小张学linux内核：二. 时间子系统

跟着上节，以tick中断为例，探究liunx时间子系统

linux 时间子系统分层框架

图片出处http://blog.csdn.net/flaoter

最底层是硬件和驱动层，每个cpu core都有自己的cpu local timer，此外SOC内部肯定会有一个用于全局的global counter。

■ 中间层是linux内核层，内核抽象出了时钟源(clocksource), 时钟事件设备(clock_event_device), tick设备(tick_device)用于时间管理。分为左右两部分，
• 右边实现计时功能。linux内核有各种time line, 包括real time clock, monotonic clock, monotonic raw clock等。clocksource提供了一个单调增加的计时器产生tick，为timeline提供时钟源。timekeeper是内核提供时间服务的基础模块，负责选择并维护最优的clocksource。
• 左边实现定时功能。clock event管理可产生event或是触发中断的定时器，** 一般而言，每个CPU形成自己的一个小系统，也就要管理自己的clock event**。tick device是基于clock event设备进行工作的，cpu管理自己的调度、进程统计等是基于tick设备的。低精度timer和高精度timer都是基于tick device生成的定时器设备，关于它们的事件和周期信号的关系在上面的图中有一个大体的介绍。

■ 最上层是linux应用层。基于timekeeping设备的是时间管理的库time lib，基于定时器设备的是定时管理的库timer lib。
出处http://blog.csdn.net/flaoter

探究tick中断路由

接着上章，我们知道tick中断的handler是tick_handle_periodic,来看看 tick中断是如何运行到这里的。arm中有一个global timer的全局定时器。我们来看看arm global timer中断驱动程序：
在linux下cat /proc/interrupts

arm global timer驱动是作为一个clocksource驱动在drivers/clocksource目录下；arm_global_timer.c中：
从驱动入口函数：global_timer_of_register开始

static int __init global_timer_of_register(struct device_node *np)
{
	struct clk *gt_clk;
	int err = 0;

	/*
	 * In A9 r2p0 the comparators for each processor with the global timer
	 * fire when the timer value is greater than or equal to. In previous
	 * revisions the comparators fired when the timer value was equal to.
	 */
	if (read_cpuid_part() == ARM_CPU_PART_CORTEX_A9
	    && (read_cpuid_id() & 0xf0000f) < 0x200000) {
		pr_warn("global-timer: non support for this cpu version.n");
		return -ENOSYS;
	}

	gt_ppi = irq_of_parse_and_map(np, 0);
	if (!gt_ppi) {
		pr_warn("global-timer: unable to parse irqn");
		return -EINVAL;
	}

	gt_base = of_iomap(np, 0);
	if (!gt_base) {
		pr_warn("global-timer: invalid base addressn");
		return -ENXIO;
	}

	gt_clk = of_clk_get(np, 0);
	if (!IS_ERR(gt_clk)) {
		err = clk_prepare_enable(gt_clk);
		if (err)
			goto out_unmap;
	} else {
		pr_warn("global-timer: clk not foundn");
		err = -EINVAL;
		goto out_unmap;
	}

	gt_clk_rate = clk_get_rate(gt_clk);
	gt_evt = alloc_percpu(struct clock_event_device);     /*分配一个clock_event_device*/
	if (!gt_evt) {
		pr_warn("global-timer: can't allocate memoryn");
		err = -ENOMEM;
		goto out_clk;
	}

	err = request_percpu_irq(gt_ppi, gt_clockevent_interrupt,
				 "gt", gt_evt);     /*1. 申请中断处理函数，绑定一个clock_event_device*/
	if (err) {
		pr_warn("global-timer: can't register interrupt %d (%d)n",
			gt_ppi, err);
		goto out_free;
	}

