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authorJohn Stultz <johnstul@us.ibm.com>2008-08-20 19:37:30 -0400
committerIngo Molnar <mingo@elte.hu>2008-08-21 03:50:24 -0400
commit2d42244ae71d6c7b0884b5664cf2eda30fb2ae68 (patch)
tree947e86ec6e2d7362daa9a170a352c035f3618d64 /include/linux/time.h
parent9a055117d3d9cb562f83f8d4cd88772761f4cab0 (diff)
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 'include/linux/time.h')
-rw-r--r--include/linux/time.h2
1 files changed, 2 insertions, 0 deletions
diff --git a/include/linux/time.h b/include/linux/time.h
index e15206a7e82e..205f974b9ebf 100644
--- a/include/linux/time.h
+++ b/include/linux/time.h
@@ -117,6 +117,7 @@ extern int do_setitimer(int which, struct itimerval *value,
117extern unsigned int alarm_setitimer(unsigned int seconds); 117extern unsigned int alarm_setitimer(unsigned int seconds);
118extern int do_getitimer(int which, struct itimerval *value); 118extern int do_getitimer(int which, struct itimerval *value);
119extern void getnstimeofday(struct timespec *tv); 119extern void getnstimeofday(struct timespec *tv);
120extern void getrawmonotonic(struct timespec *ts);
120extern void getboottime(struct timespec *ts); 121extern void getboottime(struct timespec *ts);
121extern void monotonic_to_bootbased(struct timespec *ts); 122extern void monotonic_to_bootbased(struct timespec *ts);
122 123
@@ -214,6 +215,7 @@ struct itimerval {
214#define CLOCK_MONOTONIC 1 215#define CLOCK_MONOTONIC 1
215#define CLOCK_PROCESS_CPUTIME_ID 2 216#define CLOCK_PROCESS_CPUTIME_ID 2
216#define CLOCK_THREAD_CPUTIME_ID 3 217#define CLOCK_THREAD_CPUTIME_ID 3
218#define CLOCK_MONOTONIC_RAW 4
217 219
218/* 220/*
219 * The IDs of various hardware clocks: 221 * The IDs of various hardware clocks: