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Merge branch 'timers-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 

Pull timer changes from Ingo Molnar:
 "Main changes:

   - ntp: Add CONFIG_RTC_SYSTOHC: a generic RTC driver facility
     complementing the existing CONFIG_RTC_HCTOSYS, which uses NTP to
     keep the hardware clock updated.

   - posix-timers: Fix clock_adjtime to always return timex data on
     success.  This is changing the ABI, but no breakage was expected
     and found - caution is warranted nevertheless.

   - platform persistent clock improvements/cleanups.

   - clockevents: refactor timer broadcast handling to be more generic
     and less duplicated with matching architecture code (mostly ARM
     motivated.)

   - various fixes and cleanups"

* 'timers-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  timers/x86/hpet: Use HPET_COUNTER to specify the hpet counter in vread_hpet()
  posix-cpu-timers: Fix nanosleep task_struct leak
  clockevents: Fix generic broadcast for FEAT_C3STOP
  time, Fix setting of hardware clock in NTP code
  hrtimer: Prevent hrtimer_enqueue_reprogram race
  clockevents: Add generic timer broadcast function
  clockevents: Add generic timer broadcast receiver
  timekeeping: Switch HAS_PERSISTENT_CLOCK to ALWAYS_USE_PERSISTENT_CLOCK
  x86/time/rtc: Don't print extended CMOS year when reading RTC
  x86: Select HAS_PERSISTENT_CLOCK on x86
  timekeeping: Add CONFIG_HAS_PERSISTENT_CLOCK option
  rtc: Skip the suspend/resume handling if persistent clock exist
  timekeeping: Add persistent_clock_exist flag
  posix-timers: Fix clock_adjtime to always return timex data on success
  Round the calculated scale factor in set_cyc2ns_scale()
  NTP: Add a CONFIG_RTC_SYSTOHC configuration
  MAINTAINERS: Update John Stultz's email
  time: create __getnstimeofday for WARNless calls
hifive-unleashed-5.1
Linus Torvalds 2013-02-19 19:05:45 -08:00
parent bcbd818c06 36dfbbf136
commit 266d7ad7f4
22 changed files with 245 additions and 46 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
MAINTAINERS
View File

@ -6588,7 +6588,7 @@ F: drivers/dma/dw_dmac_regs.h
F: drivers/dma/dw_dmac.c
TIMEKEEPING, NTP
M: John Stultz <johnstul@us.ibm.com>
M: John Stultz <john.stultz@linaro.org>
M: Thomas Gleixner <tglx@linutronix.de>
T: git git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git timers/core
S: Supported

2
arch/powerpc/kernel/time.c
View File

@ -669,7 +669,7 @@ int update_persistent_clock(struct timespec now)
struct rtc_time tm;
if (!ppc_md.set_rtc_time)
return 0;
return -ENODEV;
to_tm(now.tv_sec + 1 + timezone_offset, &tm);
tm.tm_year -= 1900;

1
arch/x86/Kconfig
View File

@ -107,6 +107,7 @@ config X86
select GENERIC_CLOCKEVENTS_BROADCAST if X86_64 || (X86_32 && X86_LOCAL_APIC)
select GENERIC_TIME_VSYSCALL if X86_64
select KTIME_SCALAR if X86_32
select ALWAYS_USE_PERSISTENT_CLOCK
select GENERIC_STRNCPY_FROM_USER
select GENERIC_STRNLEN_USER
select HAVE_CONTEXT_TRACKING if X86_64

1
arch/x86/kernel/rtc.c
View File

@ -149,7 +149,6 @@ unsigned long mach_get_cmos_time(void)
if (century) {
century = bcd2bin(century);
year += century * 100;
printk(KERN_INFO "Extended CMOS year: %d\n", century * 100);
} else
year += CMOS_YEARS_OFFS;

3
arch/x86/kernel/tsc.c
View File

@ -623,7 +623,8 @@ static void set_cyc2ns_scale(unsigned long cpu_khz, int cpu)
ns_now = __cycles_2_ns(tsc_now);
if (cpu_khz) {
*scale = (NSEC_PER_MSEC << CYC2NS_SCALE_FACTOR)/cpu_khz;
*scale = ((NSEC_PER_MSEC << CYC2NS_SCALE_FACTOR) +
cpu_khz / 2) / cpu_khz;
*offset = ns_now - mult_frac(tsc_now, *scale,
(1UL << CYC2NS_SCALE_FACTOR));
}

