nested interrupt handling

icoll-imx233.c

void irq_handler(void)
{
    /* read vector and notify as a read side-effect */
    uint32_t vec = HW_ICOLL_VECTOR;
    /* Save registers on IRQ stack, switch to SVC and renable interrupts */
    asm volatile(
        "mrs    lr, spsr                 \n" /* Save SPSR_irq */
        "stmfd  sp!, { lr }              \n" /* Push it on the IRQ stack */
        "msr    cpsr_c, #0x13            \n" /* Switch to SVC mode, enable IRQ */
        "stmfd  sp!, { lr }              \n" /* Save lr_SVC */
    );
    int ack_lvl = _irq_handler(vec);
    asm volatile(
        "ldmfd  sp!, { lr }              \n" /* Restore lr_SVC */
        "msr    cpsr_c, #0x92            \n" /* Mask IRQ, return to IRQ mode */
        "ldmfd  sp!, { lr }              \n" /* Pop back SPSR */
        "msr    spsr_cxsf, lr            \n" /* Restore SPSR_irq */
    );
    HW_ICOLL_LEVELACK = 1 << ack_lvl;
}