GICv2 Interrupt Auto-Deassertion Behavior on Cortex-R5
The issue at hand involves the unexpected auto-deassertion of a timer interrupt configured on IRQ#226 in a Cortex-R5 system utilizing a Generic Interrupt Controller version 2 (GICv2). The interrupt is enabled in both the distributor and CPU interface blocks, assigned to Group#1. Upon enabling the interrupt, it immediately enters a pending state, as evidenced by its appearance in the GIC CPU Interface Acknowledge Register (GICC_IAR). However, as soon as the program execution proceeds to the next instruction, the interrupt is deasserted, and the CPU fails to receive the interrupt. This behavior is unexpected and prevents the system from handling the timer interrupt as intended.
The GICv2 architecture is designed to manage interrupts efficiently, but its behavior can be influenced by several factors, including configuration settings, hardware interactions, and software handling. In this case, the auto-deassertion of the interrupt suggests that the GICv2 is not maintaining the interrupt state as expected, leading to a failure in interrupt delivery to the CPU. This issue is particularly critical in real-time systems where precise timing and interrupt handling are paramount.
To understand the root cause, it is essential to delve into the GICv2 architecture and its interaction with the Cortex-R5 processor. The GICv2 consists of two main components: the Distributor and the CPU Interface. The Distributor is responsible for managing interrupt priorities and routing interrupts to the appropriate CPU interfaces. The CPU Interface, on the other hand, handles the acknowledgment and deactivation of interrupts by the CPU. The GICC_IAR register is used to acknowledge an interrupt, and reading from this register returns the interrupt ID and acknowledges the interrupt to the GIC. The GICC_EOIR register is then used to signal the end of interrupt processing.
In the described scenario, the interrupt is acknowledged by reading the GICC_IAR register, but it is deasserted before the CPU can process it. This suggests that the GICv2 is not maintaining the interrupt state correctly, or there may be a misconfiguration in the GICv2 or Cortex-R5 settings. The following sections will explore the possible causes and provide detailed troubleshooting steps to resolve the issue.
Misconfigured GICv2 Priority Levels and Interrupt Group Settings
One of the primary reasons for the auto-deassertion of interrupts in a GICv2 system is the misconfiguration of priority levels and interrupt group settings. The GICv2 allows interrupts to be assigned to one of two groups: Group 0 (typically used for secure interrupts) and Group 1 (typically used for non-secure interrupts). Each group can have its own priority levels, and the GICv2 uses these priorities to determine which interrupt should be serviced first.
In the case of the Cortex-R5, the interrupt in question is assigned to Group#1, which is correct for a non-secure timer interrupt. However, if the priority level of the interrupt is not set appropriately, the GICv2 may not prioritize it correctly, leading to unexpected behavior such as auto-deassertion. Additionally, if the interrupt is not properly enabled in both the Distributor and CPU Interface, the GICv2 may not route the interrupt to the CPU correctly.
Another potential cause is the incorrect configuration of the GICv2’s interrupt state machine. The GICv2 uses a state machine to manage the lifecycle of interrupts, including pending, active, and inactive states. If the state machine is not configured correctly, the GICv2 may prematurely deassert an interrupt before it is processed by the CPU. This can occur if the interrupt is not properly acknowledged or if the end-of-interrupt (EOI) signal is not sent correctly.
To diagnose this issue, it is essential to verify the configuration of the GICv2 priority levels and interrupt group settings. This includes checking the GICD_IPRIORITYR and GICD_IGROUPR registers to ensure that the interrupt is assigned the correct priority and group. Additionally, the GICD_ISENABLER and GICC_CTLR registers should be checked to ensure that the interrupt is enabled in both the Distributor and CPU Interface.
Improper Handling of GICC_IAR and GICC_EOIR Registers
The GICC_IAR and GICC_EOIR registers play a critical role in the acknowledgment and deactivation of interrupts in a GICv2 system. The GICC_IAR register is used to acknowledge an interrupt, and reading from this register returns the interrupt ID and acknowledges the interrupt to the GIC. The GICC_EOIR register is then used to signal the end of interrupt processing. If these registers are not handled correctly, the GICv2 may not maintain the interrupt state as expected, leading to issues such as auto-deassertion.
In the described scenario, the interrupt is acknowledged by reading the GICC_IAR register, but it is deasserted before the CPU can process it. This suggests that the GICv2 is not maintaining the interrupt state correctly, or there may be a misconfiguration in the GICv2 or Cortex-R5 settings. One possible cause is that the GICC_EOIR register is not being written correctly after the interrupt is processed. If the EOI signal is not sent, the GICv2 may assume that the interrupt has been processed and deassert it prematurely.
Another potential cause is the incorrect handling of the GICC_IAR register. If the GICC_IAR register is read multiple times before the interrupt is processed, the GICv2 may deassert the interrupt prematurely. This can occur if the interrupt service routine (ISR) does not handle the GICC_IAR and GICC_EOIR registers correctly. Additionally, if the ISR does not complete within a reasonable time frame, the GICv2 may deassert the interrupt to prevent it from blocking other interrupts.
To diagnose this issue, it is essential to review the interrupt service routine (ISR) and ensure that the GICC_IAR and GICC_EOIR registers are handled correctly. This includes reading the GICC_IAR register once at the beginning of the ISR to acknowledge the interrupt and writing the GICC_EOIR register at the end of the ISR to signal the end of interrupt processing. Additionally, the ISR should be optimized to ensure that it completes within a reasonable time frame to prevent the GICv2 from deasserting the interrupt prematurely.
Implementing Correct GICv2 Configuration and Interrupt Handling
To resolve the issue of auto-deassertion of interrupts in a GICv2 system, it is essential to implement the correct configuration and interrupt handling procedures. This includes configuring the GICv2 priority levels and interrupt group settings correctly, as well as handling the GICC_IAR and GICC_EOIR registers properly in the interrupt service routine (ISR).
First, ensure that the interrupt is assigned the correct priority and group in the GICD_IPRIORITYR and GICD_IGROUPR registers. The priority level should be set appropriately to ensure that the interrupt is prioritized correctly by the GICv2. Additionally, the interrupt should be enabled in both the Distributor and CPU Interface by setting the appropriate bits in the GICD_ISENABLER and GICC_CTLR registers.
Next, review the interrupt service routine (ISR) and ensure that the GICC_IAR and GICC_EOIR registers are handled correctly. The GICC_IAR register should be read once at the beginning of the ISR to acknowledge the interrupt, and the GICC_EOIR register should be written at the end of the ISR to signal the end of interrupt processing. Additionally, the ISR should be optimized to ensure that it completes within a reasonable time frame to prevent the GICv2 from deasserting the interrupt prematurely.
Finally, it is essential to verify the configuration of the GICv2’s interrupt state machine. This includes checking the GICD_CTLR and GICC_CTLR registers to ensure that the GICv2 is configured correctly to maintain the interrupt state as expected. Additionally, the GICD_ISPENDR and GICD_ICPENDR registers should be checked to ensure that the interrupt is not being prematurely deasserted due to a pending state.
By implementing these steps, the issue of auto-deassertion of interrupts in a GICv2 system can be resolved, ensuring that the Cortex-R5 processor receives and processes interrupts correctly. This will enable the system to handle timer interrupts and other critical events as intended, improving the overall reliability and performance of the embedded system.
