中断线程化

 中断处理程序包括上半部硬件中断处理程序，下半部处理机制，包括软中断、tasklet、workqueue、中断线程化。

当一个外设中断发生后，内核会执行一个函数来响应该中断，这个函数通常被称为中断处理程序或中断服务例程。

上半部硬件中断处理运行在中断上下文中，要求快速完成并且退出中断。

中断线程化是实时Linux项目开发的一个新特性，目的是降低中断处理对系统实时延迟的影响。

在LInux内核里，中断具有最高优先级，只要有中断发生，内核会暂停手头的工作转向中断处理，等到所有挂起等待的中断和软终端处理完毕后才会执行进程调度，因此这个过程会造成实时任务得不到及时处理。

中断上下文总是抢占进程上下文，中断上下文不仅是中断处理程序，还包括softirq、tasklet等，中断上下文成了优化Linux实时性的最大挑战之一。

中断线程化并不是放到普通线程中，而是放到实时线程中，采用 FIFO 的调度方式，优先级为 49

---

 

request_irq调用request_threaded_irq进行中断注册，只是少了一个thread_fn参数。这也是两则的区别所在，request_irq不能注册线程化中断。

irq：Linux软件中断号，不是硬件中断号。

handler：指primary handler，也即request_irq的中断处理函数handler。

thread_fn：中断线程化的处理函数。

irqflags：中断标志位，见IRQF_*解释。

devname：中断名称。

dev_id：传递给中断处理程序的参数。

handler和thread_fn分别被赋给action->handler和action->thread_fn，组合如下：

	handler	thread_fn
1	√	√	先执行handler，然后条件执行thread_fn。
2	√	×	等同于request_irq()
3	×	√	handler=irq_default_primary_handler
4	×	×	返回-EINVAL

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;

	/*
	 * 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).
	 *
	 * Also IRQF_COND_SUSPEND only makes sense for shared interrupts and
	 * it cannot be set along with IRQF_NO_SUSPEND.
	 */
	if (((irqflags & IRQF_SHARED) && !dev_id) || //共享中断设备必须传递啊dev_id参数来区分是哪个共享外设的中断
	    (!(irqflags & IRQF_SHARED) && (irqflags & IRQF_COND_SUSPEND)) ||
	    ((irqflags & IRQF_NO_SUSPEND) && (irqflags & IRQF_COND_SUSPEND)))
		return -EINVAL;

	desc = irq_to_desc(irq);//irq-->desc
	if (!desc)
		return -EINVAL;

	if (!irq_settings_can_request(desc) ||
	    WARN_ON(irq_settings_is_per_cpu_devid(desc)))
		return -EINVAL;

	if (!handler) {
		if (!thread_fn)
			return -EINVAL;
		handler = irq_default_primary_handler;//若handler空，thread_fn非空，hander赋值一个默认函数
	}

	action = kzalloc(sizeof(struct irqaction), GFP_KERNEL);
	if (!action)
		return -ENOMEM;

	action->handler = handler;
	action->thread_fn = thread_fn;//中断线程化赋值
	action->flags = irqflags;
	action->name = devname;
	action->dev_id = dev_id;

	chip_bus_lock(desc);
	retval = __setup_irq(irq, desc, action);//最终这里
	chip_bus_sync_unlock(desc);

	if (retval)
		kfree(action);

#ifdef CONFIG_DEBUG_SHIRQ_FIXME
	if (!retval && (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;
}

 

/*
 * Internal function to register an irqaction - typically used to
 * allocate special interrupts that are part of the architecture.
 */
static int
__setup_irq(unsigned int irq, struct irq_desc *desc, struct irqaction *new)
{
    struct irqaction *old, **old_ptr;
    unsigned long flags, thread_mask = 0;
    int ret, nested, shared = 0;
    cpumask_var_t mask;

    if (!desc)
        return -EINVAL;

    if (desc->irq_data.chip == &no_irq_chip)----------------------表示没有正确初始化中断控制器，对于GICv2在gic_irq_domain_alloc()中指定chip为gic_chip。
        return -ENOSYS;
    if (!try_module_get(desc->owner))
        return -ENODEV;

    /*
     * Check whether the interrupt nests into another interrupt
     * thread.
     */
    nested = irq_settings_is_nested_thread(desc);-----------------对于设置了_IRQ_NESTED_THREAD嵌套类型的中断描述符，必须指定thread_fn。
    if (nested) {
        if (!new->thread_fn) {
            ret = -EINVAL;
            goto out_mput;
        }
        /*
         * Replace the primary handler which was provided from
         * the driver for non nested interrupt handling by the
         * dummy function which warns when called.
         */
        new->handler = irq_nested_primary_handler;
    } else {
        if (irq_settings_can_thread(desc))-----------------------判断该中断是否可以被线程化，如果没有设置_IRQ_NOTHREAD表示可以被强制线程化。
            irq_setup_forced_threading(new);
    }

