[Letux-kernel] Where does RTC actually work

Andreas Kemnade andreas at kemnade.info
Tue Jan 31 07:43:58 CET 2017


Hi,

On Mon, 30 Jan 2017 20:40:36 +0100
Andreas Kemnade <andreas at kemnade.info> wrote:

> Hi,
> 
> On Mon, 30 Jan 2017 19:28:58 +0100
> "H. Nikolaus Schaller" <hns at goldelico.com> wrote:
> 
> > 
> > > Am 30.01.2017 um 19:16 schrieb Andreas Kemnade
> > > <andreas at kemnade.info>:
> > > 
> > > Hi,
> > > 
> > > I cannot remember when I saw a device with a working RTC last
> > > time. On my semi-broken GTA04A4 it is kept across reboots at
> > > least with 3.7. Not when the battery is even removed for a short
> > > time. On the fresh GTA04A5 board it is not even kept across
> > > reboots with 4.10rc-4.
> > > 
> > > So what are the known-good combinations (kernel + device)?
> > 
> > I think on the 3.12 kernel it was tested and did work.
> > 
> I just tested what I have without bulding or copying stuff around...
> 
> I tested further and narrow it a bit:
> on my gta04a4 (with old uboot) time is kept with 4.7.0 across
> powerdown (not battery removal)
> root at gta04:~# for i in 1 2 3 4 5 6 7 8 ; do
> cat /sys/bus/iio/devices/iio\:device1/in_voltage9_input  ; sleep 1 ;
> done 3030 2863
> 3431
> 3466
> 3382
> 2727
> 3316
> 3171
> 
> And the backup battery stuff looks somewhat broken (probably
> hardware). The same kernel + same modules on a letux3704 (with new
> uboot): Voltages stable,
> clock does not survive reboots.
> 
> Verdict:
> 1. gta04a4 backup capacitor is broken
> 2. new uboot conflicts with rtc.

That verdict 2 might be too fast:
rtc has a persistent state by design which can be misleading.
So if something "poisons" the rtc, that might have consequences.
Tried old uboot + that same 4.7 kernel. Just loaded the old uboot via
pserial/ukermit
And again the 2066 problem that the kernel refuses to accept the
alternative fact that we are living in 2066.
So, that means:
The is a state of the rtc which the 4.7 kernel  cannot properly handle,
but an untouched rtc seems to be ok.
Running 4.7 kernel does not "poison" the rtc.

So how long do I have to wait until rtc looses its power?

Regards,
Andreas
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