timerkeeping_init

我们首先弄清楚两个概念时钟源和时钟事件设备（源）：

（1）时钟源

/**
 * struct clocksource - hardware abstraction for a free running counter
 *	Provides mostly state-free accessors to the underlying hardware.
 *	This is the structure used for system time.
 *
 * @name:		ptr to clocksource name
 * @list:		list head for registration
 * @rating:		rating value for selection (higher is better)
 *			To avoid rating inflation the following
 *			list should give you a guide as to how
 *			to assign your clocksource a rating
 *			1-99: Unfit for real use
 *				Only available for bootup and testing purposes.
 *			100-199: Base level usability.
 *				Functional for real use, but not desired.
 *			200-299: Good.
 *				A correct and usable clocksource.
 *			300-399: Desired.
 *				A reasonably fast and accurate clocksource.
 *			400-499: Perfect
 *				The ideal clocksource. A must-use where
 *				available.
 * @read:		returns a cycle value, passes clocksource as argument
 * @enable:		optional function to enable the clocksource
 * @disable:		optional function to disable the clocksource
 * @mask:		bitmask for two's complement
 *			subtraction of non 64 bit counters
 * @mult:		cycle to nanosecond multiplier
 * @shift:		cycle to nanosecond divisor (power of two)
 * @max_idle_ns:	max idle time permitted by the clocksource (nsecs)
 * @flags:		flags describing special properties
 * @vread:		vsyscall based read
 * @suspend:		suspend function for the clocksource, if necessary
 * @resume:		resume function for the clocksource, if necessary
 */
struct clocksource {
	/*
	 * Hotpath data, fits in a single cache line when the
	 * clocksource itself is cacheline aligned.
	 */
	cycle_t (*read)(struct clocksource *cs);
	cycle_t cycle_last;
	cycle_t mask;
	u32 mult;
	u32 shift;
	u64 max_idle_ns;

#ifdef CONFIG_IA64
	void *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
	const char *name;
	struct list_head list;
	int rating;
	cycle_t (*vread)(void);
	int (*enable)(struct clocksource *cs);
	void (*disable)(struct clocksource *cs);
	unsigned long flags;
	void (*suspend)(struct clocksource *cs);
	void (*resume)(struct clocksource *cs);

#ifdef CONFIG_CLOCKSOURCE_WATCHDOG
	/* Watchdog related data, used by the framework */
	struct list_head wd_list;
	cycle_t cs_last;
	cycle_t wd_last;
#endif
} ____cacheline_aligned;

（2）时钟事件源

/**
 * struct clock_event_device - clock event device descriptor
 * @event_handler:	Assigned by the framework to be called by the low
 *			level handler of the event source
 * @set_next_event:	set next event function
 * @next_event:		local storage for the next event in oneshot mode
 * @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)
 * @mode:		operating mode assigned by the management code
 * @features:		features
 * @retries:		number of forced programming retries
 * @set_mode:		set mode function
 * @broadcast:		function to broadcast events
 * @min_delta_ticks:	minimum delta value in ticks stored for reconfiguration
 * @max_delta_ticks:	maximum delta value in ticks stored for reconfiguration
 * @name:		ptr to clock event name
 * @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
 * @list:		list head for the management code
 */
struct clock_event_device {
	void			(*event_handler)(struct clock_event_device *);
	int			(*set_next_event)(unsigned long evt,
						  struct clock_event_device *);
	ktime_t			next_event;
	u64			max_delta_ns;
	u64			min_delta_ns;
	u32			mult;
	u32			shift;
	enum clock_event_mode	mode;
	unsigned int		features;
	unsigned long		retries;

	void			(*broadcast)(const struct cpumask *mask);
	void			(*set_mode)(enum clock_event_mode mode,
					    struct clock_event_device *);
	unsigned long		min_delta_ticks;
	unsigned long		max_delta_ticks;

	const char		*name;
	int			rating;
	int			irq;
	const struct cpumask	*cpumask;
	struct list_head	list;
} ____cacheline_aligned;

/*
 * timekeeping_init - Initializes the clocksource and common timekeeping values
 */
void __init timekeeping_init(void)
{
	struct clocksource *clock;
	unsigned long flags;
	struct timespec now, boot;


//首先从RTC中读取当前系统的时间

	read_persistent_clock(&now);
	read_boot_clock(&boot);

	write_seqlock_irqsave(&xtime_lock, flags);
//清除  NTP（网络校时协议）接口的变量。
	ntp_init();
 
//选定一个时钟
	clock = clocksource_default_clock();


	if (clock->enable)
		clock->enable(clock);
 
	timekeeper_setup_internals(clock);

//设置全局时间变量和raw_time也就是以前的monotonic时间
	xtime.tv_sec = now.tv_sec;
	xtime.tv_nsec = now.tv_nsec;
	raw_time.tv_sec = 0;
	raw_time.tv_nsec = 0;
	if (boot.tv_sec == 0 && boot.tv_nsec == 0) {
		boot.tv_sec = xtime.tv_sec;
		boot.tv_nsec = xtime.tv_nsec;
	}
	set_normalized_timespec(&wall_to_monotonic,
				-boot.tv_sec, -boot.tv_nsec);
	total_sleep_time.tv_sec = 0;
	total_sleep_time.tv_nsec = 0;
	write_sequnlock_irqrestore(&xtime_lock, flags);
}

我们着重来看系统选中的默认的时钟源是什么？

struct clocksource * __init __weak clocksource_default_clock(void)
{
    return &clocksource_jiffies;
}


struct clocksource clocksource_jiffies = {
    .name        = "jiffies",
    .rating        = 1, /* lowest valid rating*/
    .read        = jiffies_read,
    .mask        = 0xffffffff, /*32bits*/
    .mult        = NSEC_PER_JIFFY << JIFFIES_SHIFT, /* details above */
    .shift        = JIFFIES_SHIFT,
};