• John Stultz's avatar
    clocksource: introduce CLOCK_MONOTONIC_RAW · 2d42244a
    John Stultz authored
    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: default avatarJohn Stultz <johnstul@us.ibm.com>
    Signed-off-by: default avatarRoman Zippel <zippel@linux-m68k.org>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
    2d42244a
timekeeping.c 14.2 KB