Linux device driver timing and delay: a convenient method to implement time-related functions

Have you ever wondered how to write drivers for your devices in Linux? Have you ever thought about how to enable your driver to implement some time-related functions in a Linux system, such as timing, delay, timeout, etc.? If you are interested in these issues, then this article will introduce you to an effective method to achieve these goals-Linux device driver timing and delay.

Linux generates timer interrupts at regular intervals (measured by HZ) through the system hardware timer. Each interrupt causes the value jiffies of a kernel counter to accumulate, so this jiffies records the time elapsed since the system started, and then the kernel Based on this, software timers and delays are implemented.

Demo for jiffies and HZ

    #include 

    unsigned long j, stamp_1, stamp_half, stamp_n;

    j = jiffies; /* read the current value */
    stamp_1 = j + HZ; /* 1 second in the future */
    stamp_half = j + HZ/2; /* half a second */
    stamp_n = j + n * HZ / 1000; /* n milliseconds */

---

Kernel timer

The hardware clock interrupt handler will raise the TIMER_SOFTIRQ soft interrupt and run all core timers that have expired on the current processor.

Timer definition/initialization

In the Linux kernel, an instance of the timer_list structure corresponds to a timer:

    /* 当expires指定的定时器到期时间期满后，将执行function(data) */
    struct timer_list {
        unsigned long expires;           /*定时器到期时间*/
        void (*function)(unsigned long); /* 定时器处理函数 */
        unsigned long data;              /* function的参数 */
        ...
    };

    /* 定义 */
    struct timer_list my_timer;

    /* 初始化函数 */
    void init_timer(struct timer_list * timer);
    /* 初始化宏 */
    TIMER_INITIALIZER(_function, _expires, _data)

    /* 定义并初始化宏 */
    DEFINE_TIMER(_name, _function, _expires, _data)

Timer Add/Remove

    /* 注册内核定时器，将定时器加入到内核动态定时器链表中 */
    void add_timer(struct timer_list * timer);

    /* del_timer_sync()是 del_timer()的同步版，在删除一个定时器时需等待其被处理完，
       因此该函数的调用不能发生在中断上下文 */
    void del_timer(struct timer_list * timer);
    void del_timer_sync(struct timer_list * timer);

Timing time modification

    int mod_timer(struct timer_list *timer, unsigned long expires);

---

Delay

Short delay

    void ndelay(unsigned long nsecs);
    void udelay(unsigned long usecs);
    void mdelay(unsigned long msecs);

When the kernel starts, it will run a delay test program (delay loop calibration) to calculate lpj (loops per jiffy). These functions are implemented based on lpj, which is busy waiting.

Long delay

• A very intuitive way is to compare the current jiffies with the target jiffies:

  int time_after(unsigned long a, unsigned long b);    /* a after b, true */
  int time_before(unsigned long a, unsigned long b);   /* a before b */
  int time_after_eq(unsigned long a, unsigned long b); /* a after or equal b */
  int time_before_eq(unsigned long a, unsigned long b);/* a before or equal b */
  

• Asleep delay

  void msleep(unsigned int millisecs);
  unsigned long msleep_interruptible(unsigned int millisecs);
  void ssleep(unsigned int seconds);
  

  Tip: msleep() and ssleep() cannot be interrupted.

Through this article, we learned about the application and role of timing and delay in Linux device drivers, and learned how to use various timers and delay functions. We found that timing and delay are a very suitable method for the development of embedded systems, which allows us to easily implement time-related functions. Of course, timing and delay also have some precautions and limitations, such as accuracy issues and performance impacts. Therefore, when using timing and delay, we need to have certain hardware knowledge and experience, as well as good programming habits and debugging skills. I hope this article can provide you with a simple and useful guide, giving you a preliminary understanding of timing and delay.

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