[systemd-devel] Cannot find a way to get time read from RTC during boot

[Kevin P. Fleming]

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.

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 :)

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?