Kernel+LibC: Add adjtime(2)

Most systems (Linux, OpenBSD) adjust 0.5 ms per second, or 0.5 us per
1 ms tick. That is, the clock is sped up or slowed down by at most
0.05%.  This means adjusting the clock by 1 s takes 2000 s, and the
clock an be adjusted by at most 1.8 s per hour.

FreeBSD adjusts 5 ms per second if the remaining time adjustment is
>= 1 s (0.5%) , else it adjusts by 0.5 ms as well. This allows adjusting
by (almost) 18 s per hour.

Since Serenity OS can lose more than 22 s per hour (#3429), this
picks an adjustment rate up to 1% for now. This allows us to
adjust up to 36s per hour, which should be sufficient to adjust
the clock fast enough to keep up with how much time the clock
currently loses. Once we have a fancier NTP implementation that can
adjust tick rate in addition to offset, we can think about reducing
this.

adjtime is a bit old-school and most current POSIX-y OSs instead
implement adjtimex/ntp_adjtime, but a) we have to start somewhere
b) ntp_adjtime() is a fairly gnarly API. OpenBSD's adjfreq looks
like it might provide similar functionality with a nicer API. But
before worrying about all this, it's probably a good idea to get
to a place where the kernel APIs are (barely) good enough so that
we can write an ntp service, and once we have that we should write
a way to automatically evaluate how well it keeps the time adjusted,
and only then should we add improvements ot the adjustment mechanism.

1 files changed, 4 insertions, 0 deletions

diff --git a/Kernel/Time/TimeManagement.h b/Kernel/Time/TimeManagement.h
index f3a3bb0188..b6c268e3af 100644
--- a/Kernel/Time/TimeManagement.h
+++ b/Kernel/Time/TimeManagement.h
@@ -62,6 +62,9 @@ public:
 
     static timeval now_as_timeval();
 
+    timespec remaining_epoch_time_adjustment() const { return m_remaining_epoch_time_adjustment; }
+    void set_remaining_epoch_time_adjustment(const timespec& adjustment) { m_remaining_epoch_time_adjustment = adjustment; }
+
 private:
     bool probe_and_set_legacy_hardware_timers();
     bool probe_and_set_non_legacy_hardware_timers();
@@ -73,6 +76,7 @@ private:
     u32 m_ticks_this_second { 0 };
     u32 m_seconds_since_boot { 0 };
     timespec m_epoch_time { 0, 0 };
+    timespec m_remaining_epoch_time_adjustment { 0, 0 };
     RefPtr<HardwareTimerBase> m_system_timer;
     RefPtr<HardwareTimerBase> m_time_keeper_timer;
 };