	/* Register and immediately configure the timer on the boot CPU */
	err = gt_clocksource_init();     /*注册一个clocksource*/
	if (err)
		goto out_irq;
	
	err = cpuhp_setup_state(CPUHP_AP_ARM_GLOBAL_TIMER_STARTING,
				"clockevents/arm/global_timer:starting",
				gt_starting_cpu, gt_dying_cpu);    /*cpu hotplug，cpu热插拔绑定handler，当CPUHP_AP_ARM_GLOBAL_TIMER_STARTING时间触发时调用gt_starting_cpu*/
	if (err)
		goto out_irq;

	gt_delay_timer_init();

	return 0;

out_irq:
	free_percpu_irq(gt_ppi, gt_evt);
out_free:
	free_percpu(gt_evt);
out_clk:
	clk_disable_unprepare(gt_clk);
out_unmap:
	iounmap(gt_base);
	WARN(err, "ARM Global timer register failed (%d)n", err);

	return err;
}

/* Only tested on r2p2 and r3p0  */
TIMER_OF_DECLARE(arm_gt, "arm,cortex-a9-global-timer",
			global_timer_of_register);     /*声明一个驱动树节点*/

我们注意到三个步骤，
1.申请一个clock_event_device,
2.注册中断处理函数，绑定clock_event_device，中断处理函数是gt_clockevent_interrupt，且在其中会调用clock_event_device的evt_handler处理中断；
3.注册一个clocksource
4.绑定cpu热插拔时间，gt_starting_cpu，cpuhotplug我们后面在学，现在多核系统，一个主cpu （boot cpu）之外的cpu(AP cpu)可以当作一个外设来看待，在业务没那么繁忙时可以将其关掉，关闭和开启会触发cpu hotplug事件，在cpu起来时会注册一个AP global timer,并触发CPUHP_AP_ARM_GLOBAL_TIMER_STARTING事件，并调用handler：gt_starting_cpu。
我们依次来看这几个函数：
gt_clocksource_init：

static int __init gt_clocksource_init(void)
{
	writel(0, gt_base + GT_CONTROL);
	writel(0, gt_base + GT_COUNTER0);
	writel(0, gt_base + GT_COUNTER1);
	/* enables timer on all the cores */
	writel(GT_CONTROL_TIMER_ENABLE, gt_base + GT_CONTROL);

#ifdef CONFIG_CLKSRC_ARM_GLOBAL_TIMER_SCHED_CLOCK
	sched_clock_register(gt_sched_clock_read, 64, gt_clk_rate);
#endif
	return clocksource_register_hz(&gt_clocksource, gt_clk_rate);
}

1. 写timer的寄存器；2.clocksource_register_hz()
   clocksource_register_hz() 是 __clocksource_register_scale()的包装
   在include/linux/clocksource.h

static inline int clocksource_register_hz(struct clocksource *cs, u32 hz)
{
	return __clocksource_register_scale(cs, 1, hz);
}

__clocksource_register_scale在kernel/time/clocksource.c中

int __clocksource_register_scale(struct clocksource *cs, u32 scale, u32 freq)
{
	unsigned long flags;

	/* Initialize mult/shift and max_idle_ns */
	__clocksource_update_freq_scale(cs, scale, freq);

	/* Add clocksource to the clocksource list */
	mutex_lock(&clocksource_mutex);

	clocksource_watchdog_lock(&flags);
	clocksource_enqueue(cs);
	clocksource_enqueue_watchdog(cs);
	clocksource_watchdog_unlock(&flags);

	clocksource_select();
	clocksource_select_watchdog(false);
	__clocksource_suspend_select(cs);
	mutex_unlock(&clocksource_mutex);
	return 0;
}
EXPORT_SYMBOL_GPL(__clocksource_register_scale);

clocksource_enqueue将cs加入全局clocksource_list的队列中。

中断处理函数

static irqreturn_t gt_clockevent_interrupt(int irq, void *dev_id)
{
	struct clock_event_device *evt = dev_id;

	if (!(readl_relaxed(gt_base + GT_INT_STATUS) &
				GT_INT_STATUS_EVENT_FLAG))
		return IRQ_NONE;

