rt-thread的定时器管理源码分析

1 前言

rt-thread可以采用软件定时器或硬件定时器来实现定时器管理的，所谓软件定时器是指由操作系统提供的一类系统接口，它构建在硬件定时器基础之上，使系统能够提供不受数目限制的定时器服务。而硬件定时器是芯片本身提供的定时功能。一般是由外部晶振提供给芯片输入时钟，芯片向软件模块提供一组配置寄存器，接受控制输入，到达设定时间值后芯片中断控制器产生时钟中断。硬件定时器的精度一般很高，可以达到纳秒级别，并且是中断触发方式。软件定时器的精度取决于它使用的硬件定时器精度。而rt-thread操作系统在默认情况下是采用的硬件定时器的方式，用户可以通过修改宏定义#ifdef RT_USING_TIMER_SOFT来修改采用哪种。

2 rt-thread的定时器的基本工作原理

在RT-Thread定时器模块维护两个重要的全局变量，一个是当前系统的时间rt_tick（当硬件定时器中断来临时，它将加1），另一个是定时器链表rt_timer_list，系统中新创建的定时期都会被以排序的方式插入到rt_timer_list(硬件定时器模式下使用)链表中,rt_timer_list的每个节点保留了一个定时器的信息，并且在这个节点加入链表时就计算好了产生时间到达时的时间点，即tick,在rt-thread系统中如果采用软件定时器模式，则存在一定时器线程rt_thread_timer_entry，不断获取当前TICK值并与定时器链表rt_timer_list上的定时器对比判断是否时间已到，一旦发现就调用对应的回调函数，即事件处理函数进行处理，而如果采用硬件定时器管理模式的话，则该检查过程放到系统时钟中断例程中进行处理，此时，是不存在定时器线程的。如下图：注：如果采用软件定时器软件定时器，则该定时器链表为rt_soft_timer_list。

3 源码分析

3.1 数据定义

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/**
* timer structure
*/
struct rt_timer
{
struct rt_object parent; //内核对象
rt_list_t list; //链表节点
void (*timeout_func)(void *parameter); //定时器超时例程
void *parameter; //定时器例程的传入参数
rt_tick_t init_tick; //定时器的超时时间,即总共多长时间将产生超时事件
rt_tick_t timeout_tick; //定时器超时的时间点，即产生超时事件时那一该的时间点
};
typedef struct rt_timer *rt_timer_t;

3.2 rt-thread的软件定时器模式

软件定时器线程初始化及启动:

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/**
* @ingroup SystemInit
*
* This function will initialize system timer thread
*/
void rt_system_timer_thread_init(void)
{
#ifdef RT_USING_TIMER_SOFT//如果采用软件定时器管理模式，则启动定时器线程
rt_list_init(&rt_soft_timer_list);//初始化软件定时器链表
/* start software timer thread */
rt_thread_init(&timer_thread,//初始化软件定时器线程，并启动
"timer",
rt_thread_timer_entry,
RT_NULL,
&timer_thread_stack[0],
sizeof(timer_thread_stack),
RT_TIMER_THREAD_PRIO,
10);
/* startup */
rt_thread_startup(&timer_thread);
#endif
}

软件定时器线程如下：

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/* system timer thread entry */
static void rt_thread_timer_entry(void *parameter)
{
rt_tick_t next_timeout;
while (1)
{
/* get the next timeout tick */
next_timeout = rt_timer_list_next_timeout(&rt_soft_timer_list);//得到软件定时器链表上的下一个定时器的超时时间点
if (next_timeout == RT_TICK_MAX)//如果超过范围，则挂起当前线程，继续线程调度
{
/* no software timer exist, suspend self. */
rt_thread_suspend(rt_thread_self());
rt_schedule();
}
else
{
rt_tick_t current_tick;
/* get current tick */
current_tick = rt_tick_get();//获取当前时间点
if ((next_timeout - current_tick) < RT_TICK_MAX/2)//离下个中断时间点还差些时候
{
/* get the delta timeout tick */
next_timeout = next_timeout - current_tick;//计算还差多长时间
rt_thread_delay(next_timeout);//休眠一段时间
}
}
/* lock scheduler */
rt_enter_critical();//时间到，进入临界区
/* check software timer */
rt_soft_timer_check();//检查是否该产生超时事件
/* unlock scheduler */
rt_exit_critical();//退出临界区
}
}

