Kernel does not use RTC

Question

I have a PiFace RTC clock that I have setup on archlinux ARM on a rpi2.

I have edited /boot/config.txt to add

dtparam=i2c=on
dtoverlay=i2c-rtc,mcp7941x=0x6f

I have added i2c-dev and i2c:mcp7941x modules to /etc/modules-load.d/raspberrypi.conf

But the kernel does not seem to use it to update the system clock. I can see the rtc driver is loaded:

 kernel: rtc-ds1307 1-006f: registered as rtc0

and there is a /dev/rtc0 device. But the kernel does not update system clock with it.

It is only systemd-timesyncd that restores time from saved timestamp. And then later on sync time with SNTP if I have a network available.

The idea of having a RTC is to have correct time if there is no network.

But this is not used at all.

How do I tell the kernel to use the RTC to update system clock at boot ? or only if there is no internet connection ?

Enable RTC in Device Tree
– Milliways
Aug 25, 2018 at 19:11 — Milliways, Aug 25, 2018 at 19:11
Could you be more specific ?
– solsTiCe
Aug 25, 2018 at 20:53 — solsTiCe, Aug 25, 2018 at 20:53

solsTiCe · Accepted Answer · 2019-01-31 19:41:52Z

Well, an ugly hack is to make a hwclock.service like this

[Unit]
Description=Synchronize system clock from RTC

[Service]
ExecStart=/usr/bin/hwclock --hctosys
Type=simple

[Install]
WantedBy=time-sync.target

And enable it with

sudo systemctl enable hwclock.service

It will step on systemd-timesyncd's toes when network is available but should do no harm.

In the hope of a better solution ?

Another way to do it, is to use an udev rule that trigger hwclock.

in /etc/udev/rules.d/i2c-rtc.rules

KERNEL=="rtc0", SUBSYSTEM=="rtc", SUBSYSTEMS=="i2c", TAG+="systemd", ENV{SYSTEMD_WANTS}="hwclock.service"

in /etc/systemd/system/hwclock.service

[Unit]
Description=Synchronize system clock from RTC

[Service]
ExecStart=/usr/bin/hwclock --hctosys --utc
Type=oneshot

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