	/**
	 * ERRATA 740657( Global Timer can send 2 interrupts for
	 * the same event in single-shot mode)
	 * Workaround:
	 *	Either disable single-shot mode.
	 *	Or
	 *	Modify the Interrupt Handler to avoid the
	 *	offending sequence. This is achieved by clearing
	 *	the Global Timer flag _after_ having incremented
	 *	the Comparator register	value to a higher value.
	 */
	if (clockevent_state_oneshot(evt))
		gt_compare_set(ULONG_MAX, 0);       /*如果oneshot 则比较时间*/

	writel_relaxed(GT_INT_STATUS_EVENT_FLAG, gt_base + GT_INT_STATUS);
	evt->event_handler(evt);    /*调用event_device的event_handler*/

	return IRQ_HANDLED;
}

中断函数中调用clock_event_device的event_handler()处理。
那clock_event_device的event_handler在何处设置呢？
我们来看gt_starting_cpu()函数做了些什么。

static int gt_starting_cpu(unsigned int cpu)
{
	struct clock_event_device *clk = this_cpu_ptr(gt_evt);

	clk->name = "arm_global_timer";
	clk->features = CLOCK_EVT_FEAT_PERIODIC | CLOCK_EVT_FEAT_ONESHOT |
		CLOCK_EVT_FEAT_PERCPU;
	clk->set_state_shutdown = gt_clockevent_shutdown;
	clk->set_state_periodic = gt_clockevent_set_periodic;
	clk->set_state_oneshot = gt_clockevent_shutdown;
	clk->set_state_oneshot_stopped = gt_clockevent_shutdown;
	clk->set_next_event = gt_clockevent_set_next_event;
	clk->cpumask = cpumask_of(cpu);
	clk->rating = 300;
	clk->irq = gt_ppi;          
	clockevents_config_and_register(clk, gt_clk_rate,
					1, 0xffffffff);              /*注册一个clock_event_device*/
	enable_percpu_irq(clk->irq, IRQ_TYPE_NONE);        
	return 0;     
}

clockevents_config_and_register() 在kernel/time/clockevents.c

void clockevents_register_device(struct clock_event_device *dev)
{
	unsigned long flags;

	/* Initialize state to DETACHED */
	clockevent_set_state(dev, CLOCK_EVT_STATE_DETACHED);

	if (!dev->cpumask) {
		WARN_ON(num_possible_cpus() > 1);
		dev->cpumask = cpumask_of(smp_processor_id());
	}

	if (dev->cpumask == cpu_all_mask) {
		WARN(1, "%s cpumask == cpu_all_mask, using cpu_possible_mask insteadn",
		     dev->name);
		dev->cpumask = cpu_possible_mask;
	}

	raw_spin_lock_irqsave(&clockevents_lock, flags);

	list_add(&dev->list, &clockevent_devices);
	tick_check_new_device(dev);     /*检查并添加tick device*/
	clockevents_notify_released();

	raw_spin_unlock_irqrestore(&clockevents_lock, flags);
}

tick_check_new_device() 在kernel/time/tick-common.c

void tick_check_new_device(struct clock_event_device *newdev)
{
	struct clock_event_device *curdev;
	struct tick_device *td;
	int cpu;

	cpu = smp_processor_id();
	td = &per_cpu(tick_cpu_device, cpu);    /*tick_device是percpu*/
	curdev = td->evtdev;

	/* cpu local device ? */
	if (!tick_check_percpu(curdev, newdev, cpu))
		goto out_bc;

	/* Preference decision */
	if (!tick_check_preferred(curdev, newdev))
		goto out_bc;

	if (!try_module_get(newdev->owner))
		return;

	/*
	 * Replace the eventually existing device by the new
	 * device. If the current device is the broadcast device, do
	 * not give it back to the clockevents layer !
	 */
	if (tick_is_broadcast_device(curdev)) {
		clockevents_shutdown(curdev);
		curdev = NULL;
	}
	clockevents_exchange_device(curdev, newdev);
	tick_setup_device(td, newdev, cpu, cpumask_of(cpu));
	/*添加tick_device*/
	if (newdev->features & CLOCK_EVT_FEAT_ONESHOT)
		tick_oneshot_notify();
	return;

out_bc:
	/*
	 * Can the new device be used as a broadcast device ?
	 */
	tick_install_broadcast_device(newdev);
}

