Re: [QUESTION] rtc: zynqmp: CALIB_RD reset behavior differs between ZynqMP and Versal

[Alexandre Belloni <alexandre.belloni@bootlin.com>]

On 06/03/2026 12:09:40+0200, Tomas Melin wrote:
> > On Zynq UltraScale+ Devices Register Reference (UG1087) [2],
> > CALIB_RD resets to 0, so the current logic works correctly there.
> > However, this assumption does not appear to hold for Versal.
> 
> For Ultrascale+ the calibration register also gives random values after
> reset, perhaps you have noticed this:
> https://adaptivesupport.amd.com/s/article/000036886?language=en_US. Maybe
> the same can occur also on Versal.
> 
> AFAIK there is no way of knowing if the value is correct or not after reset,
> so user space helpers might be needed to maintain the calibration value at a
> desired value.
> 

Userspace is always needed to put the proper calibration, there is no
way for the kernel to know what value to put there. In the support case
above, the crystal will never be exactly 32768Hz and this value will
change over time and also depends on the temperature. The value always
needs to be computed, if your device can do NTP, chrony will provide the
proper offsets. If you don't have a way to measure the deviation, then
userspace can always forcefully set /sys/class/rtc/rtcX/offset if it
doesn't hold the correct value.
There is no need for devmem here.

-- 
Alexandre Belloni, co-owner and COO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com