ARM Cortex-A53 SError Exception Triggered by PCIe Configuration Writes
The ARM Cortex-A53 core, when integrated into a Xilinx Zynq Ultrascale MPSoC FPGA, can encounter SError exceptions during PCI Express (PCIe) configuration write accesses. This issue manifests specifically when firmware, running in bare-metal mode, attempts to perform a memory write to a System-on-Chip (SoC) peripheral register as part of PCIe enumeration. Configuration reads, however, complete successfully without triggering any faults. The SError exception is an asynchronous abort, which means it is not directly tied to the instruction causing the fault but rather arises from external or internal system conditions. This behavior is particularly problematic in prototype boards, where some units function correctly while others exhibit this fault, suggesting potential hardware design or manufacturing inconsistencies.
The SError exception is a catch-all for system errors that do not fit into other exception categories, such as prefetch aborts or data aborts. It is often triggered by conditions like bus errors, memory controller faults, or external slave errors (SLVERR). In this case, the fault occurs during a PCIe configuration write, which involves accessing the SoC’s peripheral registers. The Cortex-A53 Technical Reference Manual (TRM) provides some insight into the conditions that can trigger SError, including external access errors, but the exact root cause requires deeper investigation into the system’s memory hierarchy, PCIe subsystem, and the interaction between the ARM core and the FPGA fabric.
Memory Hierarchy and PCIe Subsystem Interaction Issues
The Cortex-A53 core relies on a coherent memory hierarchy and a well-defined interface with the PCIe subsystem to perform configuration reads and writes. When a configuration write is attempted, the core issues a memory transaction that traverses the interconnect fabric, reaches the PCIe controller, and ultimately targets the peripheral register. Several factors can disrupt this process and lead to an SError exception.
One possible cause is a misconfiguration or fault in the PCIe controller or its integration with the ARM core. The PCIe controller must correctly decode the address space, handle the transaction, and ensure that the write operation completes successfully. If the controller encounters an error, such as an unsupported address or a protocol violation, it may signal an SLVERR (slave error) back to the ARM core. This error is then propagated as an SError exception.
Another potential cause is a timing or synchronization issue in the memory hierarchy. The Cortex-A53 core uses caches to improve performance, but this introduces complexity when dealing with memory-mapped I/O (MMIO) operations, such as PCIe configuration writes. If the cache coherency mechanisms are not properly managed, stale or inconsistent data can lead to unexpected behavior. For example, a write operation might target a cached address that has not been invalidated, causing the transaction to fail.
Additionally, the physical implementation of the prototype boards may introduce variability that affects signal integrity or power delivery. Subtle differences in PCB layout, trace lengths, or decoupling capacitor placement can lead to marginal conditions that trigger faults in some units but not others. These hardware-related issues can exacerbate the problem and make it difficult to diagnose.
Diagnosing and Resolving SError Exceptions in Cortex-A53 Systems
To diagnose and resolve SError exceptions triggered by PCIe configuration writes, a systematic approach is required. This involves analyzing the Exception Syndrome Register (ESR), inspecting the memory hierarchy, and verifying the PCIe subsystem configuration.
The first step is to decode the ESR to determine the specific cause of the SError. The ESR provides detailed information about the exception, including the instruction that triggered it and the type of error. For Cortex-A53, the ESR_EL1 register contains fields such as EC (Exception Class) and ISS (Instruction Specific Syndrome), which can help identify whether the fault was caused by an external access error, a cache maintenance operation, or another condition. By examining these fields, you can narrow down the root cause and focus your investigation on the relevant subsystem.
Next, inspect the memory hierarchy and cache coherency mechanisms. Ensure that the Cortex-A53 core’s caches are properly configured for MMIO operations. This may involve disabling caching for specific address ranges or using memory barriers to enforce ordering. The Data Synchronization Barrier (DSB) and Data Memory Barrier (DMB) instructions can be used to ensure that memory operations complete in the correct order and that the cache is properly invalidated before performing PCIe configuration writes.
Verify the PCIe subsystem configuration and integration with the ARM core. Check that the PCIe controller is correctly mapped into the address space and that the address decoding logic is functioning as expected. Use a logic analyzer or debugger to capture the transaction and inspect the signals for anomalies. If the PCIe controller supports error reporting, enable these features and monitor for any error conditions.
Finally, address any potential hardware-related issues. Review the PCB design and layout to identify areas where signal integrity or power delivery could be improved. Use tools such as signal integrity simulations or power integrity analysis to validate the design and ensure that it meets the required specifications. If possible, compare the working and non-working units to identify any differences that could explain the observed behavior.
By following these steps, you can systematically diagnose and resolve SError exceptions in Cortex-A53 systems. The key is to combine software and hardware analysis techniques to identify the root cause and implement the necessary fixes. This approach ensures that the system operates reliably and that the PCIe configuration writes complete successfully without triggering faults.