已经进入tick模块，tick_setup_device()

static void tick_setup_device(struct tick_device *td,
			      struct clock_event_device *newdev, int cpu,
			      const struct cpumask *cpumask)
{
	void (*handler)(struct clock_event_device *) = NULL;
	ktime_t next_event = 0;

	/*
	 * First device setup ?
	 */
	if (!td->evtdev) {
		/*
		 * If no cpu took the do_timer update, assign it to
		 * this cpu:
		 */
		if (tick_do_timer_cpu == TICK_DO_TIMER_BOOT) {
			if (!tick_nohz_full_cpu(cpu))
				tick_do_timer_cpu = cpu;
			else
				tick_do_timer_cpu = TICK_DO_TIMER_NONE;
			tick_next_period = ktime_get();
			tick_period = NSEC_PER_SEC / HZ;
		}

		/*
		 * Startup in periodic mode first.
		 */
		td->mode = TICKDEV_MODE_PERIODIC;
		/*如果是第一个evt_handler，则则设置未周期模式*/
	} else {
		handler = td->evtdev->event_handler;   /*不是第一个event_hdanler,则获取上一个event_handler*/
		next_event = td->evtdev->next_event;
		td->evtdev->event_handler = clockevents_handle_noop;
	}

	td->evtdev = newdev;

	/*
	 * When the device is not per cpu, pin the interrupt to the
	 * current cpu:
	 */
	if (!cpumask_equal(newdev->cpumask, cpumask))
		irq_set_affinity(newdev->irq, cpumask);

	/*
	 * When global broadcasting is active, check if the current
	 * device is registered as a placeholder for broadcast mode.
	 * This allows us to handle this x86 misfeature in a generic
	 * way. This function also returns !=0 when we keep the
	 * current active broadcast state for this CPU.
	 */
	if (tick_device_uses_broadcast(newdev, cpu))
		return;

	if (td->mode == TICKDEV_MODE_PERIODIC)
		tick_setup_periodic(newdev, 0);   /*如果tick_device模式是周期的*/
	else
		tick_setup_oneshot(newdev, handler, next_event);
		/*tick_device模式是oneshot模式,该handler是tick_device的老的event_handler*/
}

用面向对象的思想，tick_device是clock_event_device的“子类”

struct tick_device {
	struct clock_event_device *evtdev;
	enum tick_device_mode mode;
};

每个cpu都有自己的tick_device;
tick_check_new_device()中
td = &per_cpu(tick_cpu_device, cpu);
tick_setup_device()给该cpu的tick_device设置一个clock_event_device;
tick_device 模式分为periodic和one shot模式
如果是初次设置，则为periodic模式，调用tick_setup_periodic(); 不是第一次设置则走one shot模式tick_setup_oneshot()：
tick_setup_periodic()分支就是我们在中断子系统章节的入口了，后面在细看，先来看tick_setup_oneshot()分支：
kernel/time/tick-oneshot.c中

void tick_setup_oneshot(struct clock_event_device *newdev,
			void (*handler)(struct clock_event_device *),
			ktime_t next_event)
{
	newdev->event_handler = handler;     /*设置event_handler*/
	clockevents_switch_state(newdev, CLOCK_EVT_STATE_ONESHOT);     /*设置clock_event_device 的state为oneshot模式*/
	clockevents_program_event(newdev, next_event, true);
}

clockevents_program_event():

int clockevents_program_event(struct clock_event_device *dev, ktime_t expires,
			      bool force)
{
	unsigned long long clc;
	int64_t delta;
	int rc;

	if (unlikely(expires < 0)) {
		WARN_ON_ONCE(1);
		return -ETIME;
	}

	dev->next_event = expires;

	if (clockevent_state_shutdown(dev))
		return 0;

	/* We must be in ONESHOT state here */
	WARN_ONCE(!clockevent_state_oneshot(dev), "Current state: %dn",
		  clockevent_get_state(dev));

