[Fw]初探linux中断系统（1）

1. 重要接口

LDD上说，“内核维护了一个中断信号线的注册表，该注册表类似于I/O端口的注册表。模块在使用中断前要先请求一个中断通道（或者中断请求IRQ），然后在使用后释放该通道。”

撇开系统如何遍历各个设备进行初始化，上面两句话说的实际上就是指两个接口函数：

externint __must_check request_irq(unsigned int irq, irq_handler_t handler, unsigned long flags, constchar*name, void*dev);

  externvoid free_irq(unsigned int, void*);

顾名思义，以上两个函数分别用于申请和释放IRQ。

而再一看，会发现其实request_irq是个“皮包”函数，它的定义是这样的：

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

所以实际上起到申请IRQ作用的，正是这个request_threaded_irq函数。一查，它位于/kernel/irq/manage.c中。

2.追随request_threaded_irq

先贴上request_threaded_irq全文

int request_threaded_irq(unsigned int irq, irq_handler_t handler,
             irq_handler_t thread_fn, unsigned long irqflags,
             constchar*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).
     */
    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;
    }

    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);
    retval = __setup_irq(irq, desc, action);
    chip_bus_sync_unlock(irq, desc);

    if (retval)
        kfree(action);

#ifdef CONFIG_DEBUG_SHIRQ
    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;
}

可以看到除去一些验证的语句，整个函数主要完成的任务是初始化了一个irqaction类型的struct和一个irq_desc类型的struct，接着对这两个struct进一步赋值和处理，便实现了IRQ申请。至此，我们有理由认为这两个struct是kernel管理IRQ的核心数据结构。因此不妨看看他们都是什么样的。

struct irq_desc {
    unsigned int        irq;
    struct timer_rand_state *timer_rand_state;
    unsigned int*kstat_irqs;
#ifdef CONFIG_INTR_REMAP
    struct irq_2_iommu      *irq_2_iommu;
#endif
    irq_flow_handler_t    handle_irq;
    struct irq_chip        *chip;
    struct msi_desc        *msi_desc;
    void*handler_data;
    void*chip_data;
    struct irqaction    *action;    /* IRQ action list */
    unsigned int        status;        /* IRQ status */

    unsigned int        depth;        /* nested irq disables */
    unsigned int        wake_depth;    /* nested wake enables */
    unsigned int        irq_count;    /* For detecting broken IRQs */
    unsigned long        last_unhandled;    /* Aging timer for unhandled count */
    unsigned int        irqs_unhandled;
    raw_spinlock_t        lock;
#ifdef CONFIG_SMP
    cpumask_var_t        affinity;
    conststruct cpumask    *affinity_hint;
    unsigned int        node;
#ifdef CONFIG_GENERIC_PENDING_IRQ
    cpumask_var_t        pending_mask;
#endif
#endif
    atomic_t        threads_active;
    wait_queue_head_t       wait_for_threads;
#ifdef CONFIG_PROC_FS
    struct proc_dir_entry    *dir;
#endif
    constchar*name;
} ____cacheline_internodealigned_in_smp;

irq_desc实际是个用于构成数组的数据结构。这里irq就是我们熟悉的irq号，每个设备申请到一个IRQ，就需要填充一个irq_desc，并由kernel放入所维护的数组中进行管理。在这些需要填充的内容里，irq_chip和irqaction是两个比较有助于理解数据结构的struct。

struct irq_chip {
    constchar*name;
    unsigned int    (*startup)(unsigned int irq);
    void        (*shutdown)(unsigned int irq);
    void        (*enable)(unsigned int irq);
    void        (*disable)(unsigned int irq);

    void        (*ack)(unsigned int irq);
    void        (*mask)(unsigned int irq);
    void        (*mask_ack)(unsigned int irq);
    void        (*unmask)(unsigned int irq);
    void        (*eoi)(unsigned int irq);

    void        (*end)(unsigned int irq);
    int        (*set_affinity)(unsigned int irq,
                    conststruct cpumask *dest);
    int        (*retrigger)(unsigned int irq);
    int        (*set_type)(unsigned int irq, unsigned int flow_type);
    int        (*set_wake)(unsigned int irq, unsigned int on);

    void        (*bus_lock)(unsigned int irq);
    void        (*bus_sync_unlock)(unsigned int irq);

    /* Currently used only by UML, might disappear one day.*/
#ifdef CONFIG_IRQ_RELEASE_METHOD
    void        (*release)(unsigned int irq, void*dev_id);
#endif
    /*
     * For compatibility, ->typename is copied into ->name.
     * Will disappear.
     */
    constchar*typename;
};

这个struct里主要定义了硬件层面上一个系统对一个IRQ的管理接口。

struct irqaction {
    irq_handler_t handler;
    unsigned long flags;
    constchar*name;
    void*dev_id;
    struct irqaction *next;
    int irq;
    struct proc_dir_entry *dir;
    irq_handler_t thread_fn;
    struct task_struct *thread;
    unsigned long thread_flags;
};

这个struct中handler定义了中断处理函数， *next指向了下一个irqaction，也就是说irqaction是以链表的形式存在的。也就是说，每一个IRQ对应一个irq_desc，而irq_desc维护着irq_chip管理了硬件层面的中断使能，同时irq_desc也维护了一个irqaction链表。

根据所查的资料，实际上，系统在处理一个中断时，会根据中断号调用irq_desc数组中的handle_irq, handle_irq再使用chip控制硬件的使能，接着调用irqaction链表，逐个调用中断处理函数。

回过头来，request一个IRQ的过程实际上就是构造irqaction项，free的过程就是移除不需要的irqaction项。

http://www.cnblogs.com/garychen2272/archive/2011/02/25/1964176.html