2
arch/x86/vdso/vclock_gettime.c
View File

@ -60,7 +60,7 @@ notrace static cycle_t vread_tsc(void)
static notrace cycle_t vread_hpet(void)
{
return readl((const void __iomem *)fix_to_virt(VSYSCALL_HPET) + 0xf0);
return readl((const void __iomem *)fix_to_virt(VSYSCALL_HPET) + HPET_COUNTER);
}
#ifdef CONFIG_PARAVIRT_CLOCK

12
drivers/rtc/Kconfig
View File

@ -20,14 +20,24 @@ if RTC_CLASS
config RTC_HCTOSYS
bool "Set system time from RTC on startup and resume"
default y
depends on !ALWAYS_USE_PERSISTENT_CLOCK
help
If you say yes here, the system time (wall clock) will be set using
the value read from a specified RTC device. This is useful to avoid
unnecessary fsck runs at boot time, and to network better.
config RTC_SYSTOHC
bool "Set the RTC time based on NTP synchronization"
default y
depends on !ALWAYS_USE_PERSISTENT_CLOCK
help
If you say yes here, the system time (wall clock) will be stored
in the RTC specified by RTC_HCTOSYS_DEVICE approximately every 11
minutes if userspace reports synchronized NTP status.
config RTC_HCTOSYS_DEVICE
string "RTC used to set the system time"
depends on RTC_HCTOSYS = y
depends on RTC_HCTOSYS = y || RTC_SYSTOHC = y
default "rtc0"
help
The RTC device that will be used to (re)initialize the system

1
drivers/rtc/Makefile
View File

@ -6,6 +6,7 @@ ccflags-$(CONFIG_RTC_DEBUG) := -DDEBUG
obj-$(CONFIG_RTC_LIB) += rtc-lib.o
obj-$(CONFIG_RTC_HCTOSYS) += hctosys.o
obj-$(CONFIG_RTC_SYSTOHC) += systohc.o
obj-$(CONFIG_RTC_CLASS) += rtc-core.o
rtc-core-y := class.o interface.o

7
drivers/rtc/class.c
View File

@ -50,6 +50,10 @@ static int rtc_suspend(struct device *dev, pm_message_t mesg)
struct rtc_device *rtc = to_rtc_device(dev);
struct rtc_time tm;
struct timespec delta, delta_delta;
if (has_persistent_clock())
return 0;
if (strcmp(dev_name(&rtc->dev), CONFIG_RTC_HCTOSYS_DEVICE) != 0)
return 0;
@ -88,6 +92,9 @@ static int rtc_resume(struct device *dev)
struct timespec new_system, new_rtc;
struct timespec sleep_time;
if (has_persistent_clock())
return 0;
rtc_hctosys_ret = -ENODEV;
if (strcmp(dev_name(&rtc->dev), CONFIG_RTC_HCTOSYS_DEVICE) != 0)
return 0;

44
drivers/rtc/systohc.c 100644
View File

@ -0,0 +1,44 @@
/*
* This program is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 as published by
* the Free Software Foundation.
*
*/
#include <linux/rtc.h>
#include <linux/time.h>
/**
* rtc_set_ntp_time - Save NTP synchronized time to the RTC
* @now: Current time of day
*
* Replacement for the NTP platform function update_persistent_clock
* that stores time for later retrieval by rtc_hctosys.
*
* Returns 0 on successful RTC update, -ENODEV if a RTC update is not
* possible at all, and various other -errno for specific temporary failure
* cases.
*
* If temporary failure is indicated the caller should try again 'soon'
*/
int rtc_set_ntp_time(struct timespec now)
{
struct rtc_device *rtc;
struct rtc_time tm;
int err = -ENODEV;
if (now.tv_nsec < (NSEC_PER_SEC >> 1))
rtc_time_to_tm(now.tv_sec, &tm);
else
rtc_time_to_tm(now.tv_sec + 1, &tm);
rtc = rtc_class_open(CONFIG_RTC_HCTOSYS_DEVICE);
if (rtc) {
/* rtc_hctosys exclusively uses UTC, so we call set_time here,
* not set_mmss. */
if (rtc->ops && (rtc->ops->set_time || rtc->ops->set_mmss))
err = rtc_set_time(rtc, &tm);
rtc_class_close(rtc);
}
return err;
}

