ARM Cortex-R52 PMU Cycle Counter Not Updating: Configuration and Debugging Guide

ARM Cortex-R52 PMU Cycle Counter Configuration and Initialization Issues

The ARM Cortex-R52 processor includes a Performance Monitoring Unit (PMU) that provides performance counters, including a cycle counter (CCNT), to measure and analyze system performance. The cycle counter is a critical tool for profiling and understanding the timing behavior of software running on the Cortex-R52. However, improper configuration or initialization of the PMU can result in the cycle counter not updating, leading to incorrect or zero readings. This issue is often encountered when enabling the PMU, configuring the cycle counter, or reading its value.

The cycle counter is controlled through a series of PMU registers, including the Performance Monitors User Enable Register (PMUSERENR), Performance Monitors Control Register (PMCR), Performance Monitors Count Enable Set Register (PMCNTENSET), and Performance Monitors Cycle Count Register (PMCCNTR). Each of these registers must be configured correctly to ensure the cycle counter operates as expected. Common pitfalls include incorrect bit settings, missing memory barriers, or improper sequencing of operations.

In the context of the Cortex-R52, the PMU is tightly integrated with the processor’s pipeline and memory system. This integration allows the PMU to accurately track events such as instruction execution, cache misses, and cycle counts. However, this tight integration also means that any misconfiguration can lead to the PMU failing to operate correctly. For example, if the PMU is not enabled or the cycle counter is not explicitly started, the PMCCNTR will remain at zero.

PMU Enable Bit, Cycle Counter Configuration, and Register Access Timing

The root cause of the cycle counter not updating on the Cortex-R52 can often be traced to one or more of the following issues: incorrect configuration of the PMU enable bit, improper initialization of the cycle counter, or timing issues related to register access.

The PMU enable bit (bit 0 of the PMCR) must be set to 1 to enable the PMU. If this bit is not set, the PMU will not function, and the cycle counter will not increment. Similarly, the cycle counter must be explicitly enabled by setting bit 31 of the PMCNTENSET register. If this bit is not set, the cycle counter will remain disabled, and the PMCCNTR will not update.

Another potential cause is the omission of memory barriers or incorrect sequencing of operations. The Cortex-R52 uses a weakly ordered memory model, which means that memory operations can be reordered by the processor. If a memory barrier is not used after configuring the PMU registers, the processor may reorder the operations, leading to incorrect behavior. For example, if the PMU enable bit is set before the cycle counter is enabled, the processor may reorder the operations, causing the cycle counter to remain disabled.

Timing issues can also arise when reading the cycle counter value. The PMCCNTR is a 32-bit register that increments with each processor cycle. If the register is read too quickly after being enabled, it may not have had time to increment, resulting in a zero value. Additionally, if the cycle counter is reset or disabled immediately after being read, the value may not reflect the actual cycle count.

Correct PMU Configuration, Cycle Counter Initialization, and Debugging Techniques

To resolve the issue of the cycle counter not updating on the Cortex-R52, follow these detailed steps to ensure proper configuration and initialization of the PMU and cycle counter.

First, enable user access to the PMU by setting bit 0 of the PMUSERENR register. This step is necessary to allow non-privileged software to access the PMU registers. Use the following assembly code to set the EN bit:

__asm__ volatile (
    "MRC p15, 0, r0, c9, c14, 0 \t\n"
    "ORR r0, r0, #0x01 \t\n"
    "MCR p15, 0, r0, c9, c14, 0 \t\n"
    "BX lr \t\n"
    : : :
);

Next, enable the PMU by setting bit 0 of the PMCR register. This step activates the PMU and allows it to start counting events. Use the following assembly code to set the E bit:

__asm__ volatile (
    "MRC p15, 0, r0, c9, c12, 0 \t\n"
    "ORR r0, r0, #0x01 \t\n"
    "MCR p15, 0, r0, c9, c12, 0 \t\n"
    "BX lr \t\n"
    : : :
);

After enabling the PMU, enable the cycle counter by setting bit 31 of the PMCNTENSET register. This step specifically enables the cycle counter to start incrementing. Use the following assembly code to set the C bit:

__asm__ volatile (
    "MOV r0, #0x80000000 \t\n"
    "MCR p15, 0, r0, c9, c12, 1 \t\n"
    "BX lr \t\n"
    : : :
);

To ensure the cycle counter starts from zero, reset it by setting bit 2 of the PMCR register. This step clears any previous value in the cycle counter. Use the following assembly code to set the C bit:

__asm__ volatile (
    "MRC p15, 0, r0, c9, c12, 0 \t\n"
    "ORR r0, r0, #0x04 \t\n"
    "MCR p15, 0, r0, c9, c12, 0 \t\n"
    "BX lr \t\n"
    : : :
);

When reading the cycle counter value, ensure that sufficient time has passed for the counter to increment. Use the following assembly code to read the PMCCNTR register:

uint32_t value = 0;
__asm__ volatile (
    "MRC p15, 0, %[result], c9, c13, 0 \t\n"
    "BX lr \t\n"
    : [result] "=&r" (value) : :
);
return value;

To avoid timing issues, insert a delay or memory barrier after enabling the cycle counter and before reading its value. This ensures that the processor has sufficient time to update the counter. Use the following assembly code to insert a memory barrier:

__asm__ volatile (
    "DSB \t\n"
    "ISB \t\n"
    : : : "memory"
);

If the cycle counter still does not update, verify that the PMU is not being disabled or reset inadvertently. Check for any code that modifies the PMCR or PMCNTENCLR registers and ensure that the cycle counter remains enabled throughout the measurement period.

Finally, if the issue persists, consider using a debugger to single-step through the PMU configuration and initialization code. This allows you to verify that each register is being set correctly and that the cycle counter is incrementing as expected. Additionally, check the processor’s documentation for any errata or special considerations related to the PMU on the Cortex-R52.

By following these steps, you can ensure that the PMU and cycle counter are configured correctly on the ARM Cortex-R52, allowing you to accurately measure and analyze system performance.