Unaligned Usage Fault in Cortex-M7 During memcpy from BKPSRAM to SRAM1

ARM Cortex-M7 Unaligned Access Fault During memcpy Operation

The issue at hand involves an unaligned usage fault occurring on an ARM Cortex-M7 processor during a memcpy operation. The memcpy function is used to copy data from the Backup SRAM (BKPSRAM) region to the SRAM1 region. This operation works flawlessly on a Cortex-M4 but triggers an unaligned access fault on the Cortex-M7 when the destination pointer is odd-aligned and the data size exceeds 4 bytes. The fault is specifically caused by an LDR instruction, which is expected to support unaligned memory transfers. This discrepancy between the Cortex-M4 and Cortex-M7 behavior raises questions about the underlying architectural differences and memory attributes that might be influencing this behavior.

The Cortex-M7 processor, unlike the Cortex-M4, has a more complex memory system with additional features such as cache, Tightly Coupled Memory (TCM), and a Memory Protection Unit (MPU). These features can significantly impact how memory accesses are handled, especially when dealing with unaligned accesses. The Cortex-M7 also has stricter requirements for memory access alignment in certain regions, particularly when dealing with device memory or regions marked with specific attributes.

The fault occurs only when the destination pointer is odd-aligned, and the data size exceeds 4 bytes. This suggests that the Cortex-M7 is attempting to perform a memory access that is not aligned to the natural boundaries of the data type being accessed. The LDR instruction, which is supposed to support unaligned accesses, is failing in this specific scenario, indicating that there might be additional constraints or configurations that need to be considered.

Memory Attribute Mismatch and MPU Configuration

One of the primary causes of this issue is the memory attribute mismatch between the source and destination regions. The BKPSRAM region, which is the source of the memcpy operation, is often mapped as a device memory region. Device memory regions have specific attributes that enforce stricter alignment requirements and do not support unaligned accesses. This is in contrast to normal memory regions, which can typically handle unaligned accesses without issues.

The Cortex-M7’s Memory Protection Unit (MPU) plays a crucial role in defining the attributes of different memory regions. By default, the BKPSRAM region might be configured with device memory attributes, which enforce aligned accesses. If the MPU is not properly configured to override these default attributes, any unaligned access to this region will result in a fault. This is particularly problematic when the destination pointer in SRAM1 is odd-aligned, as the memcpy operation might attempt to perform an unaligned read from the BKPSRAM region.

Another factor to consider is the size of the data being copied. The fault only occurs when the data size exceeds 4 bytes. This suggests that the Cortex-M7 might be attempting to perform a 64-bit access (LDRD instruction) when the data size is large enough. The LDRD instruction requires the source address to be aligned to an 8-byte boundary, and any deviation from this alignment will result in an unaligned access fault. This behavior is consistent with the observation that the fault occurs only when the data size exceeds 4 bytes.

The Cortex-M7’s cache configuration can also influence this behavior. If the cache is enabled for the BKPSRAM region, it might attempt to cache the data being read, which could lead to unaligned accesses if the cache line size is not properly aligned with the data being accessed. This is less likely to be the primary cause of the issue, but it is still a factor worth considering when troubleshooting.

Implementing Proper MPU Configuration and Data Alignment

To resolve this issue, the first step is to ensure that the MPU is properly configured to handle the memory attributes of the BKPSRAM region. The MPU should be configured to override the default device memory attributes and allow unaligned accesses. This can be achieved by defining a new MPU region for the BKPSRAM with the appropriate attributes. The following steps outline how to configure the MPU to allow unaligned accesses:

1. Define a new MPU region for BKPSRAM: The MPU region should be configured with normal memory attributes, which allow unaligned accesses. This can be done by setting the appropriate bits in the MPU Region Attribute and Size Register (MPU_RASR). The exact configuration will depend on the specific implementation, but the key is to ensure that the region is marked as normal memory rather than device memory.

2. Ensure proper alignment of the destination pointer: While the Cortex-M7 supports unaligned accesses in normal memory regions, it is still good practice to ensure that the destination pointer is properly aligned. This can be done by adjusting the pointer to the nearest aligned address before performing the memcpy operation. This will prevent any potential issues with unaligned accesses in the destination region.

3. Use a custom memcpy implementation: If the standard memcpy function is causing issues, it might be necessary to implement a custom version that takes into account the specific memory attributes and alignment requirements of the source and destination regions. This custom implementation can ensure that all accesses are properly aligned and that the memory attributes are respected.

4. Disable cache for BKPSRAM region: If the cache is enabled for the BKPSRAM region, it might be necessary to disable it to prevent any potential issues with unaligned accesses. This can be done by configuring the cache settings in the Cortex-M7’s cache control registers. Disabling the cache for the BKPSRAM region will ensure that all accesses are performed directly to the memory, without any intermediate caching.

5. Check for LDRD instruction usage: If the fault is occurring due to the use of the LDRD instruction, it might be necessary to modify the memcpy implementation to avoid using this instruction for unaligned accesses. This can be done by ensuring that the source address is always aligned to an 8-byte boundary before performing the copy operation.

By following these steps, the unaligned access fault can be resolved, and the memcpy operation can be performed without issues on the Cortex-M7. It is important to carefully consider the memory attributes and alignment requirements of both the source and destination regions when performing memory operations on the Cortex-M7, as these factors can significantly impact the behavior of the processor.

Detailed Analysis of Cortex-M7 Memory System and MPU Configuration

The Cortex-M7’s memory system is significantly more complex than that of the Cortex-M4, with additional features such as cache, TCM, and MPU. These features provide greater flexibility and performance but also introduce additional complexity when it comes to memory access and alignment. Understanding how these features interact is crucial for resolving issues such as the unaligned access fault during memcpy.

Memory Attributes and MPU Configuration

The MPU is a key component of the Cortex-M7’s memory system, allowing developers to define the attributes of different memory regions. These attributes include access permissions, cacheability, and alignment requirements. By default, certain memory regions, such as device memory, are configured with stricter alignment requirements to ensure proper operation. However, these default settings might not always be suitable for all use cases, and it might be necessary to override them using the MPU.

When configuring the MPU, it is important to consider the specific requirements of the application. For example, if the application requires unaligned accesses to a particular memory region, the MPU should be configured to allow this. This can be done by defining a new MPU region with the appropriate attributes. The following table outlines the key attributes that should be considered when configuring the MPU:

Attribute	Description
Access Permissions	Defines whether the region is readable, writable, or executable.
Cacheability	Determines whether the region is cacheable and how it is cached.
Alignment	Specifies the alignment requirements for accesses to the region.
Shareability	Defines whether the region is shared between multiple processors.
Memory Type	Specifies whether the region is normal memory, device memory, or strongly ordered.

By carefully configuring these attributes, it is possible to ensure that the memory system behaves as expected and that unaligned accesses are handled correctly.

Cache Configuration and Its Impact on Memory Access

The Cortex-M7’s cache system can significantly impact memory access behavior, particularly when dealing with unaligned accesses. The cache is designed to improve performance by storing frequently accessed data in a faster memory location. However, if the cache is not properly configured, it can lead to issues such as unaligned access faults.

When the cache is enabled for a particular memory region, it attempts to cache the data being accessed. If the data is not properly aligned with the cache line size, the cache might attempt to perform an unaligned access, which could result in a fault. To prevent this, it is important to ensure that the cache is properly configured for the specific memory region.

The following steps outline how to configure the cache to prevent unaligned access faults:

1. Disable cache for device memory regions: Device memory regions, such as BKPSRAM, should typically not be cached. This is because device memory often has specific access requirements that are not compatible with caching. Disabling the cache for these regions will ensure that all accesses are performed directly to the memory, without any intermediate caching.

2. Align data with cache line size: When accessing normal memory regions, it is important to ensure that the data is properly aligned with the cache line size. This will prevent the cache from attempting to perform unaligned accesses, which could result in a fault. The cache line size can be determined from the Cortex-M7’s cache control registers.

3. Use cache maintenance operations: If the cache is enabled for a particular memory region, it might be necessary to perform cache maintenance operations to ensure that the data is properly synchronized between the cache and the main memory. This can be done using the Cortex-M7’s cache maintenance instructions, such as DCACHE_CLEAN and DCACHE_INVALIDATE.

By following these steps, it is possible to ensure that the cache is properly configured and that unaligned access faults are prevented.

Custom memcpy Implementation for Cortex-M7

In some cases, the standard memcpy function might not be suitable for use on the Cortex-M7, particularly when dealing with unaligned accesses. In these cases, it might be necessary to implement a custom version of memcpy that takes into account the specific memory attributes and alignment requirements of the source and destination regions.

The following steps outline how to implement a custom memcpy function for the Cortex-M7:

1. Check alignment of source and destination pointers: Before performing the copy operation, it is important to check the alignment of the source and destination pointers. If either pointer is unaligned, it might be necessary to adjust the pointers to the nearest aligned address before performing the copy operation.

2. Use appropriate load/store instructions: Depending on the alignment of the source and destination pointers, it might be necessary to use different load/store instructions. For example, if the source pointer is aligned but the destination pointer is unaligned, it might be necessary to use a combination of aligned load and unaligned store instructions.

3. Handle small data sizes separately: For small data sizes (less than 4 bytes), it might be more efficient to handle the copy operation using a series of byte or half-word accesses rather than attempting to perform a word or double-word access. This will prevent any potential issues with unaligned accesses.

4. Use data synchronization barriers: If the cache is enabled for the source or destination regions, it might be necessary to use data synchronization barriers to ensure that the data is properly synchronized between the cache and the main memory. This can be done using the Cortex-M7’s data synchronization barrier (DSB) instruction.

By following these steps, it is possible to implement a custom memcpy function that is optimized for the Cortex-M7 and that handles unaligned accesses correctly.

Conclusion

The unaligned access fault during memcpy on the Cortex-M7 is a complex issue that requires a thorough understanding of the processor’s memory system and MPU configuration. By carefully configuring the MPU, cache, and memory attributes, it is possible to prevent unaligned access faults and ensure that the memcpy operation is performed correctly. Additionally, implementing a custom memcpy function that takes into account the specific requirements of the Cortex-M7 can further improve performance and reliability.

In summary, the key to resolving this issue lies in understanding the interaction between the Cortex-M7’s memory system, MPU configuration, and cache behavior. By carefully configuring these components and implementing appropriate workarounds, it is possible to ensure that the memcpy operation is performed without issues on the Cortex-M7.