ARM Cortex-R Cache Architecture and Default Configuration
The ARM Cortex-R series processors, such as the one used in the TI-AWR294x, are designed for real-time applications where deterministic performance is critical. These processors typically feature a hierarchical cache architecture, including Level 1 (L1) instruction and data caches, and sometimes Level 2 (L2) unified caches. The L1 cache is often split into separate instruction (I-cache) and data (D-cache) caches to optimize performance for both instruction fetch and data access operations.
In the case of the TI-AWR294x, the reference manual indicates that the processor supports a total of 16KB of L1 cache. However, the default configuration appears to allocate only 4KB, which is significantly less than the maximum supported capacity. This discrepancy arises because the cache configuration is determined at design time and is not dynamically adjustable through software. The registers such as CLIDR (Cache Level ID Register), CCISDR (Cache Size Identification Register), and CSSELR (Cache Size Selection Register) are used to report the cache architecture and configuration, but they do not provide a mechanism to alter the cache size or partitioning.
The CLIDR register provides information about the cache hierarchy, including the number of cache levels and whether each level has a unified cache or separate instruction and data caches. The CCISDR register reports detailed information about the cache selected by the CSSELR register. The CSSELR register allows software to select a specific cache for inspection by specifying the cache level and whether it is an instruction or data cache. However, these registers are read-only and cannot be used to modify the cache configuration.
Misconceptions About Cache Configuration Registers
One common misconception is that the cache configuration registers, such as CSOR (Cache Size Override Register), can be used to modify the cache size or partitioning. In the case of the TI-AWR294x, the user attempted to overwrite the CSOR register to allocate 8KB for the data cache and 8KB for the instruction cache. However, this approach is ineffective because the CSOR register, like other cache configuration registers, is read-only and does not support dynamic reconfiguration.
The cache size and partitioning are determined by the processor’s microarchitecture and are fixed at design time. The CSOR register, if it exists, is typically used to report the cache size and cannot be written to by software. Attempting to write to this register will have no effect on the cache configuration. This is a critical point to understand when working with ARM Cortex-R processors, as it highlights the limitations of software in modifying hardware configurations.
Another misconception is that the CSSELR register can be used to select both the instruction and data caches simultaneously. The CSSELR register allows software to select either the instruction cache or the data cache for inspection, but it does not support selecting both at the same time. This is because the instruction and data caches are separate entities within the processor’s microarchitecture, and their configurations are independent of each other.
Proper Cache Management and Configuration Techniques
Given that the cache configuration is fixed at design time, software must work within the constraints of the hardware. However, there are several techniques that can be used to optimize cache usage and ensure that the available cache is utilized effectively.
First, it is important to understand the cache architecture and configuration of the specific processor being used. This information can be obtained from the processor’s reference manual and by inspecting the cache configuration registers. Knowing the cache size, associativity, and line size is essential for optimizing memory access patterns and minimizing cache misses.
Second, software can use cache maintenance operations to ensure that the cache is used efficiently. For example, the Data Cache Clean (DCC) and Data Cache Invalidate (DCI) operations can be used to manage the contents of the data cache. Similarly, the Instruction Cache Invalidate (ICI) operation can be used to manage the contents of the instruction cache. These operations are particularly important in real-time systems where deterministic performance is critical.
Third, software can use memory barriers to ensure that memory accesses are performed in the correct order. Memory barriers are particularly important in multi-core systems where different cores may have different views of memory. The Data Synchronization Barrier (DSB) and Instruction Synchronization Barrier (ISB) operations can be used to enforce memory ordering and ensure that the processor’s view of memory is consistent.
Finally, software can use prefetching techniques to reduce cache misses and improve performance. Prefetching involves loading data into the cache before it is needed, which can reduce the latency of memory accesses. The Prefetch Data (PLD) and Prefetch Instruction (PLI) operations can be used to prefetch data and instructions into the cache.
In summary, while the cache configuration of ARM Cortex-R processors is fixed at design time, software can use a variety of techniques to optimize cache usage and ensure that the available cache is utilized effectively. Understanding the cache architecture and configuration, using cache maintenance operations, enforcing memory ordering with memory barriers, and employing prefetching techniques are all essential for achieving optimal performance in real-time systems.
Conclusion
The ARM Cortex-R series processors, such as the one used in the TI-AWR294x, feature a fixed cache configuration that is determined at design time. The cache configuration registers, such as CLIDR, CCISDR, and CSSELR, are used to report the cache architecture and configuration but cannot be used to modify the cache size or partitioning. Misconceptions about the ability to modify cache configuration through software, such as by writing to the CSOR register, are common but incorrect.
To optimize cache usage in ARM Cortex-R processors, software must work within the constraints of the hardware. This involves understanding the cache architecture and configuration, using cache maintenance operations, enforcing memory ordering with memory barriers, and employing prefetching techniques. By following these best practices, software can ensure that the available cache is utilized effectively and that the system achieves optimal performance.
In conclusion, while the cache configuration of ARM Cortex-R processors is fixed, there are several techniques that can be used to optimize cache usage and ensure that the system performs efficiently. Understanding the limitations of the hardware and employing best practices for cache management are essential for achieving optimal performance in real-time systems.
