Bug#932845: TS-219 RTC issue with Debian Buster

[Trent Piepho <tpiepho@impinj.com>]

On Fri, 2019-07-26 at 12:53 +0200, Oliver Hartkopp wrote:
> Just a thought:
> 
> There are some of these rtc drivers that set
> 
> rtc->rtc->uie_unsupported = 1;
> 
> in the case that they can't assign an irq line.
> 
> But others set
> 
> rtc->rtc->uie_unsupported = 1;
> 
> when they don't support an (alarm) trigger with 1 sec accuracy.
> 
> Wouldn't it make sense to put
> 
> +       select RTC_INTF_DEV
> +       select RTC_INTF_DEV_UIE_EMUL
> 
> in the Kconfig entries of the latter devices?

The hwclock in busybox does not use UIE.  Is it the util-linux version
that uses it?  Or systemd timedate?

I know that chrony's linux RTC support requires UIE, or UIE emulation,
to work.  chrony does not detect lack of this very well and the RTC
support just "doesn't happen" with no errors.  I had to strace it to
figure out it was waiting for UIE interrupts that never came.

Anyway, you don't really need UIE at all to use an rtc in a number of
ways.  The kernel "rtc to system clock on boot" feature doesn't need
it.  The kernel auto sync the rtc every 11 mins from NTP synced system
clock feature doesn't need it.  busybox hwclock doesn't need it.

So I suspect it's optional because it's not always needed.