uart_mcux_flexcomm: Fix TX garbage char bug in PM

There is currently a bug where when entering low power modes, there is a
garbage character sent if the UART is in the middle of a transmit when
entering low power mode. This is because the fifo interrupt happens when
last character is pulled by transmitter from fifo and then we unlock PM
constraints at that moment, instead of waiting for actual transmitter to
become idle. Fix by adding work item to check for this when needed.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>

drivers/serial/uart_mcux_flexcomm.c

@@ -51,6 +51,9 @@ struct mcux_flexcomm_config {
 	void (*rx_timeout_func)(struct k_work *work);
 	void (*tx_timeout_func)(struct k_work *work);
 #endif
+#ifdef CONFIG_PM_POLICY_DEVICE_CONSTRAINTS
+	void (*pm_unlock_work_fn)(struct k_work *);
+#endif
 };
 
 #if CONFIG_UART_ASYNC_API
@@ -90,6 +93,7 @@ struct mcux_flexcomm_data {
 #endif
 #ifdef CONFIG_PM_POLICY_DEVICE_CONSTRAINTS
 	bool pm_policy_state_lock;
+	struct k_work pm_lock_work;
 #endif
 };
 
@@ -104,13 +108,27 @@ static void mcux_flexcomm_pm_policy_state_lock_get(const struct device *dev)
 	}
 }
 
-static void mcux_flexcomm_pm_policy_state_lock_put(const struct device *dev)
+static void mcux_flexcomm_pm_unlock_if_idle(const struct device *dev)
 {
+	const struct mcux_flexcomm_config *config = dev->config;
 	struct mcux_flexcomm_data *data = dev->data;
 
-	if (data->pm_policy_state_lock) {
+	if (config->base->STAT & USART_STAT_TXIDLE_MASK) {
 		data->pm_policy_state_lock = false;
 		pm_policy_device_power_lock_put(dev);
+	} else {
+		/* can't block systemn workqueue so keep re-submitting until it's done */
+		k_work_submit(&data->pm_lock_work);
+	}
+}
+
+static void mcux_flexcomm_pm_policy_state_lock_put(const struct device *dev)
+{
+	struct mcux_flexcomm_data *data = dev->data;
+
+	if (data->pm_policy_state_lock) {
+		/* we can't block on TXidle mask in IRQ context so offload */
+		k_work_submit(&data->pm_lock_work);
 	}
 }
 #endif
@@ -1157,6 +1175,13 @@ static int mcux_flexcomm_pm_action(const struct device *dev, enum pm_device_acti
 
 static int mcux_flexcomm_init(const struct device *dev)
 {
+#if defined(CONFIG_PM_POLICY_DEVICE_CONSTRAINTS)
+	const struct mcux_flexcomm_config *config = dev->config;
+	struct mcux_flexcomm_data *data = dev->data;
+
+	k_work_init(&data->pm_lock_work, config->pm_unlock_work_fn);
+#endif
+
 	/* Rest of the init is done from the PM_DEVICE_TURN_ON action
 	 * which is invoked by pm_device_driver_init().
 	 */
@@ -1215,6 +1240,21 @@ static DEVICE_API(uart, mcux_flexcomm_driver_api) = {
 #define UART_MCUX_FLEXCOMM_IRQ_CFG_FUNC_INIT(n)
 #endif /* CONFIG_UART_MCUX_FLEXCOMM_ISR_SUPPORT */
 
+#ifdef CONFIG_PM_POLICY_DEVICE_CONSTRAINTS
+#define UART_MCUX_FLEXCOMM_PM_UNLOCK_FUNC_DEFINE(n)				\
+static void mcux_flexcomm_##n##_pm_unlock(struct k_work *work)			\
+{										\
+	const struct device *dev = DEVICE_DT_INST_GET(n);			\
+										\
+	mcux_flexcomm_pm_unlock_if_idle(dev);					\
+}
+#define UART_MCUX_FLEXCOMM_PM_UNLOCK_FUNC_BIND(n)				\
+	.pm_unlock_work_fn = mcux_flexcomm_##n##_pm_unlock,
+#else
+#define UART_MCUX_FLEXCOMM_PM_UNLOCK_FUNC_DEFINE(n)
+#define UART_MCUX_FLEXCOMM_PM_UNLOCK_FUNC_BIND(n)
+#endif /* CONFIG_PM_POLICY_DEVICE_CONSTRAINTS */
+
 #ifdef CONFIG_UART_ASYNC_API
 #define UART_MCUX_FLEXCOMM_TX_TIMEOUT_FUNC(n)					\
 	static void mcux_flexcomm_uart_##n##_tx_timeout(struct k_work *work)	\
@@ -1288,6 +1328,7 @@ static const struct mcux_flexcomm_config mcux_flexcomm_##n##_config = {		\
 	.pincfg = PINCTRL_DT_INST_DEV_CONFIG_GET(n),				\
 	UART_MCUX_FLEXCOMM_IRQ_CFG_FUNC_INIT(n)					\
 	UART_MCUX_FLEXCOMM_ASYNC_CFG(n)						\
+	UART_MCUX_FLEXCOMM_PM_UNLOCK_FUNC_BIND(n)				\
 };
 
 #define UART_MCUX_FLEXCOMM_INIT(n)						\
@@ -1298,6 +1339,8 @@ static const struct mcux_flexcomm_config mcux_flexcomm_##n##_config = {		\
 										\
 	static const struct mcux_flexcomm_config mcux_flexcomm_##n##_config;	\
 										\
+	UART_MCUX_FLEXCOMM_PM_UNLOCK_FUNC_DEFINE(n)				\
+										\
 	PM_DEVICE_DT_INST_DEFINE(n, mcux_flexcomm_pm_action);			\
 										\
 	DEVICE_DT_INST_DEFINE(n,						\