linux内核两级中断处理

    对于一个特定的irq_desc[irq]，其上的中断处理分为两级，第一级是调用irq_desc[irq].handle_irq，第二级是设备特定的中断处理例程ISR，在handle_irq的内部通过irq_desc[irq].action->handler调用。
    第一级函数在平台初始化期间被安装到irq_desc数组中，第二级函数的注册发生在设备驱动程序调用request_irq安装对应设备的中断处理例程时。第一级函数主要面向PIC的某一中断线IRQ line，第二级函数则面向该中断线上连接的具体设备。

static int mxc_gpio_probe(struct platform_device *pdev)
{
    struct device_node *np = pdev->dev.of_node;
    struct mxc_gpio_port *port;
    struct resource *iores;
    int irq_base;
    int err;

    mxc_gpio_get_hw(pdev);
    port = devm_kzalloc(&pdev->dev, sizeof(*port), GFP_KERNEL);
    if (!port)
         return -ENOMEM;

    iores = platform_get_resource(pdev, IORESOURCE_MEM, 0);
    port->base = devm_ioremap_resource(&pdev->dev, iores);
    if (IS_ERR(port->base))
         return PTR_ERR(port->base);

    port->irq_high = platform_get_irq(pdev, 1);
    port->irq = platform_get_irq(pdev, 0);
    if (port->irq < 0)
         return port->irq;

    /* disable the interrupt and clear the status */
    writel(0, port->base + GPIO_IMR);
    writel(~0, port->base + GPIO_ISR);

    if (mxc_gpio_hwtype == IMX21_GPIO)
    {
         /*
        * Setup one handler for all GPIO interrupts. Actually setting
        * the handler is needed only once, but doing it for every port
        * is more robust and easier.
        */
        irq_set_chained_handler(port->irq, mx2_gpio_irq_handler);
    }
    else
    {
         /* setup one handler for each entry */
         irq_set_chained_handler(port->irq, mx3_gpio_irq_handler);
         irq_set_handler_data(port->irq, port);
         if (port->irq_high > 0)
        {
              /* setup handler for GPIO 16 to 31 */
             irq_set_chained_handler(port->irq_high, mx3_gpio_irq_handler);
              irq_set_handler_data(port->irq_high, port);
         }
    }

    /* gpio-mxc can be a generic irq chip */
    mxc_gpio_init_gc(port, irq_base);
}
    linux内核提供两个函数 irq_set_handler 和 irq_set_chained_handler 来设置第一级中断处理函数。

static void __init mxc_gpio_init_gc(struct mxc_gpio_port *port, int irq_base)
{
    struct irq_chip_generic *gc;
    struct irq_chip_type *ct;

    gc = irq_alloc_generic_chip("gpio-mxc", 1, irq_base, port->base, handle_level_irq);
    gc->private = port;
    ct = gc->chip_types;
    ct->chip.irq_ack = irq_gc_ack_set_bit;
    ct->chip.irq_mask = irq_gc_mask_clr_bit;
    ct->chip.irq_unmask = irq_gc_mask_set_bit;
    ct->chip.irq_set_type = gpio_set_irq_type;
    ct->chip.irq_set_wake = gpio_set_wake_irq;
    ct->regs.ack = GPIO_ISR;
    ct->regs.mask = GPIO_IMR;

    irq_setup_generic_chip(gc, IRQ_MSK(32), IRQ_GC_INIT_NESTED_LOCK, IRQ_NOREQUEST,0);
}
    内核中通过irq_chip结构体来描述中断控制器，描述了中断mask，ack等成员函数。多个控制器之间还可以级联，多中断控制器的情况下，我们使用内核提供的通用的irq_chip驱动架构irq_chip_generic，目前，主流的arm芯片内部的一级中断控制器都是用了arm公司的GIC。

void handle_level_irq(unsigned int irq, struct irq_desc *desc)
{
    raw_spin_lock(&desc->lock);
    mask_ack_irq(desc);

    if (!irq_may_run(desc))
         goto out_unlock;

    desc->istate &= ~(IRQS_REPLAY | IRQS_WAITING);
    kstat_incr_irqs_this_cpu(irq, desc);

     /*
    * If its disabled or no action available
    * keep it masked and get out of here
    */
    if (unlikely(!desc->action || irqd_irq_disabled(&desc->irq_data)))
    {
         desc->istate |= IRQS_PENDING;
         goto out_unlock;
    }

    handle_irq_event(desc);
    cond_unmask_irq(desc);

out_unlock:
    raw_spin_unlock(&desc->lock);
}
    handle_level_irq  是通用中断子系统提供的标准流控回调函数，用于电平触发中断的流控处理，其他函数还有handle_simple_irq  用于简易流控处理，handle_edge_irq  用于边沿触发中断的流控处理，handle_fasteoi_irq  用于需要响应eoi的中断控制器，handle_percpu_irq  用于只在单一cpu响应的中断，handle_nested_irq  用于处理使用线程的嵌套中断。

irqreturn_t handle_irq_event(struct irq_desc *desc)
{
    struct irqaction *action = desc->action;
    irqreturn_t ret;

    desc->istate &= ~IRQS_PENDING;
    irqd_set(&desc->irq_data, IRQD_IRQ_INPROGRESS);
    raw_spin_unlock(&desc->lock);

    ret = handle_irq_event_percpu(desc, action);

    raw_spin_lock(&desc->lock);
    irqd_clear(&desc->irq_data, IRQD_IRQ_INPROGRESS);
    return ret;
}

    函数handle_irq_event 为调用设备驱动程序安装的中断处理例程做最后的准备工作，
清除IRQS_PENDING位，将当前设备的中断线设置IRQD_IRQ_INPROGRESS状态，表明
该中断线上一个中断正在被处理。

irqreturn_t handle_irq_event_percpu(struct irq_desc *desc, struct irqaction *action)
{
    irqreturn_t retval = IRQ_NONE;
    unsigned int flags = 0, irq = desc->irq_data.irq;

    do
    {
         irqreturn_t res;

         trace_irq_handler_entry(irq, action);
         res = action->handler(irq, action->dev_id);
         trace_irq_handler_exit(irq, action, res);

         if (WARN_ONCE(!irqs_disabled(),"irq %u handler %pF enabled interrupts\n",
              irq, action->handler))
         local_irq_disable();

         switch (res)
        {
              case IRQ_WAKE_THREAD:
              /*
                * Catch drivers which return WAKE_THREAD but
                * did not set up a thread function
            */
              if (unlikely(!action->thread_fn))
                {
                   warn_no_thread(irq, action);
                   break;
              }

              __irq_wake_thread(desc, action);

             /* Fall through to add to randomness */
             case IRQ_HANDLED:
              flags |= action->flags;
              break;
              default:
                   break;
          }

         retval |= res;
         action = action->next;
    } while (action);

    add_interrupt_randomness(irq, flags);

    if (!noirqdebug)
         note_interrupt(irq, desc, retval);
    return retval;
}
    函数遍历action上的中断处理例程链表，通过action->handler来调用具体设备的中断处理例程，
即通过request_irq函数安装的中断处理函数。如果返回值是IRQ_WAKE_THREAD，那么函数将调用
irq_wake_thread来唤醒action->thread表示的一个内核线程。