Disabling L2 Cache in ARM Cortex-A55: Performance Verification and Implementation

ARM Cortex-A55 L2 Cache Disabling for Performance Verification

The ARM Cortex-A55 is a highly efficient mid-range CPU core designed for power-efficient performance in embedded systems and mobile devices. It features a hierarchical cache architecture, including L1 and L2 caches, which are critical for reducing memory latency and improving overall system performance. However, in certain scenarios, such as performance verification or debugging, it may be necessary to disable the L2 cache while keeping the L1 cache operational. This allows developers to isolate the impact of the L2 cache on system performance or to test specific memory access patterns without the influence of the L2 cache.

The L1 cache in the Cortex-A55 is split into separate instruction and data caches, each typically sized at 32 KB. The L2 cache, on the other hand, is a unified cache shared across all cores in a cluster, with sizes ranging from 128 KB to 512 KB depending on the implementation. Disabling the L2 cache while keeping the L1 cache enabled requires a deep understanding of the Cortex-A55’s memory system and the ARM architecture’s cache control mechanisms.

The primary challenge in disabling the L2 cache lies in the fact that the Cortex-A55 does not provide a direct software mechanism to disable the L2 cache independently. Instead, the L2 cache is typically managed through system control registers and coherency protocols that are tightly integrated with the core’s memory subsystem. This integration ensures that cache coherency is maintained across multiple cores and caches, but it also complicates the process of selectively disabling the L2 cache.

Memory System Configuration and Cache Control Mechanisms

The Cortex-A55’s memory system is designed to optimize performance and power efficiency through a combination of hardware and software mechanisms. The L1 and L2 caches are managed through a set of system control registers, including the Cache Size Identification Register (CCSIDR), Cache Type Register (CTR), and System Control Register (SCTLR). These registers provide information about the cache hierarchy and allow software to enable or disable specific cache features.

The L2 cache in the Cortex-A55 is typically controlled through the L2 Cache Control Register (L2CTLR), which is part of the CPU’s memory-mapped register space. The L2CTLR allows software to configure various aspects of the L2 cache, including its size, associativity, and replacement policy. However, the L2CTLR does not provide a direct bit to disable the L2 cache entirely. Instead, the L2 cache can be effectively disabled by invalidating its contents and preventing new data from being cached.

To disable the L2 cache, software must first invalidate all cache lines in the L2 cache to ensure that no stale data remains. This is done by writing to the L2 Cache Invalidate Register (L2IMVA), which invalidates the cache line corresponding to a specific memory address. After invalidating the L2 cache, software can prevent new data from being cached by configuring the memory attributes in the Memory Attribute Indirection Register (MAIR) and the Translation Table Base Register (TTBR). By marking memory regions as non-cacheable, software can effectively bypass the L2 cache for those regions.

However, disabling the L2 cache in this manner can have significant performance implications. The L2 cache plays a critical role in reducing memory latency and improving data throughput, especially in multi-core systems where multiple cores may access shared data. Disabling the L2 cache can lead to increased memory latency and reduced system performance, particularly for workloads that rely heavily on memory access patterns that benefit from caching.

Implementing L2 Cache Disabling and Performance Analysis

To disable the L2 cache in the Cortex-A55, follow these steps:

1. Invalidate the L2 Cache: Before disabling the L2 cache, it is essential to invalidate all cache lines to ensure that no stale data remains. This can be done by writing to the L2 Cache Invalidate Register (L2IMVA) for each cache line. Alternatively, software can use the Data Cache Invalidate by Set/Way (DC ISW) instruction to invalidate the entire L2 cache. This step ensures that any data previously cached in the L2 cache is flushed and no longer accessible.

2. Configure Memory Attributes: After invalidating the L2 cache, software must configure the memory attributes to prevent new data from being cached. This is done by setting the appropriate bits in the Memory Attribute Indirection Register (MAIR) and the Translation Table Base Register (TTBR). By marking memory regions as non-cacheable, software can ensure that all memory accesses bypass the L2 cache. This step is critical to maintaining data consistency and preventing unintended caching of data.

3. Monitor System Performance: Once the L2 cache is disabled, it is important to monitor system performance to understand the impact of disabling the L2 cache. This can be done using performance counters and profiling tools that measure memory latency, cache misses, and overall system throughput. By comparing performance metrics with and without the L2 cache enabled, developers can gain insights into the role of the L2 cache in their specific workload and identify potential performance bottlenecks.

4. Re-enable the L2 Cache: After completing the performance verification, it is important to re-enable the L2 cache to restore normal system operation. This can be done by reversing the steps above: reconfiguring the memory attributes to allow caching and re-enabling the L2 cache through the L2 Cache Control Register (L2CTLR). It is also recommended to invalidate the L2 cache again after re-enabling it to ensure that any stale data is flushed.

The process of disabling the L2 cache in the Cortex-A55 requires careful consideration of the system’s memory architecture and the potential performance implications. While it is possible to disable the L2 cache for performance verification or debugging purposes, it is not a recommended configuration for normal operation due to the significant performance degradation that can result. Developers should use this technique judiciously and only when necessary to gain insights into the system’s memory behavior.

In conclusion, disabling the L2 cache in the ARM Cortex-A55 is a complex process that requires a deep understanding of the core’s memory system and cache control mechanisms. By following the steps outlined above, developers can effectively disable the L2 cache for performance verification while minimizing the impact on system performance. However, it is important to carefully analyze the performance implications and re-enable the L2 cache once the verification is complete to restore normal system operation.