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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/posix-timers.c
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 'kernel/posix-timers.c')
-rw-r--r--kernel/posix-timers.c15
1 files changed, 15 insertions, 0 deletions
diff --git a/kernel/posix-timers.c b/kernel/posix-timers.c
index e36d5798cbf..d3c66b53dff 100644
--- a/kernel/posix-timers.c
+++ b/kernel/posix-timers.c
@@ -223,6 +223,15 @@ static int posix_ktime_get_ts(clockid_t which_clock, struct timespec *tp)
}
/*
+ * Get monotonic time for posix timers
+ */
+static int posix_get_monotonic_raw(clockid_t which_clock, struct timespec *tp)
+{
+ getrawmonotonic(tp);
+ return 0;
+}
+
+/*
* Initialize everything, well, just everything in Posix clocks/timers ;)
*/
static __init int init_posix_timers(void)
@@ -235,9 +244,15 @@ static __init int init_posix_timers(void)
.clock_get = posix_ktime_get_ts,
.clock_set = do_posix_clock_nosettime,
};
+ struct k_clock clock_monotonic_raw = {
+ .clock_getres = hrtimer_get_res,
+ .clock_get = posix_get_monotonic_raw,
+ .clock_set = do_posix_clock_nosettime,
+ };
register_posix_clock(CLOCK_REALTIME, &clock_realtime);
register_posix_clock(CLOCK_MONOTONIC, &clock_monotonic);
+ register_posix_clock(CLOCK_MONOTONIC_RAW, &clock_monotonic_raw);
posix_timers_cache = kmem_cache_create("posix_timers_cache",
sizeof (struct k_itimer), 0, SLAB_PANIC,