Re: Cannot find a way to get time read from RTC during boot

[Lennart Poettering <lennart@xxxxxxxxxxxxxx>]

On Do, 12.03.20 07:13, Kevin P. Fleming (kevin@xxxxxxx) wrote:

> I've got some Debian Buster systems (so using the Debian systemd
> package 241-7), which have battery-backed RTCs. However the driver for
> these RTCs is loaded as a module, not built into the kernel. As a
> result the kernel's feature of reading the RTC to set the system clock
> is not available.

We generally assume that RTC drivers are built-in and the kernel does
the initial initialization of the system clock from it.

> Prior to systemd, with the 'hwclock' package installed, a udev rule
> would trigger reading of the RTC and setting the system clock when
> /dev/rtc0 appeared. With systemd running, the script run by that udev
> rule is suppressed, it doesn't do anything.

> I have systemd-timesyncd started at boot as well and syncing time with
> an NTP server; that works fine when the system clock is set to
> something reasonably close to the actual time. If it's not, then
> timesyncd can't adjust the time because it's too far off (and in
> addition I have the issue reported on GitHub where systemd-resolved
> can't resolve NTP server names due to DNSSEC failing because the clock
> is too far off...) The file that systemd-timesyncd stores for use on
> reboot helps a little, but if the system is shut off for a period of
> time (an hour or more) then the time at startup is quite far off from
> reality, which is why I have an RTC :)

Hmm, timesyncd should be able to fix the clock in any case regardless
how far things are off? Not sure I follow?

> With a system using solely systemd-provided services, what's the
> proper mechanism to get the time read from an RTC whose driver is
> loaded by systemd-modules-load.service?

There's no such mechanism. We assume that the rtc0 driver is built
into the kernel...

Lennart

--
Lennart Poettering, Berlin
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