10
fs/pstore/ram.c
View File

@ -167,12 +167,16 @@ static ssize_t ramoops_pstore_read(u64 *id, enum pstore_type_id *type,
static size_t ramoops_write_kmsg_hdr(struct persistent_ram_zone *prz)
{
char *hdr;
struct timeval timestamp;
struct timespec timestamp;
size_t len;
do_gettimeofday(&timestamp);
/* Report zeroed timestamp if called before timekeeping has resumed. */
if (__getnstimeofday(&timestamp)) {
timestamp.tv_sec = 0;
timestamp.tv_nsec = 0;
}
hdr = kasprintf(GFP_ATOMIC, RAMOOPS_KERNMSG_HDR "%lu.%lu\n",
(long)timestamp.tv_sec, (long)timestamp.tv_usec);
(long)timestamp.tv_sec, (long)(timestamp.tv_nsec / 1000));
WARN_ON_ONCE(!hdr);
len = hdr ? strlen(hdr) : 0;
persistent_ram_write(prz, hdr, len);

9
include/linux/clockchips.h
View File

@ -161,6 +161,15 @@ clockevents_calc_mult_shift(struct clock_event_device *ce, u32 freq, u32 minsec)
extern void clockevents_suspend(void);
extern void clockevents_resume(void);
#ifdef CONFIG_GENERIC_CLOCKEVENTS_BROADCAST
#ifdef CONFIG_ARCH_HAS_TICK_BROADCAST
extern void tick_broadcast(const struct cpumask *mask);
#else
#define tick_broadcast NULL
#endif
extern int tick_receive_broadcast(void);
#endif
#ifdef CONFIG_GENERIC_CLOCKEVENTS
extern void clockevents_notify(unsigned long reason, void *arg);
#else

1
include/linux/rtc.h
View File

@ -138,6 +138,7 @@ extern void rtc_device_unregister(struct rtc_device *rtc);
extern int rtc_read_time(struct rtc_device *rtc, struct rtc_time *tm);
extern int rtc_set_time(struct rtc_device *rtc, struct rtc_time *tm);
extern int rtc_set_mmss(struct rtc_device *rtc, unsigned long secs);
extern int rtc_set_ntp_time(struct timespec now);
int __rtc_read_alarm(struct rtc_device *rtc, struct rtc_wkalrm *alarm);
extern int rtc_read_alarm(struct rtc_device *rtc,
struct rtc_wkalrm *alrm);

13
include/linux/time.h
View File

@ -115,8 +115,20 @@ static inline bool timespec_valid_strict(const struct timespec *ts)
return true;
}
extern bool persistent_clock_exist;
#ifdef ALWAYS_USE_PERSISTENT_CLOCK
#define has_persistent_clock() true
#else
static inline bool has_persistent_clock(void)
{
return persistent_clock_exist;
}
#endif
extern void read_persistent_clock(struct timespec *ts);
extern void read_boot_clock(struct timespec *ts);
extern int persistent_clock_is_local;
extern int update_persistent_clock(struct timespec now);
void timekeeping_init(void);
extern int timekeeping_suspended;
@ -158,6 +170,7 @@ extern int do_setitimer(int which, struct itimerval *value,
struct itimerval *ovalue);
extern unsigned int alarm_setitimer(unsigned int seconds);
extern int do_getitimer(int which, struct itimerval *value);
extern int __getnstimeofday(struct timespec *tv);
extern void getnstimeofday(struct timespec *tv);
extern void getrawmonotonic(struct timespec *ts);
extern void getnstime_raw_and_real(struct timespec *ts_raw,

