nrf52: high standby current on nrf52840 when using MCUBOOT + UART_ASYNC_API + PM_DEVICE

[mjm987]

Describe the bug
When using CONFIG_PM_DEVICE up to Zephyr v3.7 the standby current was about 6 uA by setting pm_device_action_run(uart0, PM_DEVICE_ACTION_SUSPEND) with or without any combination of CONFIG_UART_ASYNC_API and/or CONFIG_BOOTLOADER_MCUBOOT.

But since Zephyr v4.0 the standby current after pm_device_action_run(uart0, PM_DEVICE_ACTION_SUSPEND) is between 0.5mA and 1mA when using following config combination :
CONFIG_PM_DEVICE=y + CONFIG_UART_ASYNC_API=y + CONFIG_BOOTLOADER_MCUBOOT=y

If either CONFIG_UART_ASYNC_API=n or no MCUBOOT is used, the current is as expected.

To Reproduce

1. Use eg. the Nordic nRF52840DK board (-b nrf52840dk/nrf52840).

2. Use blinky sample

3. Add in prj.conf:
   CONFIG_PM_DEVICE=y
   CONFIG_UART_ASYNC_API=y
   CONFIG_SERIAL=y
   CONFIG_LOG=y
   CONFIG_BOOTLOADER_MCUBOOT=y

4. Replace in main.c::main() the printk() statement by:
   pm_device_action_run(DEVICE_DT_GET(DT_NODELABEL(uart0)), PM_DEVICE_ACTION_RESUME);
   LOG_INF("LED state: %s\n", led_state ? "ON" : "OFF");
   while (log_process()) ;
   pm_device_action_run(DEVICE_DT_GET(DT_NODELABEL(uart0)), PM_DEVICE_ACTION_SUSPEND);

Expected behavior
The standby current should be about 6uA

Impact
The standby current is about 500uA ... 1000uA

Environment (please complete the following information):

Zephyr >= v4.0 (tested with v4.0 and v4.1)

[carlescufi]

@nvlsianpu and @nordicjm could MCUboot be leaving a peripheral turned on?
@nordic-krch any clues?
@bjarki-andreasen adding you here since there's PM device involved

[nordic-krch]

if it's only when MCUBOOT is present then i would check this: #75703 uart driver does not attempt to do any cleanup for the bootloader. If it is needed application need to do that before UART is initialized.

[mjm987]

Unfortunately the cleanup as shown in #75703 did not help.