Detailed explanation of Linux kernel timer and delay work driver development

Linux kernel timers and delay work are two commonly used mechanisms to implement scheduled tasks and delayed execution tasks. They allow the driver to execute specific functions at the appropriate time point to adapt to the needs and characteristics of the hardware device. But how do you properly use Linux kernel timers to work with delays? This article will introduce the basic knowledge and skills of Linux kernel timer and delay work driver development from both theoretical and practical aspects, as well as some common problems and solutions.

Kernel timer

The timer on the software ultimately relies on the hardware clock. Simply put, the kernel will detect whether each timer registered to the kernel has expired after the clock interrupt occurs. If it expires, the corresponding registration function will be called back. Do this as an interrupt to the bottom half. In fact, the clock interrupt handler triggers the TIMER_SOFTIRQ soft interrupt and runs all timers that have expired on the current processor.
Device drivers that want to obtain time information and require timing services can use kernel timers.

jiffies

To talk about the kernel timer, we must first talk about an important concept of time in the kernel: jiffies variable, as the basis of the kernel clock, jiffies will increase by 1 every fixed time , called adding a beat. This fixed interval is implemented by timer interrupts. How many timer interrupts are generated per second is determined by the HZ macro defined in . In this way, it can be passed jiffies obtains a period of time, for example jiffies/HZ represents the number of seconds since system startup. The next two seconds are (jiffies/HZ 2). The kernel uses jiffies for timing. Seconds are converted into jiffies: seconds*HZ, so the unit is jiffy and the current time is used as the basis for timing 2 seconds: ( jiffies/HZ 2)*HZ=jiffies 2*HZIf you want to get the current time, you can use **do_gettimeofday()**. This function fills a struct timeval structure with a resolution close to subtle.

//kernel/time/timekeeping.c
 473 /**
 474  * do_gettimeofday - Returns the time of day in a timeval
 475  * @tv:         pointer to the timeval to be set
 476  *
 477  * NOTE: Users should be converted to using getnstimeofday()
 478  */
 479 void do_gettimeofday(struct timeval *tv)   

In order to allow the hardware enough time to complete some tasks, the driver often needs to delay the execution of specific code for a period of time. Depending on the length of the delay, long delay and # are used in kernel development. ##Short delay Two concepts. The definition of long delay is: delay time > multiple jiffies. To achieve long delay, you can use the method of querying jiffies:

time_before(jiffies, new_jiffies);
time_after(new_jiffiesmjiffies);

**The definition of short delay is: the delay event is close to or shorter than one jiffy. To achieve short delay, you can call

udelay();
mdelay();

These two functions are busy waiting functions, which consume a lot of CPU time. The former uses software loops to delay a specified number of microseconds, and the latter uses the nesting of the former to achieve millisecond-level delays.

Timer

The driver can register a kernel timer to specify a function to be executed at a certain time in the future. The timer starts counting when it is registered to the kernel, and the registered function will be executed after the specified time is reached. That is, the timeout value is a jiffies value. When the jiffies value is greater than timer->expires, timer->function will be executed. The API is as follows

//定一个定时器
struct timer_list my_timer;

//初始化定时器
void init_timer(struct timer_list *timer);
mytimer.function = my_function;
mytimer.expires = jiffies +HZ;

//增加定时器
void add_timer(struct timer_list *timer);

//删除定时器
int del_tiemr(struct timer_list *timer);

Example

static struct timer_list tm;
struct timeval oldtv;

void callback(unsigned long arg)
{
    struct timeval tv;
    char *strp = (char*)arg;
    do_gettimeofday(&tv);
    printk("%s: %ld, %ld\n", __func__,
        tv.tv_sec - oldtv.tv_sec,
        tv.tv_usec- oldtv.tv_usec);
    oldtv = tv;
    tm.expires = jiffies+1*HZ;
    add_timer(&tm);
}

static int __init demo_init(void)
{
    init_timer(&tm);
    do_gettimeofday(&oldtv);
    tm.function= callback;
    tm.data    = (unsigned long)"hello world";
    tm.expires = jiffies+1*HZ;
    add_timer(&tm);
    return 0;
}

Delayed work

In addition to using the kernel timer to complete scheduled delay work, the Linux kernel also provides a set of encapsulated "shortcuts" -

delayed_work, which is similar to the kernel timer and essentially uses work queues and timing. Device implementation,

//include/linux/workqueue.h
100 struct work_struct {           
101         atomic_long_t data;
102         struct list_head entry;
103         work_func_t func;
104 #ifdef CONFIG_LOCKDEP
105         struct lockdep_map lockdep_map;
106 #endif
107 };
113 struct delayed_work {              
114         struct work_struct work;
115         struct timer_list timer;
116 
117         /* target workqueue and CPU ->timer uses to queue ->work */
118         struct workqueue_struct *wq;
119         int cpu;
120 };

“

struct work_struct
–103–>需要延迟执行的函数, typedef void (work_func_t)(struct work_struct work);

”

至此，我们可以使用一个delayed_work对象以及相应的调度API实现对指定任务的延时执行

//注册一个延迟执行
591 static inline bool schedule_delayed_work(struct delayed_work *dwork,unsigned long delay)
//注销一个延迟执行
2975 bool cancel_delayed_work(struct delayed_work *dwork)    

和内核定时器一样，延迟执行只会在超时的时候执行一次，如果要实现循环延迟，只需要在注册的函数中再次注册一个延迟执行函数。

schedule_delayed_work(&work,msecs_to_jiffies(poll_interval));

本文从理论和实践两方面，详细介绍了Linux内核定时器与延迟工作驱动开发的基本知识和技巧。我们首先了解了Linux内核定时器与延迟工作的概念、原理、特点和API函数，然后学习了如何使用Linux内核定时器与延迟工作来实现按键事件的检测和处理。最后，我们介绍了一些在Linux内核定时器与延迟工作驱动开发过程中可能遇到的问题，以及相应的解决方法。

通过本文，我们希望能够帮助你掌握Linux内核定时器与延迟工作驱动开发的基本方法和技巧，为你在嵌入式Linux领域的进一步学习和工作打下坚实的基础。

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