36
kernel/hrtimer.c
View File

@ -642,21 +642,9 @@ static inline void hrtimer_init_hres(struct hrtimer_cpu_base *base)
* and expiry check is done in the hrtimer_interrupt or in the softirq.
*/
static inline int hrtimer_enqueue_reprogram(struct hrtimer *timer,
struct hrtimer_clock_base *base,
int wakeup)
struct hrtimer_clock_base *base)
{
if (base->cpu_base->hres_active && hrtimer_reprogram(timer, base)) {
if (wakeup) {
raw_spin_unlock(&base->cpu_base->lock);
raise_softirq_irqoff(HRTIMER_SOFTIRQ);
raw_spin_lock(&base->cpu_base->lock);
} else
__raise_softirq_irqoff(HRTIMER_SOFTIRQ);
return 1;
}
return 0;
return base->cpu_base->hres_active && hrtimer_reprogram(timer, base);
}
static inline ktime_t hrtimer_update_base(struct hrtimer_cpu_base *base)
@ -737,8 +725,7 @@ static inline int hrtimer_switch_to_hres(void) { return 0; }
static inline void
hrtimer_force_reprogram(struct hrtimer_cpu_base *base, int skip_equal) { }
static inline int hrtimer_enqueue_reprogram(struct hrtimer *timer,
struct hrtimer_clock_base *base,
int wakeup)
struct hrtimer_clock_base *base)
{
return 0;
}
@ -997,8 +984,21 @@ int __hrtimer_start_range_ns(struct hrtimer *timer, ktime_t tim,
*
* XXX send_remote_softirq() ?
*/
if (leftmost && new_base->cpu_base == &__get_cpu_var(hrtimer_bases))
hrtimer_enqueue_reprogram(timer, new_base, wakeup);
if (leftmost && new_base->cpu_base == &__get_cpu_var(hrtimer_bases)
&& hrtimer_enqueue_reprogram(timer, new_base)) {
if (wakeup) {
/*
* We need to drop cpu_base->lock to avoid a
* lock ordering issue vs. rq->lock.
*/
raw_spin_unlock(&new_base->cpu_base->lock);
raise_softirq_irqoff(HRTIMER_SOFTIRQ);
local_irq_restore(flags);
return ret;
} else {
__raise_softirq_irqoff(HRTIMER_SOFTIRQ);
}
}
unlock_hrtimer_base(timer, &flags);

23
kernel/posix-cpu-timers.c
View File

@ -1417,8 +1417,10 @@ static int do_cpu_nanosleep(const clockid_t which_clock, int flags,
while (!signal_pending(current)) {
if (timer.it.cpu.expires.sched == 0) {
/*
* Our timer fired and was reset.
* Our timer fired and was reset, below
* deletion can not fail.
*/
posix_cpu_timer_del(&timer);
spin_unlock_irq(&timer.it_lock);
return 0;
}
@ -1436,9 +1438,26 @@ static int do_cpu_nanosleep(const clockid_t which_clock, int flags,
* We were interrupted by a signal.
*/
sample_to_timespec(which_clock, timer.it.cpu.expires, rqtp);
posix_cpu_timer_set(&timer, 0, &zero_it, it);
error = posix_cpu_timer_set(&timer, 0, &zero_it, it);
if (!error) {
/*
* Timer is now unarmed, deletion can not fail.
*/
posix_cpu_timer_del(&timer);
}
spin_unlock_irq(&timer.it_lock);
while (error == TIMER_RETRY) {
/*
* We need to handle case when timer was or is in the
* middle of firing. In other cases we already freed
* resources.
*/
spin_lock_irq(&timer.it_lock);
error = posix_cpu_timer_del(&timer);
spin_unlock_irq(&timer.it_lock);
}
if ((it->it_value.tv_sec | it->it_value.tv_nsec) == 0) {
/*
* It actually did fire already.

2
kernel/posix-timers.c
View File

@ -997,7 +997,7 @@ SYSCALL_DEFINE2(clock_adjtime, const clockid_t, which_clock,
err = kc->clock_adj(which_clock, &ktx);
if (!err && copy_to_user(utx, &ktx, sizeof(ktx)))
if (err >= 0 && copy_to_user(utx, &ktx, sizeof(ktx)))
return -EFAULT;
return err;

8
kernel/time.c
View File

@ -114,6 +114,12 @@ SYSCALL_DEFINE2(gettimeofday, struct timeval __user *, tv,
return 0;
}
/*
* Indicates if there is an offset between the system clock and the hardware
* clock/persistent clock/rtc.
*/
int persistent_clock_is_local;
/*
* Adjust the time obtained from the CMOS to be UTC time instead of
* local time.
@ -135,6 +141,8 @@ static inline void warp_clock(void)
struct timespec adjust;
adjust = current_kernel_time();
if (sys_tz.tz_minuteswest != 0)
persistent_clock_is_local = 1;
adjust.tv_sec += sys_tz.tz_minuteswest * 60;
do_settimeofday(&adjust);
}

