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9e8912e04e
In order for ntpd to correctly synchronize the clocks, the frequency of the system clock must not be off by more than 500 ppm (or, put another way, 1:2000), or ntpd will end up giving up on trying to synchronize properly, and ends up reseting the clock in jumps instead. The fast TSC PIT calibration sometimes failed this test - it was assuming that the PIT reads always took about one microsecond each (2us for the two reads to get a 16-bit timer), and that calibrating TSC to the PIT over 15ms should thus be sufficient to get much closer than 500ppm (max 2us error on both sides giving 4us over 15ms: a 270 ppm error value). However, that assumption does not always hold: apparently some hardware is either very much slower at reading the PIT registers, or there was other noise causing at least one machine to get 700+ ppm errors. So instead of using a fixed 15ms timing loop, this changes the fast PIT calibration to read the TSC delta over the individual PIT timer reads, and use the result to calculate the error bars on the PIT read timing properly. We then successfully calibrate the TSC only if the maximum error bars fall below 500ppm. In the process, we also relax the timing to allow up to 25ms for the calibration, although it can happen much faster depending on hardware. Reported-and-tested-by: Jesper Krogh <jesper@krogh.cc> Cc: john stultz <johnstul@us.ibm.com> Cc: Thomas Gleixner <tglx@linutronix.de> Acked-by: Ingo Molnar <mingo@elte.hu> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> |
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