把握linux内核设计思想（二）：硬中断及中断处理

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操作系统负责管理硬件设备。为了使系统和硬件设备的协同工作不减少机器性能。系统和硬件的通信使用中断的机制，也就是让硬件在须要的时候向内核发出信号，这样使得内核不用去轮询设备而导致做非常多无用功。

        中断使得硬件能够发出通知给处理器。硬件设备生成中断的时候并不考虑与处理器的时钟同步，中断能够随时产生。

也就是说，内核随时可能由于新到来的中断而被打断。

当接收到一个中断后，中断控制器会给处理器发送一个电信号，处理器检測到该信号便中断自己当前工作而处理中断。

        在响应一个中断时。内核会运行一个函数，该函数叫做中断处理程序或中断服务例程（ISR）。

中断处理程序运行与中断上下文，中断上下文中运行的代码不可堵塞，应该高速运行，这样才干保证尽快恢复被中断的代码的运行。中断处理程序是管理硬件驱动的驱动程序的组成部分，假设设备使用中断。那么对应的驱动程序就注冊一个中断处理程序。

        在驱动程序中,通常使用request_irq()来注冊中断处理程序。该函数在文件<include/linux/interrupt.h>中声明：

extern int __must_check
request_irq(unsigned int irq, irq_handler_t handler, unsigned long flags,
        const char *name, void *dev);

第一个參数为要分配的中断号；第二个參数为指向中断处理程序的指针。第三个參数为中断处理标志。该函数实现例如以下：

static inline int __must_check
request_irq(unsigned int irq, irq_handler_t handler, unsigned long flags,
        const char *name, void *dev)
{
    return request_threaded_irq(irq, handler, NULL, flags, name, dev);
}

int request_threaded_irq(unsigned int irq, irq_handler_t handler,
                         irq_handler_t thread_fn, unsigned long irqflags,
                         const char *devname, void *dev_id)
{
        struct irqaction *action;
        struct irq_desc *desc;
        int retval;
        /*
         * handle_IRQ_event() always ignores IRQF_DISABLED except for
         * the _first_ irqaction (sigh).  That can cause oopsing, but
         * the behavior is classified as "will not fix" so we need to
         * start nudging drivers away from using that idiom.
         */
        if ((irqflags & (IRQF_SHARED|IRQF_DISABLED)) ==
                                        (IRQF_SHARED|IRQF_DISABLED)) {
                pr_warning(
                  "IRQ %d/%s: IRQF_DISABLED is not guaranteed on shared IRQs\n",
                        irq, devname);
        }
#ifdef CONFIG_LOCKDEP
        /*
         * Lockdep wants atomic interrupt handlers:
         */
        irqflags |= IRQF_DISABLED;
#endif
        /*
         * Sanity-check: shared interrupts must pass in a real dev-ID,
         * otherwise we'll have trouble later trying to figure out
         * which interrupt is which (messes up the interrupt freeing
         * logic etc).
         */
        if ((irqflags & IRQF_SHARED) && !dev_id)
                return -EINVAL;
        desc = irq_to_desc(irq);
        if (!desc)
                return -EINVAL;
        if (desc->status & IRQ_NOREQUEST)
                return -EINVAL;
        if (!handler) {
                if (!thread_fn)
                        return -EINVAL;
                handler = irq_default_primary_handler;
        }
        //分配一个irqaction
        action = kzalloc(sizeof(struct irqaction), GFP_KERNEL);
        if (!action)
                return -ENOMEM;
        action->handler = handler;
        action->thread_fn = thread_fn;
        action->flags = irqflags;
        action->name = devname;
        action->dev_id = dev_id;
        chip_bus_lock(irq, desc);

        //将创建并初始化完在的action增加desc
        retval = __setup_irq(irq, desc, action);
        chip_bus_sync_unlock(irq, desc);
        if (retval)
                kfree(action);
#ifdef CONFIG_DEBUG_SHIRQ
        if (irqflags & IRQF_SHARED) {
                /*
                 * It's a shared IRQ -- the driver ought to be prepared for it
                 * to happen immediately, so let's make sure....
                 * We disable the irq to make sure that a 'real' IRQ doesn't
                 * run in parallel with our fake.
                 */
                unsigned long flags;
                disable_irq(irq);
                local_irq_save(flags);
                handler(irq, dev_id);
                local_irq_restore(flags);
                enable_irq(irq);
        }
#endif
        return retval;
}

以下看一下中断处理程序的实例，以rtc驱动程序为例，代码位于<drivers/char/rtc.c>中。当RTC驱动装载时，rtc_init()函数会被调用来初始化驱动程序。包含注冊中断处理函数：

    /*
     * XXX Interrupt pin #7 in Espresso is shared between RTC and
     * PCI Slot 2 INTA# (and some INTx# in Slot 1).
     */
    if (request_irq(rtc_irq, rtc_interrupt, IRQF_SHARED, "rtc",
            (void *)&rtc_port)) {
        rtc_has_irq = 0;
        printk(KERN_ERR "rtc: cannot register IRQ %d\n", rtc_irq);
        return -EIO;
    }

