Linux hrtimer的实现

1. Linux hrtimer的实现方案

       Linux hrtimer的实现是依赖硬件(通过可编程定时器来实现)的支持的，而且此定时器有自己的专用寄存器， 硬中断和频率。比如我的板子上的对应参数如下：

    Timer at Vir:0xE0100200 = Phy:0xE0100200, using Irq:27, at Freq:250000000，由此可见，其频率为250MHz，所以其精度为:1/250000000=4ns，比系统时钟jiffy(HZ=100,精度为10ms)的精度高得太多了。可是支持此高精度timer是需要付出硬件成本的。即它是一个硬件时钟。这里所说的硬件时钟特指的是硬件计时器时钟。

 

2. 硬件时钟 数据结构
　　和硬件计时器（本文又称作硬件时钟，区别于软件时钟）相关的数据结构主要有两个：
　　struct clocksource ：对硬件设备的抽象，描述时钟源信息

struct clocksource {/* * First part of structure is read mostly */char *name;struct list_head list;int rating;cycle_t (*read)(struct clocksource *cs);int (*enable)(struct clocksource *cs);void (*disable)(struct clocksource *cs);cycle_t mask;u32 mult;u32 shift;u64 max_idle_ns;unsigned long flags;cycle_t (*vread)(void);void (*suspend)(struct clocksource *cs);void (*resume)(struct clocksource *cs);#ifdef CONFIG_IA64void *fsys_mmio;        /* used by fsyscall asm code */#define CLKSRC_FSYS_MMIO_SET(mmio, addr)      ((mmio) = (addr))#else#define CLKSRC_FSYS_MMIO_SET(mmio, addr)      do { } while (0)#endif/* * Second part is written at each timer interrupt * Keep it in a different cache line to dirty no * more than one cache line. */cycle_t cycle_last ____cacheline_aligned_in_smp;#ifdef CONFIG_CLOCKSOURCE_WATCHDOG/* Watchdog related data, used by the framework */struct list_head wd_list;cycle_t wd_last;#endif};

　　struct clock_event_device ：时钟的事件信息，包括当硬件时钟中断发生时要执行那些操作（实际上保存了相应函数的指针）。本文将该结构称作为“时钟事件设备”。

/** * struct clock_event_device - clock event device descriptor * @name:ptr to clock event name * @features:features * @max_delta_ns:maximum delta value in ns * @min_delta_ns:minimum delta value in ns * @mult:nanosecond to cycles multiplier * @shift:nanoseconds to cycles divisor (power of two) * @rating:variable to rate clock event devices * @irq:IRQ number (only for non CPU local devices) * @cpumask:cpumask to indicate for which CPUs this device works * @set_next_event:set next event function * @set_mode:set mode function * @event_handler:Assigned by the framework to be called by the low *level handler of the event source * @broadcast:function to broadcast events * @list:list head for the management code * @mode:operating mode assigned by the management code * @next_event:local storage for the next event in oneshot mode * @retries:number of forced programming retries */struct clock_event_device {const char*name;unsigned intfeatures;u64max_delta_ns;u64min_delta_ns;u32mult;u32shift;intrating;intirq;const struct cpumask*cpumask;int(*set_next_event)(unsigned long evt,  struct clock_event_device *);void(*set_mode)(enum clock_event_mode mode,    struct clock_event_device *);void(*event_handler)(struct clock_event_device *);void(*broadcast)(const struct cpumask *mask);struct list_headlist;enum clock_event_modemode;ktime_tnext_event;unsigned longretries;}; 

　 上述两个结构内核源代码中有较详细的注解，分别位于文件 clocksource.h 和 clockchips.h 中。需要特别注意的是结构 clock_event_device 的成员 event_handler ，它指定了当硬件时钟中断发生时，内核应该执行那些操作，也就是真正的时钟中断处理函数。
　　Linux 内核维护了两个链表，分别存储了系统中所有时钟源的信息和时钟事件设备的信息。这两个链表的表头在内核中分别是 clocksource_list 和 clockevent_devices 。

 

3. hrtimer是如何实现的呢？

    下文就为之一一描述。

3.1 初始化hrtimer硬件定时器

3.1.1 设置硬件中断

     前面已经看到，它有一个硬件中断，为了使此硬件中断能正常工作，肯定需要设置一个硬件中断，其参考代码如下： 

static unsigned long my_timer_irqnbr = 25;  //硬件中断号static struct irqaction my_timer_irqaction = {.name= "My HrTimer",.flags= IRQF_DISABLED | IRQF_TIMER | IRQF_IRQPOLL,.handler= my_timer_interrupt_handler, //中断处理函数};setup_irq(my_timer_irqnbr, &my_timer_irqaction);

