Re: PROBLEM: Linux updates RTC secretly when clock synchronizes

[Riley Williams <rhw@MemAlpha.cx>]

Hi Pavel.

>> PROBLEM: Linux updates RTC secretly when clock synchronizes.
>> 
>> Please CC replies etc to Ian Maclaine-cross <iml@debian.org>.
>> 
>> When /usr/sbin/ntpd synchronizes the Linux kernel (or system) clock
>> using the Network Time Protocol the kernel time is accurate to a few
>> milliseconds. Linux then sets the Real Time (or Hardware or CMOS)
>> Clock to this time at approximately 11 minute intervals. Typical
>> RTCs drift less than 10 s/day so rebooting causes only millisecond
>> errors.
>>
>> Linux currently does not record the 11 minute updates to a log file.
>> Clock programs (like hwclock) cannot correct RTC drift at boot time
>> without knowing when the RTC was last set. If NTP service is available
>> after a long shutdown, ntpd may step the time. Worse after a longer
>> shutdown ntpd may drop out or even synchronize to the wrong timezone.
>> The workarounds are clumsy.
>>
>> Please find following my small patch for
>> linux/arch/i386/kernel/time.c which adds a KERN_NOTICE of each 11
>> minute update to the RTC. This is just for i386 machines at present.
>> A script can search the logs for the last set time of the RTC and
>> update /etc/adjtime. Hwclock can then correct the RTC for drift and
>> set the kernel clock.

> That seems as very wrong solution.

> What about just making kernel only _read_ system clock, and never
> set it? That looks way cleaner to me.

It is cleaner. However, I feel that the RTC code should printk (at least
as KERN_DEBUG if not as KERN_NOTICE) whenever the RTC is written to.
It's too important a subsystem to be left hidden like it currently is.

Best wishes from Riley.