This demonstrates the benefit of the write buffer in reducing wait states for write operations.

3. Write Buffer Enabled and MPU Enabled with Basic Settings: When the MPU is enabled with basic settings (e.g., FLASH_MEMORY_ATT = MPU_RASR_C_Msk, PERIPHERALS_ATT = MPU_RASR_B_Msk | MPU_RASR_S_Msk, INT_SRAM_MEMORY_ATT = 0), the performance is slightly worse than the write buffer-only configuration, with clock cycles measured at 1,695,451. This suggests that the MPU introduces some overhead, even with minimal configuration.

4. Write Buffer Enabled and MPU Enabled with SRAM Cacheable Attribute: Enabling the cacheable attribute for SRAM (INT_SRAM_MEMORY_ATT = MPU_RASR_C_Msk | MPU_RASR_S_Msk) results in performance similar to the write buffer-only configuration, with clock cycles measured at 1,676,825. This indicates that the cacheable attribute can mitigate some of the MPU’s overhead.

5. Write Buffer Enabled and MPU Enabled with SRAM Bufferable and Cacheable Attributes: Finally, enabling both the bufferable and cacheable attributes for SRAM (INT_SRAM_MEMORY_ATT = MPU_RASR_B_Msk | MPU_RASR_C_Msk | MPU_RASR_S_Msk) yields performance nearly identical to the write buffer-only configuration, with clock cycles measured at 1,676,832. This confirms that the bufferable attribute has little additional impact on SRAM performance when the cacheable attribute is already enabled.

These results highlight the importance of carefully configuring the MPU and write buffer to balance performance and functionality. While enabling the write buffer generally improves performance, the MPU’s impact depends on the specific attributes configured for each memory region.

Optimizing MPU and Write Buffer Configuration for Cortex-M4 Systems

To achieve optimal performance and reliability in Cortex-M4 systems, consider the following recommendations for configuring the MPU and write buffer:

1. Enable the Write Buffer for Peripheral Regions: For peripheral regions, enabling the bufferable attribute (B=1) can significantly improve write performance. However, be aware of the potential for imprecise bus faults and ensure that your error-handling routines can accommodate asynchronous fault reporting.

2. Use Cacheable Attribute for SRAM: For internal SRAM, enabling the cacheable attribute (C=1) can improve performance by allowing the write buffer to be used effectively. The bufferable attribute (B=1) provides minimal additional benefit for SRAM and can typically be omitted unless specific performance requirements dictate otherwise.

3. Minimize MPU Overhead: When configuring the MPU, aim to minimize the number of regions and attributes to reduce overhead. Use the simplest configuration that meets your security and performance requirements.

4. Test and Measure Performance: As demonstrated in the test scenarios, the impact of MPU and write buffer configuration can vary depending on the application. Use performance measurement tools, such as cycle counters or debuggers, to evaluate different configurations and identify the optimal setup for your specific use case.

5. Consult Manufacturer Documentation: The behavior of memory attributes can vary depending on the specific microcontroller implementation. Consult the manufacturer’s documentation, such as STMicroelectronics’ STM32 reference manuals, for details on how memory attributes are handled in your target device.

By following these guidelines, you can effectively configure the MPU and write buffer to maximize the performance and reliability of your Cortex-M4-based embedded systems. Understanding the nuances of bufferable and cacheable attributes, as well as their interaction with the MPU, is key to achieving optimal system behavior.