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Date: Sun, 14 Dec 2003 09:19:03 +1100 (EST)
From: Bruce Evans <bde@zeta.org.au>
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To: Poul-Henning Kamp <phk@phk.freebsd.dk>
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cc: Freebsd Current <current@FreeBSD.org>
Subject: Re: SC520 and reboot 
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On Sat, 13 Dec 2003, Poul-Henning Kamp wrote:
> If you run adjkerntz -a it fiddles the CMOS RTC clock and possibly the
> "century" byte I think.  If your BIOS uses a checksum on the RTC and
> adjkerntz doesn't know about it, your BIOS will think the contents is
> trashed.

Actually, adjkerntz -a uses the MI interface resettodr() to set the
hardware clock, CMOS or otherwise.  This has the same tiny chance of
trashing the clock than the MI syscall settimeofday() and a better
chance of setting it to the current time, since it has a smaller race
than settimeofday().  The CPU_ADJKERNTZ sysctl exists just to minimize
this race, but it has never really worked in at least the i386
implementation since resettodr() doesn't wait to sync with the hardware
so it sets a time that is wrong by an average half a second.

Bruce