     设置中断之后，中断处理函数也有了。

 

3.1.2 初始化硬件时钟相关寄存器并注册此硬件时钟到系统中

 

static struct clocksource myclocksource = {.name= "my_hrtimer_src",.rating = 300,.read= my_get_cycles, //读取COUNT寄存器以获取cycle value.mask= CLOCKSOURCE_MASK(64),.flags= CLOCK_SOURCE_IS_CONTINUOUS,};static void __init my_clocksource_init(void){unsigned long ctrl = 0;unsigned long count = (my_timer_freq / HZ);        ...writel(count, my_timer_vaddr + MY_TIMER_COMPARATOR_LOW);writel(count, my_timer_vaddr + MY_TIMER_AUTO_INCREMENT);ctrl = (MY_TIMER_CTRL_IRQ_ENA   | MY_TIMER_CTRL_COMP_ENA |        MY_TIMER_CTRL_TIMER_ENA | MY_TIMER_CTRL_AUTO_INC);writel(ctrl, my_timer_vaddr + MY_TIMER_CONTROL);        ...clocksource_calc_mult_shift(&myclocksource, my_timer_freq, 4);                //向系统注册我的硬件时钟,即把它加入clocksource_listclocksource_register(&myclocksource);}

3.1.3 初始化时钟事件设备并注册到系统中

static struct clock_event_device myclockevent = {.name= "my_timer_evt",.features= CLOCK_EVT_FEAT_PERIODIC,.set_mode= my_set_mode,  //通过写寄存器设置clock_event_mode.set_next_event= my_set_next_event, // 通过写寄存器写下一个事件.rating= 300,.cpumask= cpu_all_mask,};static void __init my_clockevents_init(unsigned int timer_irq){    myclockevent.irq = timer_irq;    clockevents_calc_mult_shift(&myclockevent, my_timer_freq, 4);    myclockevent.max_delta_ns = clockevent_delta2ns(0xffffffff, &myclockevent);    myclockevent.min_delta_ns = clockevent_delta2ns(0xf, &myclockevent);    //注册我的时钟事件设备，即把它加入clockevent_devices链表    clockevents_register_device(&myclockevent);}

3.2 硬件中处理函数my_timer_interrupt_handler

static irqreturn_t my_timer_interrupt_handler(int irq, void *dev_id){struct clock_event_device *evt = &myclockevent;/* clear the interrupt */writel(value, register_addr);evt->event_handler(evt);return IRQ_HANDLED;}

硬件中断处理函数很简单，它直接调用clockevent的event_handler函数。前面的初始化中并没有初始化此event_handler,很显然是在使用过程中进行动态初始化的。下面看看hrtimer中是如何初始化此event_handler的。

 

4. hrtimer如何初始化clock_event_device的event_handler?

 

hrtimer的中断处理函数，很自然地想到了hrtimer_interrupt，哪这个东东与clock_event_device有关系吗?

 

此软中断TIMER_SOFTIRQ在run_local_timers函数中通过调用raise_softirq(TIMER_SOFTIRQ);来触发。(注:raise_softirq->raise_softirq_irqoff->__raise_softirq_irqoff)

 

init_timers(中调用open_softirq(TIMER_SOFTIRQ, run_timer_softirq);)
run_timer_softirq->
hrtimer_run_pending(Called from timer softirq every jiffy, expire hrtimers，check如果hrtimer_hres_enabled is on<=1>,则执行下面的代码切换到高精度模式)->
hrtimer_switch_to_hres->
tick_init_highres->
tick_switch_to_oneshot(hrtimer_interrupt)
<把hrtimer_interrupt赋值给dev->event_handler,即dev->event_handler = handler;>

 

       看到没有？在每一次时钟软中断处理函数中，都会尝试把hrtimer切换到高精度模式，如果满足条件，就切换，切换之后高精度模式就被激活了，在hrtimer_run_pending检查是否被激活，如果被激活了，下面的代码就不用执行了。

 

5. hrtimer高精度模式下真正的中断处理函数

    hrtimer_interrupt

 

6. hrtimer高精度式的触发过程

    以下以nanosleep为例：

SYSCALL_DEFINE2(nanosleep, struct timespec __user *, rqtp, struct timespec __user *, rmtp)->
hrtimer_nanosleep->
do_nanosleep->
hrtimer_start_expires->
hrtimer_start_range_ns->
__hrtimer_start_range_ns->
enqueue_hrtimer(insert into rb_tree) then  hrtimer_enqueue_reprogram-> hrtimer_reprogram->
tick_program_event->
tick_dev_program_event->
clockevents_program_event->
dev->set_next_event((unsigned long) clc, dev)<调用my clock_event_device的set_next_event方法设置register>