目录
前言
15. 定时器
本章内容为驱动基石之一。
驱动只提供功能，不提供策略。
原文：https://www.cnblogs.com/lizhuming/p/14922233.html
参考源码路径：includelinux imer.h
参考例子就明白如何使用了。
15.1 内核函数汇总
KERNEL 使用。
说明：以下只是给出部分 API 而已。为 Linux5.12.8 版本。还有其它 API 可以实现内核定时器功能，具体参考内核文档。
setup_timer(): 设置定时器。add_timer(): 向内核添加定时器。mod_timer(): 修改定时器超时时间。del_timer(): 删除定时器。
15.2 内核滴答
获取 CONFIG_HZ:
- 在 Linux 内核源码根目录上找到 .config 配置文件。打开该文件找到宏
CONFIG_HZ，该宏定义了内核滴答（tick）频率。 - 命令行：进入 /boot，查找文件 config-5.8.0-53-geberic（各个系统，文件名不一样，看前缀即可） 内容。
- 如命令：
grep ^CONFIG_HZ /boot/config-$(uname -r)。
- APP：
sysconf(_SC_CLK_TCK)。
内核 tick：
每跳动一次，内核全局值 jiffies 就会累加 1。
定时器的时间值就是基于 jiffies 的。
所以，其实也可以比较设定的 滴答值 和 jiffies 来判断是否超时。由此，内核推荐使用以下 4 个 API：
/*
* These inlines deal with timer wrapping correctly. You are
* strongly encouraged to use them
* 1. Because people otherwise forget
* 2. Because if the timer wrap changes in future you won't have to
* alter your driver code.
*
* time_after(a,b) returns true if the time a is after time b.
*
* Do this with "<0" and ">=0" to only test the sign of the result. A
* good compiler would generate better code (and a really good compiler
* wouldn't care). Gcc is currently neither.
*/
#define time_after(a,b)
(typecheck(unsigned long, a) &&
typecheck(unsigned long, b) &&
((long)((b) - (a)) < 0))
#define time_before(a,b) time_after(b,a)
#define time_after_eq(a,b)
(typecheck(unsigned long, a) &&
typecheck(unsigned long, b) &&
((long)((a) - (b)) >= 0))
#define time_before_eq(a,b) time_after_eq(b,a)
15.3 相关结构体
timer_list:
struct timer_list {
/*
* All fields that change during normal runtime grouped to the
* same cacheline
*/
struct hlist_node entry;
unsigned long expires; // 超时时间
void (*function)(struct timer_list *); // 回调函数
u32 flags; // 标志
#ifdef CONFIG_LOCKDEP
struct lockdep_map lockdep_map;
#endif
};
15.4 setup_timer() 设置定时器
功能：设置定时器。初始化 timer_list 结构体。
函数原型：
/**
* timer_setup - prepare a timer for first use
* @timer: the timer in question
* @callback: the function to call when timer expires
* @flags: any TIMER_* flags
*
* Regular timer initialization should use either DEFINE_TIMER() above,
* or timer_setup(). For timers on the stack, timer_setup_on_stack() must
* be used and must be balanced with a call to destroy_timer_on_stack().
*/
#define timer_setup(timer, callback, flags)
__init_timer((timer), (callback), (flags))
15.5 add_timer() 向内核添加定时器
功能：向内核添加定时器。
函数原型：
/**
* add_timer - start a timer
* @timer: the timer to be added
*
* The kernel will do a ->function(@timer) callback from the
* timer interrupt at the ->expires point in the future. The
* current time is 'jiffies'.
*
* The timer's ->expires, ->function fields must be set prior calling this
* function.
*
* Timers with an ->expires field in the past will be executed in the next
* timer tick.
*/
void add_timer(struct timer_list *timer)
{
BUG_ON(timer_pending(timer));
__mod_timer(timer, timer->expires, MOD_TIMER_NOTPENDING);
}
EXPORT_SYMBOL(add_timer);
15.6 mod_timer() 修改定时器超时时间
功能：修改定时器超时时间。
注：注释说明中，修改超时时间相当于删除定时器再重新添加到内核。说明，在添加到内核后就不能简单的使用 imer->expires = expires; 来修改超时时间啦。
函数原型：
/**
* mod_timer - modify a timer's timeout
* @timer: the timer to be modified
* @expires: new timeout in jiffies
*
* mod_timer() is a more efficient way to update the expire field of an
* active timer (if the timer is inactive it will be activated)
*
* mod_timer(timer, expires) is equivalent to:
*
* del_timer(timer); timer->expires = expires; add_timer(timer);
*
* Note that if there are multiple unserialized concurrent users of the
* same timer, then mod_timer() is the only safe way to modify the timeout,
* since add_timer() cannot modify an already running timer.
*
* The function returns whether it has modified a pending timer or not.
* (ie. mod_timer() of an inactive timer returns 0, mod_timer() of an
* active timer returns 1.)
*/
int mod_timer(struct timer_list *timer, unsigned long expires)
{
return __mod_timer(timer, expires, 0);
}
EXPORT_SYMBOL(mod_timer);
15.7 del_timer 删除定时器
功能：删除定时器。
函数原型：
/**
* del_timer - deactivate a timer.
* @timer: the timer to be deactivated
*
* del_timer() deactivates a timer - this works on both active and inactive
* timers.
*
* The function returns whether it has deactivated a pending timer or not.
* (ie. del_timer() of an inactive timer returns 0, del_timer() of an
* active timer returns 1.)
*/
int del_timer(struct timer_list *timer)
{
struct timer_base *base;
unsigned long flags;
int ret = 0;
debug_assert_init(timer);
if (timer_pending(timer)) {
base = lock_timer_base(timer, &flags);
ret = detach_if_pending(timer, base, true);
raw_spin_unlock_irqrestore(&base->lock, flags);
}
return ret;
}
EXPORT_SYMBOL(del_timer);