Messages in this thread | | | Date | Tue, 27 Aug 2019 18:31:34 +0200 | From | Alexandre Belloni <> | Subject | Re: New kernel interface for sys_tz and timewarp? |
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On 27/08/2019 18:27:32+0200, Arnd Bergmann wrote: > On Tue, Aug 20, 2019 at 8:58 PM Alexandre Belloni > <alexandre.belloni@bootlin.com> wrote: > > > > 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. > > Would you use the regular RTC_SET_TIME ioctl for that, or > add a new RTC_SYS_TO_HC command that takes an explicit > offset? It sounds to me that the synchronization bit (actually > waiting for the right moment to update the rtc registers) is better > done in the kernel, while the decision about the offset and when > to call into the driver is better done in user space. >
The existing ioctls are fine to do that, see: https://git.kernel.org/pub/scm/linux/kernel/git/abelloni/rtc-tools.git/tree/rtc-sync.c
-- Alexandre Belloni, Bootlin Embedded Linux and Kernel engineering https://bootlin.com
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