处理程序函数rtc_interrupt()：

/*
 *  A very tiny interrupt handler. It runs with IRQF_DISABLED set,
 *  but there is possibility of conflicting with the set_rtc_mmss()
 *  call (the rtc irq and the timer irq can easily run at the same
 *  time in two different CPUs). So we need to serialize
 *  accesses to the chip with the rtc_lock spinlock that each
 *  architecture should implement in the timer code.
 *  (See ./arch/XXXX/kernel/time.c for the set_rtc_mmss() function.)
 */
static irqreturn_t rtc_interrupt(int irq, void *dev_id)
{
    /*
     *  Can be an alarm interrupt, update complete interrupt,
     *  or a periodic interrupt. We store the status in the
     *  low byte and the number of interrupts received since
     *  the last read in the remainder of rtc_irq_data.
     */
    spin_lock(&rtc_lock);    //保证rtc_irq_data不被SMP机器上其它处理器同一时候訪问
    rtc_irq_data += 0x100;
    rtc_irq_data &= ~0xff;
    if (is_hpet_enabled()) {
        /*
         * In this case it is HPET RTC interrupt handler
         * calling us, with the interrupt information
         * passed as arg1, instead of irq.
         */
        rtc_irq_data |= (unsigned long)irq & 0xF0;
    } else {
        rtc_irq_data |= (CMOS_READ(RTC_INTR_FLAGS) & 0xF0);
    }
    if (rtc_status & RTC_TIMER_ON)
        mod_timer(&rtc_irq_timer, jiffies + HZ/rtc_freq + 2*HZ/100);
    spin_unlock(&rtc_lock);
    /* Now do the rest of the actions */
    spin_lock(&rtc_task_lock);    //避免rtc_callback出现系统情况，RTC驱动同意注冊一个回调函数在每一个RTC中断到来时运行。
    if (rtc_callback)
        rtc_callback->func(rtc_callback->private_data);
    spin_unlock(&rtc_task_lock);
    wake_up_interruptible(&rtc_wait);
    kill_fasync(&rtc_async_queue, SIGIO, POLL_IN);
    return IRQ_HANDLED;
}

        在内核中，中断的旅程開始于提前定义入口点，这类似于系统调用。

对于每条中断线。处理器都会跳到相应的一个唯一的位置。这样，内核就能够知道所接收中断的IRQ号了。初始入口点仅仅是在栈中保存这个号，并存放当前寄存器的值(这些值属于被中断的任务)；然后，内核调用函数do_IRQ().从这里開始。大多数中断处理代码是用C写的。

do_IRQ()的声明例如以下：

unsigned int do_IRQ(struct pt_regs regs)

       由于C的调用惯例是要把函数參数放在栈的顶部。因此pt_regs结构包括原始寄存器的值，这些值是曾经在汇编入口例程中保存在栈上的。中断的值也会得以保存，所以，do_IRQ()能够将它提取出来，X86的代码为：

int irq = regs.orig_eax & 0xff

       计算出中断号后，do_IRQ()对所接收的中断进行应答，禁止这条线上的中断传递。在普通的PC机器上，这些操作是由mask_and_ack_8259A()来完毕的，该函数由do_IRQ()调用。接下来，do_IRQ()须要确保在这条中断线上有一个有效的处理程序，并且这个程序已经启动可是当前没有执行。假设这种话， do_IRQ()就调用handle_IRQ_event()来执行为这条中断线所安装的中断处理程序，函数位于<kernel/irq/handle.c>:

/**
 * handle_IRQ_event - irq action chain handler
 * @irq:    the interrupt number
 * @action: the interrupt action chain for this irq
 *
 * Handles the action chain of an irq event
 */
irqreturn_t handle_IRQ_event(unsigned int irq, struct irqaction *action)
{
    irqreturn_t ret, retval = IRQ_NONE;
    unsigned int status = 0;

    //假设没有设置IRQF_DISABLED。将CPU中断打开。应该尽量避免中断关闭情况，本地中断关闭情况下会导致中断丢失。
    if (!(action->flags & IRQF_DISABLED))
        local_irq_enable_in_hardirq();

    do {    //遍历执行中断处理程序
        trace_irq_handler_entry(irq, action);
        ret = action->handler(irq, action->dev_id);
        trace_irq_handler_exit(irq, action, ret);

        switch (ret) {
        case IRQ_WAKE_THREAD:
            /*
             * Set result to handled so the spurious check
             * does not trigger.
             */
            ret = IRQ_HANDLED;

            /*
             * 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;
            }

            /*
             * Wake up the handler thread for this
             * action. In case the thread crashed and was
             * killed we just pretend that we handled the
             * interrupt. The hardirq handler above has
             * disabled the device interrupt, so no irq
             * storm is lurking.
             */
            if (likely(!test_bit(IRQTF_DIED,
                         &action->thread_flags))) {
                set_bit(IRQTF_RUNTHREAD, &action->thread_flags);
                wake_up_process(action->thread);
            }
            /* Fall through to add to randomness */
        case IRQ_HANDLED:
            status |= action->flags;
            break;

        default:
            break;
        }

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

    if (status & IRQF_SAMPLE_RANDOM)
        add_interrupt_randomness(irq);
    local_irq_disable();//关中断

    return retval;
}

         前面说到中断应该尽快运行完，以保证被中断代码可以尽快的恢复运行。但其实中断通常有非常多工作要做。包含应答、重设硬件、数据拷贝、处理请求、发送请求等。

为了求得平衡，内核把中断处理工作分成两半。中断处理程序是上半部——接收到中断就開始运行。可以稍后完毕的工作推迟到下半部操作，下半部在合适的时机被开中段运行。比如网卡收到数据包时马上发出中断，内核运行网卡已注冊的中断处理程序，此处工作就是通知硬件拷贝最新的网络数据包到内存，然后将控制权交换给系统之前被中断的任务，其它的如处理和操作数据包等任务被放到随后的下半部中去运行。

下一节我们将了解中断处理的下半部。