Interrupt Management

Exceptions and Interrupts

XWOS classifies interrupts into EXC and IRQ:

• EXC refers to interrupts defined by the architecture, called Exceptions in some architectures, with negative interrupt numbers;
• IRQ refers to peripheral interrupts, such as UART interrupts, etc., with interrupt numbers represented as 0 and positive values.

Kernel Interrupts

XWOS kernel interrupts include:

• Context switch interrupt: used to switch threads;
• Tick timer interrupt: used to provide periodic timer interrupts;
• Scheduler service interrupt: used to start scheduling, exit threads, freeze threads, power management, and other operations.

Interrupt Priority

XWOS requirements for interrupt priority:

Context switch interrupt is the lowest priority interrupt in the system
Context switch interrupt <= Tick timer interrupt <= Scheduler service interrupt

CPU Interrupt Control

A CPU can only operate its own interrupt control, not the interrupts of other CPUs. The following functions affect whichever CPU they run on:

• xwos_cpuirq_enable_lc(): Enable local CPU interrupts (non-nestable)
• xwos_cpuirq_disable_lc(): Disable local CPU interrupts (non-nestable)
• xwos_cpuirq_save_lc(): Save then disable local CPU interrupt state (nestable)
• xwos_cpuirq_restore_lc(): Restore local CPU interrupt state (nestable)
• xwos_cpuirq_suspend_lc(): Suspend local CPU interrupts (nestable)
• xwos_cpuirq_resume_lc(): Resume local CPU interrupts (nestable)
• xwos_cpuirq_test_lc(): Test local CPU interrupt state

When porting XWOS to a specific chip, the OS porting implementation layer (XWOSIMPL) must provide the concrete implementation of the above functions:

• Source files: xwosimpl_irq.h/xwosimpl_irq.c
  • xwospl_cpuirq_enable_lc(): corresponds to xwos_cpuirq_enable_lc()
  • xwospl_cpuirq_disable_lc(): corresponds to xwos_cpuirq_disable_lc()
  • xwospl_cpuirq_save_lc(): corresponds to xwos_cpuirq_save_lc()
  • xwospl_cpuirq_restore_lc(): corresponds to xwos_cpuirq_restore_lc()
  • xwospl_cpuirq_test_lc(): corresponds to xwos_cpuirq_test_lc()

Usage Limitations

Disabling and enabling the CPU global interrupt control must ensure that nesting does not occur. For example, the following nested code is incorrect because when critical section 1 ends, it will unexpectedly enable CPU interrupts, thus failing to guarantee the safety of critical section 0:

void func1(void)
{
        xwos_cpuirq_disable_lc();
        /* ... critical section 1 ... */
        xwos_cpuirq_enable_lc();
}

void func0(void)
{
        xwos_cpuirq_disable_lc();
        /* ... critical section 0 ... */
        func1(); /* ERROR!!! nesting occurs */
        /* CPU interrupts are unexpectedly enabled here */
        /* ... critical section 0 ... */
        xwos_cpuirq_enable_lc();
}

When interrupt-disable code nests, the CPU global interrupt state must be saved and restored. The above example can be corrected to:

void func1(void)
{
        xwreg_t cpuirq;

        xwos_cpuirq_save_lc(&cpuirq);
        /* ... critical section 1 ... */
        xwos_cpuirq_restore_lc(cpuirq);
}

void func0(void)
{
        xwreg_t cpuirq;

        xwos_cpuirq_save_lc(&cpuirq);
        /* ... critical section 0 ... */
        func1();
        /* ... critical section 0 ... */
        xwos_cpuirq_restore_lc(cpuirq);
}

The Suspend/Resume versions of CAPI can also be used:

void func1(void)
{
        xwos_cpuirq_suspend_lc(&cpuirq);
        /* ... critical section 1 ... */
        xwos_cpuirq_resume_lc(cpuirq);
}

void func0(void)
{
        xwos_cpuirq_suspend_lc(&cpuirq);
        /* ... critical section 0 ... */
        func1();
        /* ... critical section 0 ... */
        xwos_cpuirq_resume_lc(cpuirq);
}

Peripheral Interrupts

Getting the Current Code’s Interrupt Number

XWOS provides CAPI xwos_irq_get_id() to obtain the interrupt number of the current interrupt. If this CAPI is used outside an interrupt context, it returns an error code:

• -ENOTISRCTX: current context is not an interrupt

Therefore, this CAPI can also be used to determine whether the current context is an interrupt context.

When porting XWOS to a specific chip, the OS porting implementation layer (XWOSIMPL) must provide the implementation of the above function:

• Source files: xwosimpl_irq.h/xwosimpl_irq.c
  • xwospl_irq_get_id(): corresponds to xwos_irq_get_id()

Other Peripheral Interrupt CAPI

• xwos_irq_enable(): Enable a specific peripheral interrupt
• xwos_irq_disable(): Disable a specific peripheral interrupt
• xwos_irq_save(): Save the state of a peripheral interrupt, then disable it
• xwos_irq_restore(): Restore the state of a peripheral interrupt

When porting XWOS to a specific chip, the OS porting implementation layer (XWOSIMPL) must provide the implementation of the above functions:

• Source files: xwosimpl_irq.h/xwosimpl_irq.c
  • xwospl_irq_enable(): corresponds to xwos_irq_enable()
  • xwospl_irq_disable(): corresponds to xwos_irq_disable()
  • xwospl_irq_save(): corresponds to xwos_irq_save()
  • xwospl_irq_restore(): corresponds to xwos_irq_restore()

CAPI Reference

• Header <xwos/osal/irq.h>