Hello Ulrich, On Friday, May 9, 2008 at 10:21:44 +0200, Ulrich Windl wrote:
> How does hwclock know when the RTC was updated last? [...] how does > hwclock know the drift? Asuming it has exclusive ownership of the RTC? Yes. More exactly assuming exclusive write access to both the RTC and a small adjtime file, where hwclock records the last setting epoch and its estimate of the drift rate. > Did you consider that on Multi-Boot systems a different OS might have > run that also updates the RTC? That's indeed a problem, but not unsolvable. Sometimes it's possible to share the adjfile between the different OSes, and then hwclock can work nearly as well as on single-boot. Sometimes adjfile sharing poses some difficulties, or hwclock just can't run (MS Windows comes to mind). Then it's a real problem: drift can't be well corrected. But the pure kernel method still doesn't do any better. Halt a Linux-box in the evening, multiboot it on Windows next morning: The time at startup will also be wrong. > IMHO exchanging systemtime with the RTC by a user-program is broken. This user-program is 1000 times more accurate in most practical usage cases. And not worse even in the most uncomfortable corner cases. > hwclock is unnecessary if NTP is used and the kernel handles the RTC > properly (IMHO). The problem is that the kernel handles the RTC quite poorly. The day when kernels will be able to read and write the hardware clock down to some microseconds, and manage its drift, you might become right. That's not today. Serge. -- Serge point Bets arobase laposte point net _______________________________________________ questions mailing list [email protected] https://lists.ntp.org/mailman/listinfo/questions