9
kernel/time/Kconfig
View File

@ -12,6 +12,11 @@ config CLOCKSOURCE_WATCHDOG
config ARCH_CLOCKSOURCE_DATA
bool
# Platforms has a persistent clock
config ALWAYS_USE_PERSISTENT_CLOCK
bool
default n
# Timekeeping vsyscall support
config GENERIC_TIME_VSYSCALL
bool
@ -38,6 +43,10 @@ config GENERIC_CLOCKEVENTS_BUILD
default y
depends on GENERIC_CLOCKEVENTS
# Architecture can handle broadcast in a driver-agnostic way
config ARCH_HAS_TICK_BROADCAST
bool
# Clockevents broadcasting infrastructure
config GENERIC_CLOCKEVENTS_BROADCAST
bool

22
kernel/time/ntp.c
View File

@ -15,6 +15,7 @@
#include <linux/time.h>
#include <linux/mm.h>
#include <linux/module.h>
#include <linux/rtc.h>
#include "tick-internal.h"
@ -483,8 +484,7 @@ out:
return leap;
}
#ifdef CONFIG_GENERIC_CMOS_UPDATE
#if defined(CONFIG_GENERIC_CMOS_UPDATE) || defined(CONFIG_RTC_SYSTOHC)
static void sync_cmos_clock(struct work_struct *work);
static DECLARE_DELAYED_WORK(sync_cmos_work, sync_cmos_clock);
@ -510,14 +510,26 @@ static void sync_cmos_clock(struct work_struct *work)
}
getnstimeofday(&now);
if (abs(now.tv_nsec - (NSEC_PER_SEC / 2)) <= tick_nsec / 2)
fail = update_persistent_clock(now);
if (abs(now.tv_nsec - (NSEC_PER_SEC / 2)) <= tick_nsec / 2) {
struct timespec adjust = now;
fail = -ENODEV;
if (persistent_clock_is_local)
adjust.tv_sec -= (sys_tz.tz_minuteswest * 60);
#ifdef CONFIG_GENERIC_CMOS_UPDATE
fail = update_persistent_clock(adjust);
#endif
#ifdef CONFIG_RTC_SYSTOHC
if (fail == -ENODEV)
fail = rtc_set_ntp_time(adjust);
#endif
}
next.tv_nsec = (NSEC_PER_SEC / 2) - now.tv_nsec - (TICK_NSEC / 2);
if (next.tv_nsec <= 0)
next.tv_nsec += NSEC_PER_SEC;
if (!fail)
if (!fail || fail == -ENODEV)
next.tv_sec = 659;
else
next.tv_sec = 0;

38
kernel/time/tick-broadcast.c
View File

@ -18,6 +18,7 @@
#include <linux/percpu.h>
#include <linux/profile.h>
#include <linux/sched.h>
#include <linux/smp.h>
#include "tick-internal.h"
@ -86,6 +87,22 @@ int tick_is_broadcast_device(struct clock_event_device *dev)
return (dev && tick_broadcast_device.evtdev == dev);
}
static void err_broadcast(const struct cpumask *mask)
{
pr_crit_once("Failed to broadcast timer tick. Some CPUs may be unresponsive.\n");
}
static void tick_device_setup_broadcast_func(struct clock_event_device *dev)
{
if (!dev->broadcast)
dev->broadcast = tick_broadcast;
if (!dev->broadcast) {
pr_warn_once("%s depends on broadcast, but no broadcast function available\n",
dev->name);
dev->broadcast = err_broadcast;
}
}
/*
* Check, if the device is disfunctional and a place holder, which
* needs to be handled by the broadcast device.
@ -105,6 +122,7 @@ int tick_device_uses_broadcast(struct clock_event_device *dev, int cpu)
*/
if (!tick_device_is_functional(dev)) {
dev->event_handler = tick_handle_periodic;
tick_device_setup_broadcast_func(dev);
cpumask_set_cpu(cpu, tick_get_broadcast_mask());
tick_broadcast_start_periodic(tick_broadcast_device.evtdev);
ret = 1;
@ -116,15 +134,33 @@ int tick_device_uses_broadcast(struct clock_event_device *dev, int cpu)
*/
if (!(dev->features & CLOCK_EVT_FEAT_C3STOP)) {
int cpu = smp_processor_id();
cpumask_clear_cpu(cpu, tick_get_broadcast_mask());
tick_broadcast_clear_oneshot(cpu);
} else {
tick_device_setup_broadcast_func(dev);
}
}
raw_spin_unlock_irqrestore(&tick_broadcast_lock, flags);
return ret;
}
#ifdef CONFIG_GENERIC_CLOCKEVENTS_BROADCAST
int tick_receive_broadcast(void)
{
struct tick_device *td = this_cpu_ptr(&tick_cpu_device);
struct clock_event_device *evt = td->evtdev;
if (!evt)
return -ENODEV;
if (!evt->event_handler)
return -EINVAL;
evt->event_handler(evt);
return 0;
}
#endif
/*
* Broadcast the event to the cpus, which are set in the mask (mangled).
*/