    /*
     * Create a handler thread when a thread function is supplied
     * and the interrupt does not nest into another interrupt
     * thread.
     */
    if (new->thread_fn && !nested) {-----------------------------对不支持嵌套的线程化中断创建一个内核线程，实时SCHED_FIFO，优先级为50的实时线程。
        struct task_struct *t;
        static const struct sched_param param = {
            .sched_priority = MAX_USER_RT_PRIO/2,
        };

        t = kthread_create(irq_thread, new, "irq/%d-%s", irq,
                   new->name);-----------------------------------由irq、中断号、中断名组成的中断线程名，处理函数是irq_thread()。
        if (IS_ERR(t)) {
            ret = PTR_ERR(t);
            goto out_mput;
        }

        sched_setscheduler_nocheck(t, SCHED_FIFO, &param);

        get_task_struct(t);
        new->thread = t;

        set_bit(IRQTF_AFFINITY, &new->thread_flags);
    }

    if (!alloc_cpumask_var(&mask, GFP_KERNEL)) {
        ret = -ENOMEM;
        goto out_thread;
    }

    /*
     * Drivers are often written to work w/o knowledge about the
     * underlying irq chip implementation, so a request for a
     * threaded irq without a primary hard irq context handler
     * requires the ONESHOT flag to be set. Some irq chips like
     * MSI based interrupts are per se one shot safe. Check the
     * chip flags, so we can avoid the unmask dance at the end of
     * the threaded handler for those.
     */
    if (desc->irq_data.chip->flags & IRQCHIP_ONESHOT_SAFE)----------表示该中断控制器不支持中断嵌套，所以flags去掉IRQF_ONESHOT。
        new->flags &= ~IRQF_ONESHOT;

    raw_spin_lock_irqsave(&desc->lock, flags);
    old_ptr = &desc->action;
    old = *old_ptr;
    if (old) {-----------------------------------------------------old指向desc->action指向的链表，old不为空说明已经有中断添加到中断描述符irq_desc中，说明这是一个共享中断。shared=1。
...
        /* add new interrupt at end of irq queue */
        do {
            /*
             * Or all existing action->thread_mask bits,
             * so we can find the next zero bit for this
             * new action.
             */
            thread_mask |= old->thread_mask;
            old_ptr = &old->next;
            old = *old_ptr;
        } while (old);
        shared = 1;
    }

    /*
     * Setup the thread mask for this irqaction for ONESHOT. For
     * !ONESHOT irqs the thread mask is 0 so we can avoid a
     * conditional in irq_wake_thread().
     */
    if (new->flags & IRQF_ONESHOT) {
        /*
         * Unlikely to have 32 resp 64 irqs sharing one line,
         * but who knows.
         */
        if (thread_mask == ~0UL) {
            ret = -EBUSY;
            goto out_mask;
        }

        new->thread_mask = 1 << ffz(thread_mask);

    } else if (new->handler == irq_default_primary_handler &&---------非IRQF_ONESHOT类型中断，且handler使用默认irq_default_primary_handler(),如果中断触发类型是LEVEL，如果中断出发后不清中断容易引发中断风暴。提醒驱动开发者，没有primary handler且中断控制器不支持硬件oneshot，必须显式指定IRQF_ONESHOT表示位。
           !(desc->irq_data.chip->flags & IRQCHIP_ONESHOT_SAFE)) {

        pr_err("Threaded irq requested with handler=NULL and !ONESHOT for irq %d\n",
               irq);
        ret = -EINVAL;
        goto out_mask;
    }

    if (!shared) {-------------------------------------------------非共享中断情况
        ret = irq_request_resources(desc);
        if (ret) {
            pr_err("Failed to request resources for %s (irq %d) on irqchip %s\n",
                   new->name, irq, desc->irq_data.chip->name);
            goto out_mask;
        }

        init_waitqueue_head(&desc->wait_for_threads);

        /* Setup the type (level, edge polarity) if configured: */
        if (new->flags & IRQF_TRIGGER_MASK) {
            ret = __irq_set_trigger(desc, irq,-------------------调用gic_chip->irq_set_type设置中断触发类型。
                    new->flags & IRQF_TRIGGER_MASK);

            if (ret)
                goto out_mask;
        }

        desc->istate &= ~(IRQS_AUTODETECT | IRQS_SPURIOUS_DISABLED | \
                  IRQS_ONESHOT | IRQS_WAITING);
        irqd_clear(&desc->irq_data, IRQD_IRQ_INPROGRESS);---------清IRQD_IRQ_INPROGRESS标志位

        if (new->flags & IRQF_PERCPU) {
            irqd_set(&desc->irq_data, IRQD_PER_CPU);
            irq_settings_set_per_cpu(desc);
        }

        if (new->flags & IRQF_ONESHOT)
            desc->istate |= IRQS_ONESHOT;

        if (irq_settings_can_autoenable(desc))
            irq_startup(desc, true);
        else
            /* Undo nested disables: */
            desc->depth = 1;

