Who sets date/time automatically from the hardware clock at boot?

Question

When Linux is booting, the current time must be fetched from a RTC chip and then set to Linux system date/time. I can do this manually by running hwclock --hctosys. But where exactly does this happen automatically at boot time? Is there a process taking care of this? Does the kernel do this job?

Answer 1

Method 1: Linux Kernel loads time from RTC chip

One can set CONFIG_RTC_HCTOSYS in kernel config to enable this feature. When enabled, the Linux Kernel will take care of reading stored time in rtc0 and setting it to system time. With CONFIG_RTC_HCTOSYS_DEVICE="rtc0" the device can be altered.

For this to work the according RTC driver must be compiled as built-in module. This can be determined as follows:

$ cat /boot/config-$(uname -r) | grep -i RTC_HCTOSYS

Method 2: udev calls hwclock as soon as /dev/rtcX is created

The package util-linux provides a udev rule file at /usr/lib/udev/rules.d/85-hwclock.rules which can be copied to /etc/udev/rules.d/ (as illustrated below). The udev daemon will then call /usr/lib/udev/hwclock-set automatically as soon as /dev/rtc0 is created. You can change rtc0 in 85-hwclock.rules if needed.

$ sudo cp /usr/lib/udev/rules.d/85-hwclock.rules /etc/udev/rules.d/85-hwclock.rules

Note: Both methods accommodate the warnings in man hwclock against using the --hctosys option with hwclock on a running system. Method 2 is probably easiest for most.