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authorJohn Stultz <johnstul@us.ibm.com>2008-08-20 16:37:30 -0700
committerIngo Molnar <mingo@elte.hu>2008-08-21 09:50:24 +0200
commit2d42244ae71d6c7b0884b5664cf2eda30fb2ae68 (patch)
tree947e86ec6e2d7362daa9a170a352c035f3618d64 /kernel/time
parent9a055117d3d9cb562f83f8d4cd88772761f4cab0 (diff)
downloadlinux-2d42244ae71d6c7b0884b5664cf2eda30fb2ae68.tar.bz2
clocksource: introduce CLOCK_MONOTONIC_RAW
In talking with Josip Loncaric, and his work on clock synchronization (see btime.sf.net), he mentioned that for really close synchronization, it is useful to have access to "hardware time", that is a notion of time that is not in any way adjusted by the clock slewing done to keep close time sync. Part of the issue is if we are using the kernel's ntp adjusted representation of time in order to measure how we should correct time, we can run into what Paul McKenney aptly described as "Painting a road using the lines we're painting as the guide". I had been thinking of a similar problem, and was trying to come up with a way to give users access to a purely hardware based time representation that avoided users having to know the underlying frequency and mask values needed to deal with the wide variety of possible underlying hardware counters. My solution is to introduce CLOCK_MONOTONIC_RAW. This exposes a nanosecond based time value, that increments starting at bootup and has no frequency adjustments made to it what so ever. The time is accessed from userspace via the posix_clock_gettime() syscall, passing CLOCK_MONOTONIC_RAW as the clock_id. Signed-off-by: John Stultz <johnstul@us.ibm.com> Signed-off-by: Roman Zippel <zippel@linux-m68k.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Ingo Molnar <mingo@elte.hu>
Diffstat (limited to 'kernel/time')
-rw-r--r--kernel/time/timekeeping.c44
1 files changed, 44 insertions, 0 deletions
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 83d3555a6998..5099c95b8aa2 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -75,6 +75,9 @@ static void clocksource_forward_now(void)
nsec = cyc2ns(clock, cycle_delta);
timespec_add_ns(&xtime, nsec);
+
+ nsec = ((s64)cycle_delta * clock->mult_orig) >> clock->shift;
+ clock->raw_time.tv_nsec += nsec;
}
/**
@@ -183,6 +186,8 @@ static void change_clocksource(void)
clocksource_forward_now();
+ new->raw_time = clock->raw_time;
+
clock = new;
clock->cycle_last = 0;
clock->cycle_last = clocksource_read(new);
@@ -205,6 +210,39 @@ static inline void change_clocksource(void) { }
#endif
/**
+ * getrawmonotonic - Returns the raw monotonic time in a timespec
+ * @ts: pointer to the timespec to be set
+ *
+ * Returns the raw monotonic time (completely un-modified by ntp)
+ */
+void getrawmonotonic(struct timespec *ts)
+{
+ unsigned long seq;
+ s64 nsecs;
+ cycle_t cycle_now, cycle_delta;
+
+ do {
+ seq = read_seqbegin(&xtime_lock);
+
+ /* read clocksource: */
+ cycle_now = clocksource_read(clock);
+
+ /* calculate the delta since the last update_wall_time: */
+ cycle_delta = (cycle_now - clock->cycle_last) & clock->mask;
+
+ /* convert to nanoseconds: */
+ nsecs = ((s64)cycle_delta * clock->mult_orig) >> clock->shift;
+
+ *ts = clock->raw_time;
+
+ } while (read_seqretry(&xtime_lock, seq));
+
+ timespec_add_ns(ts, nsecs);
+}
+EXPORT_SYMBOL(getrawmonotonic);
+
+
+/**
* timekeeping_valid_for_hres - Check if timekeeping is suitable for hres
*/
int timekeeping_valid_for_hres(void)
@@ -466,6 +504,12 @@ void update_wall_time(void)
second_overflow();
}
+ clock->raw_time.tv_nsec += clock->raw_interval;
+ if (clock->raw_time.tv_nsec >= NSEC_PER_SEC) {
+ clock->raw_time.tv_nsec -= NSEC_PER_SEC;
+ clock->raw_time.tv_sec++;
+ }
+
/* accumulate error between NTP and clock interval */
clock->error += tick_length;
clock->error -= clock->xtime_interval << (NTP_SCALE_SHIFT - clock->shift);