45
kernel/time/timekeeping.c
View File

@ -29,6 +29,9 @@ static struct timekeeper timekeeper;
/* flag for if timekeeping is suspended */
int __read_mostly timekeeping_suspended;
/* Flag for if there is a persistent clock on this platform */
bool __read_mostly persistent_clock_exist = false;
static inline void tk_normalize_xtime(struct timekeeper *tk)
{
while (tk->xtime_nsec >= ((u64)NSEC_PER_SEC << tk->shift)) {
@ -264,19 +267,18 @@ static void timekeeping_forward_now(struct timekeeper *tk)
}
/**
* getnstimeofday - Returns the time of day in a timespec
* __getnstimeofday - Returns the time of day in a timespec.
* @ts: pointer to the timespec to be set
*
* Returns the time of day in a timespec.
* Updates the time of day in the timespec.
* Returns 0 on success, or -ve when suspended (timespec will be undefined).
*/
void getnstimeofday(struct timespec *ts)
int __getnstimeofday(struct timespec *ts)
{
struct timekeeper *tk = &timekeeper;
unsigned long seq;
s64 nsecs = 0;
WARN_ON(timekeeping_suspended);
do {
seq = read_seqbegin(&tk->lock);
@ -287,6 +289,26 @@ void getnstimeofday(struct timespec *ts)
ts->tv_nsec = 0;
timespec_add_ns(ts, nsecs);
/*
* Do not bail out early, in case there were callers still using
* the value, even in the face of the WARN_ON.
*/
if (unlikely(timekeeping_suspended))
return -EAGAIN;
return 0;
}
EXPORT_SYMBOL(__getnstimeofday);
/**
* getnstimeofday - Returns the time of day in a timespec.
* @ts: pointer to the timespec to be set
*
* Returns the time of day in a timespec (WARN if suspended).
*/
void getnstimeofday(struct timespec *ts)
{
WARN_ON(__getnstimeofday(ts));
}
EXPORT_SYMBOL(getnstimeofday);
@ -640,12 +662,14 @@ void __init timekeeping_init(void)
struct timespec now, boot, tmp;
read_persistent_clock(&now);
if (!timespec_valid_strict(&now)) {
pr_warn("WARNING: Persistent clock returned invalid value!\n"
" Check your CMOS/BIOS settings.\n");
now.tv_sec = 0;
now.tv_nsec = 0;
}
} else if (now.tv_sec || now.tv_nsec)
persistent_clock_exist = true;
read_boot_clock(&boot);
if (!timespec_valid_strict(&boot)) {
@ -718,11 +742,12 @@ void timekeeping_inject_sleeptime(struct timespec *delta)
{
struct timekeeper *tk = &timekeeper;
unsigned long flags;
struct timespec ts;
/* Make sure we don't set the clock twice */
read_persistent_clock(&ts);
if (!(ts.tv_sec == 0 && ts.tv_nsec == 0))
/*
* Make sure we don't set the clock twice, as timekeeping_resume()
* already did it
*/
if (has_persistent_clock())
return;
write_seqlock_irqsave(&tk->lock, flags);