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>

1 files changed, 3 insertions, 0 deletions

diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
index f0a7fb984413..f88d32f8ff7c 100644
--- a/include/linux/clocksource.h
+++ b/include/linux/clocksource.h
@@ -79,6 +79,7 @@ struct clocksource {
         /* timekeeping specific data, ignore */
         cycle_t cycle_interval;
         u64     xtime_interval;
+        u32     raw_interval;
         /*
          * Second part is written at each timer interrupt
          * Keep it in a different cache line to dirty no
@@ -87,6 +88,7 @@ struct clocksource {
         cycle_t cycle_last ____cacheline_aligned_in_smp;
         u64 xtime_nsec;
         s64 error;
+        struct timespec raw_time;
 
 #ifdef CONFIG_CLOCKSOURCE_WATCHDOG
         /* Watchdog related data, used by the framework */
@@ -215,6 +217,7 @@ static inline void clocksource_calculate_interval(struct clocksource *c,
 
         /* Go back from cycles -> shifted ns, this time use ntp adjused mult */
         c->xtime_interval = (u64)c->cycle_interval * c->mult;
+        c->raw_interval = ((u64)c->cycle_interval * c->mult_orig) >> c->shift;
 }