	/* Shortcut for clockevent devices that can deal with ktime. */
	if (dev->features & CLOCK_EVT_FEAT_KTIME)
		return dev->set_next_ktime(expires, dev);

	delta = ktime_to_ns(ktime_sub(expires, ktime_get()));
	if (delta <= 0)
		return force ? clockevents_program_min_delta(dev) : -ETIME;

	delta = min(delta, (int64_t) dev->max_delta_ns);
	delta = max(delta, (int64_t) dev->min_delta_ns);

	clc = ((unsigned long long) delta * dev->mult) >> dev->shift;
	rc = dev->set_next_event((unsigned long) clc, dev);
	/*设置下一次触发事件的时刻*/
	return (rc && force) ? clockevents_program_min_delta(dev) : rc;
}

clock_source_device 中set_next_event是在和时设置？
回到gt_starting_cpu() dricers/clockcource/arm_global_timer.c

clk->set_next_event = gt_clockevent_set_next_event;

我们来看gt_clockevent_set_next_event()做了什么？

static int gt_clockevent_set_next_event(unsigned long evt,
					struct clock_event_device *unused)
{
	gt_compare_set(evt, 0);
	return 0;
}

static void gt_compare_set(unsigned long delta, int periodic)
{
	u64 counter = gt_counter_read();
	unsigned long ctrl;

	counter += delta;
	ctrl = GT_CONTROL_TIMER_ENABLE;
	writel_relaxed(ctrl, gt_base + GT_CONTROL);
	writel_relaxed(lower_32_bits(counter), gt_base + GT_COMP0);
	writel_relaxed(upper_32_bits(counter), gt_base + GT_COMP1);

	if (periodic) {
		writel_relaxed(delta, gt_base + GT_AUTO_INC);
		ctrl |= GT_CONTROL_AUTO_INC;
	}

	ctrl |= GT_CONTROL_COMP_ENABLE | GT_CONTROL_IRQ_ENABLE;
	writel_relaxed(ctrl, gt_base + GT_CONTROL);
}

硬件寄存器操作，设置下一次中断产生的时刻。设置下一次中断产生调用tick_device的老的event_handler。
我们再看看periodic分支
tick_setup_periodic():

void tick_setup_periodic(struct clock_event_device *dev, int broadcast)
{
	tick_set_periodic_handler(dev, broadcast);      /*设置event_hdanler*/

	/* Broadcast setup ? */
	if (!tick_device_is_functional(dev))
		return;

	if ((dev->features & CLOCK_EVT_FEAT_PERIODIC) &&
	    !tick_broadcast_oneshot_active()) {
		clockevents_switch_state(dev, CLOCK_EVT_STATE_PERIODIC);
	} else {
		unsigned long seq;
		ktime_t next;

		do {
			seq = read_seqbegin(&jiffies_lock);
			next = tick_next_period;
		} while (read_seqretry(&jiffies_lock, seq));

		clockevents_switch_state(dev, CLOCK_EVT_STATE_ONESHOT);

		for (;;) {
			if (!clockevents_program_event(dev, next, false))
				return;
			next = ktime_add(next, tick_period);
		}
	}
}

tick_setup_periodic(dev, broadcast); 设置clock_event_device的event_handler 为tick_handle_periodic
kernel/time/tick-internal.h

static inline void tick_set_periodic_handler(struct clock_event_device *dev, int broadcast)
{
	dev->event_handler = tick_handle_periodic;
}

这样timer中断中调用clock_event_device->event_handler,就进入tick_handle_periodic()函数。
低速时钟使用periodic模式。这样tick中断的路由就已完成，接下来进入tick_handle_periodic()处理一些cpu调度和统计信息，我们在调度子系统中学习。

高速时钟概念待补充

tick_init()函数

https://www.cnblogs.com/arnoldlu/p/7078228.html
https://blog.csdn.net/cc289123557/article/details/53728173
https://www.jb51.net/article/133579.htm

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