检查是否产生中断函数rt_soft_timer_check函数如下定义:

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/**
* This function will check timer list, if a timeout event happens, the
* corresponding timeout function will be invoked.
*/
void rt_soft_timer_check(void)
{
rt_tick_t current_tick;
rt_list_t *n;
struct rt_timer *t;
RT_DEBUG_LOG(RT_DEBUG_TIMER, ("software timer check enter\n"));
current_tick = rt_tick_get();//得到当前时间点
for (n = rt_soft_timer_list.next; n != &(rt_soft_timer_list);)//得到下一定时器节点
{
t = rt_list_entry(n, struct rt_timer, list);//t指向rt_timer定时器
/*
* It supposes that the new tick shall less than the half duration of
* tick max.
*/
if ((current_tick - t->timeout_tick) < RT_TICK_MAX / 2)//如果当前的时间点超过定时器的超时时间点
{
RT_OBJECT_HOOK_CALL(rt_timer_timeout_hook, (t));//使用钩子函数
/* move node to the next */
n = n->next;//指向下一定时器
/* remove timer from timer list firstly */
rt_list_remove(&(t->list));//移除当前定时器
/* call timeout function */
t->timeout_func(t->parameter);//产生定时器超时事件，调用对应处理函数
/* re-get tick */
current_tick = rt_tick_get();//再次获取当前时间点
RT_DEBUG_LOG(RT_DEBUG_TIMER, ("current tick: %d\n", current_tick));
if ((t->parent.flag & RT_TIMER_FLAG_PERIODIC) &&//如果当前定时器是周期性定时器，则将其再次按序放入软件定时器链表
(t->parent.flag & RT_TIMER_FLAG_ACTIVATED))
{
/* start it */
t->parent.flag &= ~RT_TIMER_FLAG_ACTIVATED;//置标志为非激活状态
rt_timer_start(t);//再次将定时器t放入软件定时器链表末尾
}
else
{
/* stop timer */
t->parent.flag &= ~RT_TIMER_FLAG_ACTIVATED;//置标志为非激活状态
}
}
else break; /* not check anymore */
}
RT_DEBUG_LOG(RT_DEBUG_TIMER, ("software timer check leave\n"));
}

上面代码中，为什么定时器里判断超时的条件是((current_tick - t→timeout_tick) < RT_TICK_MAX/2)？

因为系统时钟溢出后会自动回绕。取定时器比较最大值是定时器最大值的一半，即RT_TICK_MAX/2（在比较两个定时器值时，值是32位无符号数，相减运算将会自动回绕）。系统支持的定时器最大长度就是RT_TICK_MAX的一半：即248天(10ms/tick)，124天(5ms/tick)，24.5天(1ms/tick)，以下内容相同道理。

其上rt_timer_start函数如下定义:

