nRF54L: uptime does not reset across reboots

[JordanYates]

Describe the bug

On the nRF54L, resetting the application without removing power does not result in the internal counters starting again from 0.

To Reproduce

1. Update zephyr/samples/hello_world to:

#include <stdio.h>
#include <zephyr/kernel.h>

int main(void)
{
	while(1) {
		printf("Hello World! %s %6d\n", CONFIG_BOARD_TARGET, k_uptime_seconds());
		k_sleep(K_SECONDS(1));
	}
	return 0;
}

2. west build -b nrf54l15dk/nrf54l15/cpuapp zephyr/samples/hello_world/
3. west flash
4. Wait several seconds
5. nrfutil device reset
6. Observe the serial output

Expected behavior

Application should start counting from 0 on each reset.

Impact

Application thinks it's been running for much longer than it has been.
Absolute timestamps not constructed without reference to the current uptime will be broken.
i.e. K_TIMEOUT_ABS_MS(5000) will expire immediately upon reboot.

Logs and console output

Environment (please complete the following information):

• Zephyr v4.1 and main

Additional context

k_uptime_seconds() is not special in any way, it simply retrieves the system tick counter and does a division.
This implies that the system tick counter is not being reset across reboots.

The nRF54L series does not use the same system tick source as previous nRF SoCs, it uses the GRTC (nrf_grtc_timer).

[nordicjm]

Ping @nordic-krch

[nordic-krch]

@adamkondraciuk can you check that?

[adamkondraciuk]

GRTC is not cleared on every reboot by default and we don't want to do that, however I agree that uptime should be cleared on every reboot - we're working on it.
For now you can use this PR which adds configuration that clears the GRTC on driver initialization:
#89664
To use it just add CONFIG_NRF_GRTC_CLEAR_AT_INIT=y in your .conf file, or build it using command:
west build -b nrf54l15dk/nrf54l15/cpuapp zephyr/samples/hello_world/ -- -DCONFIG_NRF_GRTC_CLEAR_AT_INIT=y