uart: nrfx_uarte: drivers/serial/uart_nrfx_uarte.c: uarte_nrfx_isr_int disables uarte TXSTOPPED if PM_DEVICE_RUNTIME enabled during interrupt driven output

[helpfulchicken]

Describe the bug

Interrupt driven UART output fails when using nRF UARTE in v4.0 if CONFIG_PM_DEVICE_RUNTIME is enabled while not using low power mode for the UART.

Platform: nRF, such as nRF52840DK

I've narrowed it down to a specific set of changes from a specific commit: 3ce19da
https://github.com/zephyrproject-rtos/zephyr/aabd3a1826d750f2ae4b271894837a91f42186bc/drivers/serial/uart_nrfx_uarte.c#L351-L372

zephyr/drivers/serial/uart_nrfx_uarte.c

Lines 343 to 364 in 3ce19da

	bool txstopped = nrf_uarte_event_check(uarte, NRF_UARTE_EVENT_TXSTOPPED);
	if (txstopped && (IS_ENABLED(CONFIG_PM_DEVICE_RUNTIME) || LOW_POWER_ENABLED(config))) {
	unsigned int key = irq_lock();
	if (IS_ENABLED(CONFIG_PM_DEVICE_RUNTIME) &&
	(data->flags & UARTE_FLAG_POLL_OUT)) {
	data->flags &= ~UARTE_FLAG_POLL_OUT;
	pm_device_runtime_put(dev);
	} else {
	nrf_uarte_disable(uarte);
	}
	#ifdef UARTE_INTERRUPT_DRIVEN
	if (!data->int_driven || data->int_driven->fifo_fill_lock == 0)
	#endif
	{
	nrf_uarte_int_disable(uarte, NRF_UARTE_INT_TXSTOPPED_MASK);
	}
	irq_unlock(key);
	}

Prior to this commit, this conditional branch only executes nrf_uarte_disable after a TxStopped event if the UART was configured for low power.

Now this conditional branch also executes nrf_uarte_disable on line 358 after a TxStopped event if CONFIG_PM_DEVICE_RUNTIME is enabled while transmit is NOT polling based. This breaks the prior interrupt based transmit behaviour where a user could use their interrupt driven callback to load data into the uart fifo after each TxStopped event without the uarte unexpectedly being disabled.

The commit originally introducing that nrf_uarte_disable call (f075fee) talks about adding low power mode for the uarte peripheral, so I believe the intent is that it is only called if the uart is actually configured for low power mode, not as a side effect of CONFIG_PM_DEVICE_RUNTIME=y plus using interrupt driven transmit.

By the same logic, the nrf_uarte_int_disable section highlighted below might be unintentionally called, but it would be easy to workaround by re-enabling the tx irq from the user uart irq callback.

zephyr/drivers/serial/uart_nrfx_uarte.c

Lines 356 to 361 in 3ce19da

	#ifdef UARTE_INTERRUPT_DRIVEN
	if (!data->int_driven || data->int_driven->fifo_fill_lock == 0)
	#endif
	{
	nrf_uarte_int_disable(uarte, NRF_UARTE_INT_TXSTOPPED_MASK);
	}

---

Suggested fix:
Add a condition to 'else' branch requiring LOW_POWER_ENABLED(config), so that as originally intended it is only called after the TXSTOPPED event if the uart is configured for low power.

	if (txstopped && (IS_ENABLED(CONFIG_PM_DEVICE_RUNTIME) || LOW_POWER_ENABLED(config))) {
		unsigned int key = irq_lock();

		if (IS_ENABLED(CONFIG_PM_DEVICE_RUNTIME) &&
		    (data->flags & UARTE_FLAG_POLL_OUT)) {
			data->flags &= ~UARTE_FLAG_POLL_OUT;
			pm_device_runtime_put(dev);
		} else if (LOW_POWER_ENABLED(config)){
			nrf_uarte_disable(uarte);
		}

Regression

• This is a regression.

Steps to reproduce

Reproduction notes:

• take an interrupt driven UART program, target a nRF board with a UARTE peripheral
• use interrupt driven UART to send data, like filling the uart fifo from the user irq callback
• enable CONFIG_PM_DEVICE_RUNTIME
• ensure the devicetree for the UART has compatible = "nordic,nrf-uarte" and doesn't have low-power-enable
• note that the nrf uarte peripheral gets disabled as soon as a TXSTOPPED event is processed, so refilling the uart fifo from the user irq callback doesn't get the expected result, even if the uarte tx irq is re-enabled by the user.

Relevant log output

Impact

Major – Severely degrades functionality; workaround is difficult or unavailable.

Environment

OS: Windows
Toolchain: Zephyr 4.0

Additional Context

nRF family target,
uarte peripheral enabled, (compatible = "nordic,nrf-uarte"), without low power enabled
config enabling CONFIG_PM_DEVICE_RUNTIME and CONFIG_UART_INTERRUPT_DRIVEN