2.3 系统调用
timekeeper提供一系列的系统调用，使得用户空间可以获取想要的时间。下面简单的介绍一下clock_gettime系统调用
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
系统在初始化是会调用init_posix_timers等函数来初始化clock_gettime系统调用所需要的相关数据结构。这里，调用clock_gettime获取时间时，需要的是k_clock结构中的clock_get回调函数。对于clock_gettime的which_clock参数，系统支持获取包括xtime,boot time,monotonic time,raw monotonic time以及进程或者线程运行时间等共十种方式。对于获取xtime和monotonic time，which_clock有两种设置，分别是带_COARSE和不带两种方式。
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;
}
从函数实现可以看到，当带_COARSE后缀时，函数直接返回xtime。而不带_COARSE后缀时，函数得首先统计当期时刻和上次更新xtime时的时间差，将时间差与xtime之和返回。从这点看，带后缀的比不带后缀的效率要高。当需要的时间不需要太精确时，可以使用带_COARSE后缀的参数来获取时间，这样可以略微提升应用的运行速度。当获取时间的操作很频繁时，作用尤其明显。
,Android自动化测试之Robotium--基础操作.pdf)
