#3359 NORM First D: Time server for XOs.

[Zarro Boogs per Child]

#3359: Time server for XOs.
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  Reporter:  cscott         |       Owner:  wad                   
      Type:  task           |      Status:  new                   
  Priority:  normal         |   Milestone:  First Deployment, V1.0
 Component:  school server  |     Version:                        
Resolution:                 |    Keywords:                        
  Verified:  0              |  
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Comment(by gnu):

 Is the essence of the problem that the kernel copies the RTC into its idea
 of the time, but the RTC only has 1-second resolution?  If that's it,
 there's a pretty good fix:

 It should be possible to set the kernel clock accurately, by watching the
 realtime clock tick to another second.  This could even be done
 incrementally, second by second, sneaking up on the instant that it ticks.
 (Gen-2 should use a realtime clock with a counter and alarm that ticks off
 much smaller units.)

 So on wakeup, you set the time to the RTC time and 0 microseconds (say
 04:06:05.0000.  Set an RTC alarm to occur at the next second (04:06:06);
 you'll get an interrupt right after it ticks.  Your kernel time will be
 maybe 04:06:05.314000.  Now you know that time was too conservative; so
 bump forward your idea of what time it is to be 04:06:06.00000.  You're
 almost synchronized with the tick, but you had some interrupt latency, so
 you're still a little slow.  Set an internal timer to wake you up 99% of
 the way through the next second (at 04:06:06.99000), and when you get that
 interrupt, sit in a tight loop and watch it tick over to 04:06:07.
 Advance your internal clock to 04:06:07.00000 right then, and you'll be
 accurate to within the time it takes to do one cycle of a tight loop
 reading the RTC.  In less than three seconds, you've probably recovered
 the time "lost" in suspend -- and without ever making the clock run
 backward.  Think of this as adjtime for suspended clocks.

 Then you won't need NTP to sync it to somewhere else.  (Of course, this
 kind of clock fiddling will give NTP some concern about the stability of
 your local oscillator.  Check in with Dave Mills, truechimer wizard, for
 how to make this work best.)

-- 
Ticket URL: <https://dev.laptop.org/ticket/3359#comment:3>
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