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/**
* This function will start the timer
*
* @param timer the timer to be started
*
* @return the operation status, RT_EOK on OK, -RT_ERROR on error
*/
rt_err_t rt_timer_start(rt_timer_t timer)
{
struct rt_timer *t;
register rt_base_t level;
rt_list_t *n, *timer_list;
/* timer check */
RT_ASSERT(timer != RT_NULL);
if (timer->parent.flag & RT_TIMER_FLAG_ACTIVATED)//如果传入的定时器已经激活，则直接返回错误
return -RT_ERROR;
RT_OBJECT_HOOK_CALL(rt_object_take_hook, (&(timer->parent)));//使用钩子函数
/*
* get timeout tick,
* the max timeout tick shall not great than RT_TICK_MAX/2
*/
RT_ASSERT(timer->init_tick < RT_TICK_MAX / 2);
timer->timeout_tick = rt_tick_get() + timer->init_tick;//得到定时器超时的时间点
/* disable interrupt */
level = rt_hw_interrupt_disable();//关中断
#ifdef RT_USING_TIMER_SOFT//如果采用的是软件定时器管理模式，则将定时器加入到rt_soft_timer_list中
if (timer->parent.flag & RT_TIMER_FLAG_SOFT_TIMER)
{
/* insert timer to soft timer list */
timer_list = &rt_soft_timer_list;
}
else
#endif
{
/* insert timer to system timer list */
timer_list = &rt_timer_list;
}
for (n = timer_list->next; n != timer_list; n = n->next)//将定时器按序加入到定时器链表中
{
t = rt_list_entry(n, struct rt_timer, list);
/*
* It supposes that the new tick shall less than the half duration of
* tick max.
*/
if ((t->timeout_tick - timer->timeout_tick) < RT_TICK_MAX / 2)
{
rt_list_insert_before(n, &(timer->list));//将定时器timer插入到t之前
break;
}
}
/* no found suitable position in timer list */
if (n == timer_list)//没有找到合适的位置，则放到链表头
{
rt_list_insert_before(n, &(timer->list));
}
timer->parent.flag |= RT_TIMER_FLAG_ACTIVATED;//置定时器为激活状态
/* enable interrupt */
rt_hw_interrupt_enable(level);
#ifdef RT_USING_TIMER_SOFT
if (timer->parent.flag & RT_TIMER_FLAG_SOFT_TIMER)//如果系统采用的是软件定时器管理模式，且软件定时器线程处理ready状态，则恢复此线程
{
/* check whether timer thread is ready */
if (timer_thread.stat != RT_THREAD_READY)
{
/* resume timer thread to check soft timer */
rt_thread_resume(&timer_thread);//恢复定时器线程
rt_schedule();//开始线程调度
}
}
#endif
return -RT_EOK;
}

软件定时器管理模式的源码分析完了，接下来介绍RTT的硬件定时器管理模式。

3.3 RTT的硬件定时器管理模式

硬件定时器管理模式顾名思义，就是说与硬件相关，因此，不用的MCU，其部分源码是不一样的，因为其要采用MCU的系统时钟中断例程来实现。

以STM32F2XX为例，先找到其启动汇编，位置在：RTT/bsp/stm32f2xx/Libraries/CMSIS/CM3/DeviceSupport/ST/STM32F2xx/startup/arm/startup_stm32f2xx.s

找到中断向量:

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DCD SysTick_Handler ; SysTick Handler

这是系统时钟中断向量，再找到其中断例程实现：

在bsp/stm32f2xx/drivers/board.c文件中:

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/**
* This is the timer interrupt service routine.
*
*/
void SysTick_Handler(void)//系统时钟中断例程
{
/* enter interrupt */
rt_interrupt_enter();
rt_tick_increase();
/* leave interrupt */
rt_interrupt_leave();
}

其中rt_tick_increase函数在RTT/src/clock.c文件中的实现如下:

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/**
* This function will notify kernel there is one tick passed. Normally,
* this function is invoked by clock ISR.
*/
void rt_tick_increase(void)
{
struct rt_thread *thread;
/* increase the global tick */
++ rt_tick;//全局rt_tick加1
/* check time slice */
thread = rt_thread_self();//得到当前正在运行的线程
-- thread->remaining_tick;//纯种剩下时间减1
if (thread->remaining_tick == 0)//如果线程剩余时间为0，即调度时间已到
{
/* change to initialized tick */
thread->remaining_tick = thread->init_tick;//将线程剩余时间重新设置初始化值
/* yield */
rt_thread_yield();//调度时间到，切换到其它线程
}
/* check timer */
rt_timer_check();//检查硬件定时器链表是否有定时器产生超时事件
}

其中rt_timer_check函数在RTT/src/timer.c文件中如下定义:

