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SubjectRe: Why touch the CMOS clock?
On  6 May 96 at 14:21, Johan Myréen wrote:

> I noticed the 1.3.98 patch contains the following warning:
> +Also, if the kernel time is synchronized with an external source, the
> +kernel will write the time back to the CMOS clock every 11 minutes. In
> +the process of doing this, the kernel briefly turns off RTC periodic
> +interrupts, so be aware of this if you are doing serious work. If you
> +don't synchronize the kernel time with an external source (via ntp or
> +whatever) then the kernel will keep its hands off the RTC, allowing you
> +exclusive access to the device for your applications.
> My question is: Why does the kernel have to update the RTC? It is my
> understanding that the kernel only reads the RTC at boot time and does its
> timekeeping using the timer interrupt. (Correct me if I'm wrong.) Why can't
> the RTC be left running on its own and the RTC time be used only as an
> approximation of the real time of day when the machine is power-cycled? You
> can't trust the RTC anyway if the machine is down for a longer period of
> time.

The RTC usually is more precise than the interrupt driven clock. In
Addidion as long as the RTC runs local time, you will have to change
it when entering/leaving DST. Despite of that I've seen several
occasions where the CMOS clock was not being updated even when the
kernel clock was synchronized.

I think the RTC should be updated, even when the hardware is somewhat
broken: Setting the time shouldn't affect periodic interrupts.

Maybe itimer just should preempt a running task so that you don't
have to use /dev/rtc.

> Johan Myreen

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