Messages in this thread | | | Date | Tue, 20 Aug 2019 20:58:30 +0200 | From | Alexandre Belloni <> | Subject | Re: New kernel interface for sys_tz and timewarp? |
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On 19/08/2019 13:09:03+0200, Karel Zak wrote: > On Wed, Aug 14, 2019 at 11:32:08AM +0200, Alexandre Belloni wrote: > > On 14/08/2019 11:09:36+0200, Lennart Poettering wrote: > > > On Mi, 14.08.19 10:31, Arnd Bergmann (arnd@arndb.de) wrote: > > > > > > > - glibc stops passing the caller timezone argument to the kernel > > > > - the distro kernel disables CONFIG_RTC_HCTOSYS, > > > > CONFIG_RTC_SYSTOHC and CONFIG_GENERIC_CMOS_UPDATE > > > > > > What's the benefit of letting userspace do this? It sounds a lot more > > > fragile to leave this syncing to userspace if the kernel can do this > > > trivially on its own. > > Good point, why CONFIG_RTC_SYSTOHC has been added to the kernel? > > If I good remember than it's because synchronize userspace hwclock > with rtc is pretty fragile and frustrating. We have improved this > hwclock code many times and it will never be perfect. See for example > hwclock --delay= option, sometimes hwclock has no clue about RTC behaviour. >
With a bit of care, we can reliably set the rtc to the system time from userspace. It takes a bit of time (up to 2 seconds) but it can be reliably set with an accuracy of a few ms on a slow system and an rtc on a slow bus or a few ns with a fast system and a fast bus. I know I did say I would implement it in hwclock and I still didn't (sorry) but we could do better than the --delay option.
> > It does it trivially and badly: > > > > - hctosys will always think the RTC is in UTC so if the RTC is in > > local time, you will anyway have up to 12 hours difference until > > userspace fixes that. > > Cannot we provide all necessary information for example on kernel > command line, or/and as rtc module option? >
We could but from a distro point of view, would that be convenient?
-- Alexandre Belloni, Bootlin Embedded Linux and Kernel engineering https://bootlin.com
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