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/**
* This function will check timer list, if a timeout event happens, the
* corresponding timeout function will be invoked.
*
* @note this function shall be invoked in operating system timer interrupt.
*/
void rt_timer_check(void)
{
struct rt_timer *t;
rt_tick_t current_tick;
register rt_base_t level;
RT_DEBUG_LOG(RT_DEBUG_TIMER, ("timer check enter\n"));
current_tick = rt_tick_get();
/* disable interrupt */
level = rt_hw_interrupt_disable();
while (!rt_list_isempty(&rt_timer_list))
{
t = rt_list_entry(rt_timer_list.next, struct rt_timer, list);
/*
* It supposes that the new tick shall less than the half duration of
* tick max.
*/
if ((current_tick - t->timeout_tick) < RT_TICK_MAX/2)
{
RT_OBJECT_HOOK_CALL(rt_timer_timeout_hook, (t));
/* remove timer from timer list firstly */
rt_list_remove(&(t->list));
/* call timeout function */
t->timeout_func(t->parameter);
/* re-get tick */
current_tick = rt_tick_get();
RT_DEBUG_LOG(RT_DEBUG_TIMER, ("current tick: %d\n", current_tick));
if ((t->parent.flag & RT_TIMER_FLAG_PERIODIC) &&
(t->parent.flag & RT_TIMER_FLAG_ACTIVATED))
{
/* start it */
t->parent.flag &= ~RT_TIMER_FLAG_ACTIVATED;
rt_timer_start(t);
}
else
{
/* stop timer */
t->parent.flag &= ~RT_TIMER_FLAG_ACTIVATED;
}
}
else
break;
}
/* enable interrupt */
rt_hw_interrupt_enable(level);
RT_DEBUG_LOG(RT_DEBUG_TIMER, ("timer check leave\n"));
}

此函数与rt_soft_timer_check基本大致相同，只不过一个是查找硬件定时器链表rt_timer_list，一个是查找rt_soft_timer_list.

在此，硬件定时器管理模式基本上介绍完毕，接下来介绍一些定时器接口.

4 定时器接口

4.1 定时器初始化

静态初始化定义器

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/**
* This function will initialize a timer, normally this function is used to
* initialize a static timer object.
*
* @param timer the static timer object
* @param name the name of timer
* @param timeout the timeout function
* @param parameter the parameter of timeout function
* @param time the tick of timer
* @param flag the flag of timer
*/
void rt_timer_init(rt_timer_t timer,
const char *name,
void (*timeout)(void *parameter),
void *parameter,
rt_tick_t time,
rt_uint8_t flag)
{
/* timer check */
RT_ASSERT(timer != RT_NULL);
/* timer object initialization */
rt_object_init((rt_object_t)timer, RT_Object_Class_Timer, name);//初始化内核对象
_rt_timer_init(timer, timeout, parameter, time, flag);
}

_rt_timer_init函数如下定义:

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static void _rt_timer_init(rt_timer_t timer,
void (*timeout)(void *parameter),
void *parameter,
rt_tick_t time,
rt_uint8_t flag)
{
/* set flag */
timer->parent.flag = flag;//置flag
/* set deactivated */
timer->parent.flag &= ~RT_TIMER_FLAG_ACTIVATED;//初始化时，设置为非激活状态
timer->timeout_func = timeout;//设置超时事件处理函数
timer->parameter = parameter;//超时事件处理函数的传入参数
timer->timeout_tick = 0;//定时器的超时时间点初始化时为0
timer->init_tick = time;//置超时时间
/* initialize timer list */
rt_list_init(&(timer->list));//初始化本身节点
}

动态创建定时器

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/**
* This function will create a timer
*
* @param name the name of timer
* @param timeout the timeout function
* @param parameter the parameter of timeout function
* @param time the tick of timer
* @param flag the flag of timer
*
* @return the created timer object
*/
rt_timer_t rt_timer_create(const char *name,
void (*timeout)(void *parameter),
void *parameter,
rt_tick_t time,
rt_uint8_t flag)
{
struct rt_timer *timer;
/* allocate a object */
timer = (struct rt_timer *)rt_object_allocate(RT_Object_Class_Timer, name);//动态分配定时器内核对象
if (timer == RT_NULL)
{
return RT_NULL;
}
_rt_timer_init(timer, timeout, parameter, time, flag);//调用上述的初始化接口
return timer;
}

