[RISC-V][Linux Kernel] How Interrupts Are Handled

No matter what project you are working on, you will likely develop a device driver. To develop a good device driver, it is essential to understand how interrupts work. Additionally, understanding interrupts is crucial for optimizing performance and handling preemption effectively.

In a RISC-V processor-based system, what functions does the Linux kernel call to handle an interrupt? Let's explore the overall execution flow.

Interrupt Trigger

Interrupts can be triggered by various peripheral devices. In a RISC-V-based architecture, external peripheral-generated interrupts are managed by the Platform-Level Interrupt Controller (PLIC). The PLIC consists of a gateway and a PLIC core, which together route interrupts to the appropriate processing unit.

When an interrupt occurs, the PLIC routes it to a RISC-V Hart (hardware thread), which then triggers an exception. The general execution flow is as follows:

Function Call Flow

[1] Handling Exceptions

When an interrupt occurs, the Linux kernel jumps to the handle_exception label. This is because, in a RISC-V-based Linux system, the handle_exception label is set as the exception vector address.

• Inside handle_exception, the following operations take place:
• The register set is pushed onto the stack to save the current CPU state.
• The scause register is checked. If its highest bit is set to 1, the kernel calls the do_irq() function to handle the interrupt.

?? Note: In the RISC-V architecture, whenever an exception occurs, the scause register is updated with an exception code that indicates the type of exception.

[2] Handling Different Types of Interrupts

In RISC-V, interrupts are categorized into three types:

• CLIC (Core Local Interrupt Controller): Handles timer interrupts and software-generated interrupts.
• PLIC (Platform-Level Interrupt Controller): Handles interrupts from external peripheral devices.

Since most device drivers interact with external peripherals, the Linux kernel eventually calls the plic_handle_irq() function to handle these types of interrupts.

Summary

• When an interrupt occurs, the handle_exception function is executed first.
• If the highest bit of the scause register is set to 1, the do_irq() function is called.
• Depending on the interrupt type, different handlers are executed: Timer and software-generated interrupts use CLIC. External device interrupts use PLIC, which is processed by plic_handle_irq().

Understanding how interrupts are processed in the RISC-V Linux kernel is crucial for efficient driver development and system optimization.