时间管理 2> 长延迟

 4种方法：
1> 忙等待：

void 1_delay(u32 sec)
{
    unsigned long end_point = jiffies + sec*HZ;

    while(!time_before_eq(end_point, jiffies)){
        /*nothing*/;
    }
}

#define time_after(a,b) \
          (typecheck(unsigned long, a) && \
          typecheck(unsigned long, b) && \
          ((long)(b) - (long)(a) < 0))        //学会这招！！,复习有符号到无符号的转换
 #define time_before(a,b) time_after(b,a)      //如此巧妙

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2> 让出CPU：

void 2_delay(u32 sec)
  {
          unsigned long end_point = jiffies + sec*HZ;

          while(!time_before_eq(end_point, jiffies)){
                  schedule();
         }
 }

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3> 等待队列实现：

void 3_delay(u32 s)
 {
          wait_queue_head_t head;
          init_waitqueue_head(&head);

          wait_event_interruptible_timeout(head, 0, s*HZ);
  }

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4> 最理想的实现：

void 4_delay(u32 s)
{
        set_current_state(TASK_INTERRUPTIBLE);
        schedule_timeout(s*HZ);
}