POLL机制

static DECLARE_WAIT_QUEUE_HEAD(queue_wait);
……
static unsigned int drv_poll(struct file *fp, poll_table * wait)
{
    Unsigned int ret = 0;
    printk("%s %s line %d\n", __FILE__, __FUNCTION__, __LINE__);
    poll_wait(fp, &queue_wait, wait);
     if(atomic_read(&dev->releasekey)) { /* 按键按下 */
     ret = POLLIN | POLLRDNORM;        /* 返回 PLLIN */
 }
     return ret;
}
……
/* 定义自己的file_operations结构体 */
static struct file_operations gpio_key_drv = {
    .owner   = THIS_MODULE,
    .read    = gpio_key_drv_read,
    .poll    = gpio_key_drv_poll,
};
……

……
while (1)
    {
        /* 3. 读文件 */
        ret = poll(fds, 1, timeout_ms);
        if ((ret == 1) && (fds[0].revents & POLLIN))
        {
            read(fd, &val, 4);
            printf("get button : 0x%x\n", val);
        }
        else
        {
            printf("timeout\n");
        }
    }
……

app: poll
      |
drv:sys_poll
      |
      — do_sys_poll(struct pollfd __user * ufds, unsigned int nfds, struct timespec * end_time)
        |  
        - poll_initwait(&table);  >  实际效果：令函数指针 table.pt.qproc = __pollwait，这个函数指针最终会传递给poll_wait函数调用中的wait->qproc
        |
        - do_poll(head, &table, end_time);
        |
        _ for ( walk = head; walk; walk = walk->next )
          {
                for (; pfd != pfd_end; pfd++) {    /* 可以监测多个驱动设备所产生的事件 */
                    if (do_pollfd(pfd, pt)) {  
                      |
                      _  mask = file->f_op->poll(file, pwait); > 实际效果：进入我们写的drivers_poll(file,pwait)
                                |
                                 _  poll_wait(file, &button_waitq, wait); > 实际效果：进入__pollwait(file, &button_waitq, wait)，也就是将进程挂接到button_waitq等待队列下
                                          |
                                          _ if (p && p->_qproc && wait_address)
                                                p->_qproc(filp, wait_address, p); > 将线程放入等待队列
                                            |
                                             _ add_wait_queue()
                                |
                                —  mask赋值 ; return mask; /* 返回事件类型 */
                         pollfd->revents = mask;    /* 将实际事件类型返回 */
                         count++;
                         pt->_qproc = NULL;         //这个地方清空，下次调用驱动poll，就不会将进程挂入等待队列
                     } 
                 } 
               if (count || timed_out) /* 如果有事件发生，或者超时，则跳出poll */ 
                  break; 
               if (!poll_schedule_timeout(wait, TASK_INTERRUPTIBLE, to, slack)) /* 如果没有事件发生，那么陷入休眠状态 */ 
                  timed_out = 1; 
          }