4.2 脱离和删除

脱离:

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/**
* This function will detach a timer from timer management.
*
* @param timer the static timer object
*
* @return the operation status, RT_EOK on OK; RT_ERROR on error
*/
rt_err_t rt_timer_detach(rt_timer_t timer)
{
register rt_base_t level;
/* timer check */
RT_ASSERT(timer != RT_NULL);
/* disable interrupt */
level = rt_hw_interrupt_disable();//关中断
/* remove it from timer list */
rt_list_remove(&(timer->list));//从定时器链表中移除
/* enable interrupt */
rt_hw_interrupt_enable(level);//开中断
rt_object_detach((rt_object_t)timer);//脱离内核对象
return -RT_EOK;
}

删除动态创建的定时器

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/**
* This function will delete a timer and release timer memory
*
* @param timer the timer to be deleted
*
* @return the operation status, RT_EOK on OK; RT_ERROR on error
*/
rt_err_t rt_timer_delete(rt_timer_t timer)
{
register rt_base_t level;
/* timer check */
RT_ASSERT(timer != RT_NULL);
/* disable interrupt */
level = rt_hw_interrupt_disable();//关中断
/* remove it from timer list */
rt_list_remove(&(timer->list));//从定时器链表中移除
/* enable interrupt */
rt_hw_interrupt_enable(level);//开中断
rt_object_delete((rt_object_t)timer);//删除动态创建的定时器内核对象
return -RT_EOK;
}

4.3 启动定时器

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/**
* This function will start the timer
*
* @param timer the timer to be started
*
* @return the operation status, RT_EOK on OK, -RT_ERROR on error
*/
rt_err_t rt_timer_start(rt_timer_t timer)

此接口已在上面介绍软件定时器模式时已有分析，这里就不再重复了。

4.4 停止定时器

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/**
* This function will stop the timer
*
* @param timer the timer to be stopped
*
* @return the operation status, RT_EOK on OK, -RT_ERROR on error
*/
rt_err_t rt_timer_stop(rt_timer_t timer)
{
register rt_base_t level;
/* timer check */
RT_ASSERT(timer != RT_NULL);
if (!(timer->parent.flag & RT_TIMER_FLAG_ACTIVATED))//如果定时器已经为非激活状态
return -RT_ERROR;
RT_OBJECT_HOOK_CALL(rt_object_put_hook, (&(timer->parent)));//使用钩子函数
/* disable interrupt */
level = rt_hw_interrupt_disable();//关中断
/* remove it from timer list */
rt_list_remove(&(timer->list));//从定时器链表中移除
/* enable interrupt */
rt_hw_interrupt_enable(level);//开中断
/* change stat */
timer->parent.flag &= ~RT_TIMER_FLAG_ACTIVATED;//置非激活状态
return RT_EOK;
}

4.5 控制

此接口是用来修改一个定时器的参数，如下代码：

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/**
* This function will get or set some options of the timer
*
* @param timer the timer to be get or set
* @param cmd the control command
* @param arg the argument
*
* @return RT_EOK
*/
rt_err_t rt_timer_control(rt_timer_t timer, rt_uint8_t cmd, void *arg)
{
/* timer check */
RT_ASSERT(timer != RT_NULL);
switch (cmd)
{
case RT_TIMER_CTRL_GET_TIME://获取时间参数
*(rt_tick_t *)arg = timer->init_tick;
break;
case RT_TIMER_CTRL_SET_TIME://修改时间参数
timer->init_tick = *(rt_tick_t *)arg;
break;
case RT_TIMER_CTRL_SET_ONESHOT://修改定时器模式为单次触发定时器
timer->parent.flag &= ~RT_TIMER_FLAG_PERIODIC;
break;
case RT_TIMER_CTRL_SET_PERIODIC://修改定时器为周期触发定时器
timer->parent.flag |= RT_TIMER_FLAG_PERIODIC;
break;
}
return RT_EOK;
}

完!