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

[Michał Zegan]

Seems like rtc drivers as module is quite often a thing for embedded.
But not sure where this should be solved, maybe at initramfs? If one is
unwilling to build all rtc drivers into the kernel (the case of generic
kernels where you would have to build all of them in)...

W dniu 31.03.2020 o 16:29, Lennart Poettering pisze:
> On Do, 12.03.20 07:13, Kevin P. Fleming (kevin at km6g.us) 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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