2.3 系統(tǒng)調用

timekeeper提供一系列的系統(tǒng)調用，使得用戶空間可以獲取想要的時間。下面簡單的介紹一下clock_gettime系統(tǒng)調用

SYSCALL_DEFINE2(clock_gettime, const clockid_t, which_clock,

struct timespec __user *,tp)

{

struct k_clock *kc = clockid_to_kclock(which_clock);

struct timespec kernel_tp;

int error;

if (!kc)

return -EINVAL;

error = kc->clock_get(which_clock, &kernel_tp);

if (!error && copy_to_user(tp, &kernel_tp, sizeof (kernel_tp)))

error = -EFAULT;

return error;

}

static __init int init_posix_timers(void)

{

struct k_clock clock_realtime = {

.clock_getres ??= hrtimer_get_res,

.clock_get ?????= posix_clock_realtime_get,

.clock_set ?????= posix_clock_realtime_set,

.clock_adj ?????= posix_clock_realtime_adj,

.nsleep ????????= common_nsleep,

.nsleep_restart = hrtimer_nanosleep_restart,

.timer_create ??= common_timer_create,

.timer_set ?????= common_timer_set,

.timer_get ?????= common_timer_get,

.timer_del ?????= common_timer_del,

};

struct k_clock clock_monotonic = {

.clock_getres ??= hrtimer_get_res,

.clock_get ?????= posix_ktime_get_ts,

.nsleep ????????= common_nsleep,

.nsleep_restart = hrtimer_nanosleep_restart,

.timer_create ??= common_timer_create,

.timer_set ?????= common_timer_set,

.timer_get ?????= common_timer_get,

.timer_del ?????= common_timer_del,

};

struct k_clock clock_monotonic_raw = {

.clock_getres ??= hrtimer_get_res,

.clock_get ?????= posix_get_monotonic_raw,

};

struct k_clock clock_realtime_coarse = {

.clock_getres ??= posix_get_coarse_res,

.clock_get ?????= posix_get_realtime_coarse,

};

struct k_clock clock_monotonic_coarse = {

.clock_getres ??= posix_get_coarse_res,

.clock_get ?????= posix_get_monotonic_coarse,

};

struct k_clock clock_boottime = {

.clock_getres ??= hrtimer_get_res,

.clock_get ?????= posix_get_boottime,

.nsleep ????????= common_nsleep,

.nsleep_restart = hrtimer_nanosleep_restart,

.timer_create ??= common_timer_create,

.timer_set ?????= common_timer_set,

.timer_get ?????= common_timer_get,

.timer_del ?????= common_timer_del,

};

posix_timers_register_clock(CLOCK_REALTIME, &clock_realtime);

posix_timers_register_clock(CLOCK_MONOTONIC, &clock_monotonic);

posix_timers_register_clock(CLOCK_MONOTONIC_RAW, &clock_monotonic_raw);

posix_timers_register_clock(CLOCK_REALTIME_COARSE, &clock_realtime_coarse);

posix_timers_register_clock(CLOCK_MONOTONIC_COARSE, &clock_monotonic_coarse);

posix_timers_register_clock(CLOCK_BOOTTIME, &clock_boottime);

posix_timers_cache = kmem_cache_create("posix_timers_cache",

sizeof (struct k_itimer), 0, SLAB_PANIC, NULL);

idr_init(&posix_timers_id);

return 0;

}

#define CLOCK_REALTIME ?????????????????0

#define CLOCK_MONOTONIC ????????????????1

#define CLOCK_PROCESS_CPUTIME_ID ???????2

#define CLOCK_THREAD_CPUTIME_ID ????????3

#define CLOCK_MONOTONIC_RAW ????????????4

#define CLOCK_REALTIME_COARSE ??????????5

#define CLOCK_MONOTONIC_COARSE ?????????6

#define CLOCK_BOOTTIME ?????????????????7

#define CLOCK_REALTIME_ALARM ???????????8

#define CLOCK_BOOTTIME_ALARM ???????????9

系統(tǒng)在初始化是會調用init_posix_timers等函數來初始化clock_gettime系統(tǒng)調用所需要的相關數據結構。這里，調用clock_gettime獲取時間時，需要的是k_clock結構中的clock_get回調函數。對于clock_gettime的which_clock參數，系統(tǒng)支持獲取包括xtime,boot time,monotonic time,raw monotonic time以及進程或者線程運行時間等共十種方式。對于獲取xtime和monotonic time，which_clock有兩種設置，分別是帶_COARSE和不帶兩種方式。

2.3.1 _COARSE作用

static int posix_clock_realtime_get(clockid_t which_clock, struct timespec *tp)

{

ktime_get_real_ts(tp);

return 0;

}

#define ktime_get_real_ts(ts) ??getnstimeofday(ts)

void getnstimeofday(struct timespec *ts)

{

unsigned long seq;

s64 nsecs;

WARN_ON(timekeeping_suspended);

do {

seq = read_seqcount_begin(&xtime_seq);

*ts = xtime;

nsecs = timekeeping_get_ns();

/* If arch requires, add in gettimeoffset() */

nsecs += arch_gettimeoffset();

} while (read_seqcount_retry(&xtime_seq, seq));

timespec_add_ns(ts, nsecs);

}

static inline s64 timekeeping_get_ns(void)

{

cycle_t cycle_now, cycle_delta;

struct clocksource *clock;

/* read clocksource: */

clock = timekeeper.clock;

cycle_now = clock->read(clock);

/* calculate the delta since the last update_wall_time: */

cycle_delta = (cycle_now - clock->cycle_last) & clock->mask;

/* return delta convert to nanoseconds using ntp adjusted mult. */

return clocksource_cyc2ns(cycle_delta, timekeeper.mult,

timekeeper.shift);

}

static int posix_get_realtime_coarse(clockid_t which_clock, struct timespec *tp)

{

*tp = current_kernel_time();

return 0;

}

struct timespec current_kernel_time(void)

{

struct timespec now;

unsigned long seq;

do {

seq = read_seqcount_begin(&xtime_seq);

now = xtime;

} while (read_seqcount_retry(&xtime_seq, seq));

return now;

}

從函數實現可以看到，當帶_COARSE后綴時，函數直接返回xtime。而不帶_COARSE后綴時，函數得首先統(tǒng)計當期時刻和上次更新xtime時的時間差，將時間差與xtime之和返回。從這點看，帶后綴的比不帶后綴的效率要高。當需要的時間不需要太精確時，可以使用帶_COARSE后綴的參數來獲取時間，這樣可以略微提升應用的運行速度。當獲取時間的操作很頻繁時，作用尤其明顯。

總結

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