        /* Exclude IRQ from balancing if requested */
        if (new->flags & IRQF_NOBALANCING) {
            irq_settings_set_no_balancing(desc);
            irqd_set(&desc->irq_data, IRQD_NO_BALANCING);
        }

        /* Set default affinity mask once everything is setup */
        setup_affinity(irq, desc, mask);

    } else if (new->flags & IRQF_TRIGGER_MASK) {
..
    }

    new->irq = irq;
    *old_ptr = new;

    irq_pm_install_action(desc, new);

    /* Reset broken irq detection when installing new handler */
    desc->irq_count = 0;
    desc->irqs_unhandled = 0;

    /*
     * Check whether we disabled the irq via the spurious handler
     * before. Reenable it and give it another chance.
     */
    if (shared && (desc->istate & IRQS_SPURIOUS_DISABLED)) {
        desc->istate &= ~IRQS_SPURIOUS_DISABLED;
        __enable_irq(desc, irq);
    }

    raw_spin_unlock_irqrestore(&desc->lock, flags);

    /*
     * Strictly no need to wake it up, but hung_task complains
     * when no hard interrupt wakes the thread up.
     */
    if (new->thread)
        wake_up_process(new->thread);------------------------------如果该中断被线程化，那么就唤醒该内核线程。这里每个中断对应一个线程。

    register_irq_proc(irq, desc);----------------------------------创建/proc/irq/xxx/目录及其节点。
    new->dir = NULL;
    register_handler_proc(irq, new);-------------------------------以action->name创建目录
    free_cpumask_var(mask);

    return 0;
...
}

中断线程什么时候触发执行，在中断上半部硬中断处理完后，如果上半部返回了IRQ_WAKE_THREAD则调用__irq_wake_thread。

以常用spi 中断为例，spi，lpi中断handle_fasteoi_irq。

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);//执行struct irqaction的handler函数
		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://上半部返回这个的话，一般是使用 request_threaded_irq
			/*
			 * Catch drivers which return WAKE_THREAD but
			 * did not set up a thread function
			 */
			if (unlikely(!action->thread_fn)) {
				warn_no_thread(irq, action);//若没有设置thread_fn，打印
				break;
			}

			__irq_wake_thread(desc, action);//中断线程化 唤醒

			/* Fall through to add to randomness */
		case IRQ_HANDLED:
			flags |= action->flags;//已经处理完毕，可以结束
			break;

		default:
			break;
		}

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

	add_interrupt_randomness(irq, flags);

	if (!noirqdebug)
		note_interrupt(irq, desc, retval);
	return retval;
}

 

void __irq_wake_thread(struct irq_desc *desc, struct irqaction *action)
{
	/*
	 * In case the thread crashed and was killed we just pretend that
	 * we handled the interrupt. The hardirq handler has disabled the
	 * device interrupt, so no irq storm is lurking.
	 */
	if (action->thread->flags & PF_EXITING)
		return;

	/*
	 * Wake up the handler thread for this action. If the
	 * RUNTHREAD bit is already set, nothing to do.
	 */
	if (test_and_set_bit(IRQTF_RUNTHREAD, &action->thread_flags))//若已经对IRQF_RUNTHREAD置位，表示已经处于唤醒中，该函数直接返回
		return;

	
	desc->threads_oneshot |= action->thread_mask;

	 */
	atomic_inc(&desc->threads_active);//活跃中断线程计数

	wake_up_process(action->thread);//唤醒action的thread内核线程
}

/*
 * Interrupt handler thread
 */
static int irq_thread(void *data)//创建后就执行了，会在while中睡眠
{
	struct callback_head on_exit_work;
	struct irqaction *action = data;
	struct irq_desc *desc = irq_to_desc(action->irq);
	irqreturn_t (*handler_fn)(struct irq_desc *desc,
			struct irqaction *action);

	if (force_irqthreads && test_bit(IRQTF_FORCED_THREAD,
					&action->thread_flags))
		handler_fn = irq_forced_thread_fn;
	else
		handler_fn = irq_thread_fn;//函数指针

	init_task_work(&on_exit_work, irq_thread_dtor);、//task work
	task_work_add(current, &on_exit_work, false);

	irq_thread_check_affinity(desc, action);

	while (!irq_wait_for_interrupt(action)) {
		irqreturn_t action_ret;

		irq_thread_check_affinity(desc, action);

		action_ret = handler_fn(desc, action);//唤醒之后执行irq_thread_fn
		if (action_ret == IRQ_HANDLED)
			atomic_inc(&desc->threads_handled);

		wake_threads_waitq(desc);//中断线程执行完了，所以唤醒等待队列头上的进程
	}

	/*
	 * This is the regular exit path. __free_irq() is stopping the
	 * thread via kthread_stop() after calling
	 * synchronize_irq(). So neither IRQTF_RUNTHREAD nor the
	 * oneshot mask bit can be set. We cannot verify that as we
	 * cannot touch the oneshot mask at this point anymore as
	 * __setup_irq() might have given out currents thread_mask
	 * again.
	 */
	task_work_cancel(current, irq_thread_dtor);
	return 0;
}