drivers: serial: stm32: fix non-LP UARTs with PM

Re-configure the registers if they re erased because of a STOP2 low power
mode.

Return the error code when re-initializing the UART after a STANDBY low
power mode.

Fix few typos and comments.

Run clang format on uart_stm32_pm_action()

Signed-off-by: Mickael Bosch <[email protected]>

Author: Mickael Bosch

drivers/serial/uart_stm32.c

@@ -2224,13 +2224,13 @@ static int uart_stm32_registers_configure(const struct device *dev)
 	/* Wait until TEACK flag is set */
 	while (!(LL_USART_IsActiveFlag_TEACK(usart))) {
 	}
-#endif /* !USART_ISR_TEACK */
+#endif /* USART_ISR_TEACK */
 
 #ifdef USART_ISR_REACK
 	/* Wait until REACK flag is set */
 	while (!(LL_USART_IsActiveFlag_REACK(usart))) {
 	}
-#endif /* !USART_ISR_REACK */
+#endif /* USART_ISR_REACK */
 
 	return 0;
 }
@@ -2301,14 +2301,12 @@ static void uart_stm32_suspend_setup(const struct device *dev)
 	LL_USART_ClearFlag_ORE(usart);
 }
 
-static int uart_stm32_pm_action(const struct device *dev,
-			       enum pm_device_action action)
+static int uart_stm32_pm_action(const struct device *dev, enum pm_device_action action)
 {
 	const struct uart_stm32_config *config = dev->config;
 	struct uart_stm32_data *data = dev->data;
 	int err;
 
-
 	switch (action) {
 	case PM_DEVICE_ACTION_RESUME:
 		/* Set pins to active state */
@@ -2317,21 +2315,34 @@ static int uart_stm32_pm_action(const struct device *dev,
 			return err;
 		}
 
-		/* Enable clock */
-		err = clock_control_on(data->clock,
-					(clock_control_subsys_t)&config->pclken[0]);
+		/* Enable bus clock */
+		err = clock_control_on(data->clock, (clock_control_subsys_t)&config->pclken[0]);
 		if (err < 0) {
 			LOG_ERR("Could not enable (LP)UART clock");
 			return err;
 		}
 
-		if ((IS_ENABLED(CONFIG_PM_S2RAM)) &&
-			(!LL_USART_IsEnabled(config->usart))) {
+		if (!LL_USART_IsEnabled(config->usart)) {
 			/* When exiting low power mode, check whether UART is enabled.
-			 * If not, it means we are exiting Suspend to RAM mode (STM32
-			 * Standby), and the driver needs to be reinitialized.
+			 * If not, it means the peripheral has been powered down
+			 * by the low-power mode. If suspend-to-RAM is enabled,
+			 * assume the entire SoC has been powered down and do a
+			 * full re-initialization. Otherwise, assume that the
+			 * low-power mode shut down power to the UART but not
+			 * critical peripherals (CPU, GPIO, RCC), which means
+			 * we only have to reconfigure this UART instance.
+			 *
+			 * STOP2 on STM32WLE5 is an example of such low-power mode.
 			 */
-			uart_stm32_init(dev);
+			if (IS_ENABLED(CONFIG_PM_S2RAM)) {
+				err = uart_stm32_init(dev);
+			} else {
+				err = uart_stm32_registers_configure(dev);
+			}
+
+			if (err < 0) {
+				return err;
+			}
 		}
 		break;
 	case PM_DEVICE_ACTION_SUSPEND: