drivers: serial: ns16550: Fix TX IRQ not triggered when FIFO is empty

When uart_ns16550_irq_tx_enable() is called and the TX FIFO is already
empty, no new interrupt is generated, causing data transmission to stall
in some cases. This patch introduces a workaround to simulate an ISR
callback if the FIFO is empty when enabling the TX IRQ.

Signed-off-by: Jacky Lee <jacky.lee@egistec.com>

drivers/serial/Kconfig.ns16550

@@ -103,6 +103,17 @@ config UART_NS16550_WA_ISR_REENABLE_INTERRUPT
 	  the IER is being toggled to re-assert interrupts at the end of ISR
 	  to nudge the host interrupt controller to fire the ISR again.
 
+config UART_NS16550_WA_TX_FIFO_EMPTY_INTERRUPT
+	bool "Callback directly when the TX FIFO is already empty"
+	default y if SHELL_BACKEND_SERIAL
+	depends on UART_INTERRUPT_DRIVEN
+	help
+	  When calling uart_ns16550_irq_tx_enable() to wait for the TX FIFO
+	  ready interrupt, but this interrupt will only be triggered if the
+	  current state is not empty. Therefore, if the current state is
+	  empty, you need to solve this problem by calling the callback
+	  function directly.
+
 endmenu
 
 endif # UART_NS16550

drivers/serial/uart_ns16550.c

@@ -369,6 +369,10 @@ struct uart_ns16550_dev_data {
 	void *cb_data;	/**< Callback function arg */
 #endif
 
+#ifdef CONFIG_UART_NS16550_WA_TX_FIFO_EMPTY_INTERRUPT
+	uint8_t sw_tx_irq; /**< software tx ready flag */
+#endif
+
 #if UART_NS16550_DLF_ENABLED
 	uint8_t dlf;		/**< DLF value */
 #endif
@@ -933,6 +937,10 @@ static void uart_ns16550_poll_out(const struct device *dev,
 
 	ns16550_outbyte(dev_cfg, THR(dev), c);
 
+#ifdef CONFIG_UART_NS16550_WA_TX_FIFO_EMPTY_INTERRUPT
+	data->sw_tx_irq = 0; /**< clean up */
+#endif
+
 	k_spin_unlock(&data->lock, key);
 }
 
@@ -980,6 +988,12 @@ static int uart_ns16550_fifo_fill(const struct device *dev,
 		ns16550_outbyte(dev_cfg, THR(dev), tx_data[i]);
 	}
 
+#ifdef CONFIG_UART_NS16550_WA_TX_FIFO_EMPTY_INTERRUPT
+	if (i != 0) {
+		data->sw_tx_irq = 0; /**< clean up */
+	}
+#endif
+
 	k_spin_unlock(&data->lock, key);
 
 	return i;
@@ -1042,6 +1056,26 @@ static void uart_ns16550_irq_tx_enable(const struct device *dev)
 #endif
 	ns16550_outbyte(dev_cfg, IER(dev), ns16550_inbyte(dev_cfg, IER(dev)) | IER_TBE);
 
+#ifdef CONFIG_UART_NS16550_WA_TX_FIFO_EMPTY_INTERRUPT
+	if (ns16550_inbyte(dev_cfg, LSR(dev)) & LSR_THRE) {
+		k_spin_unlock(&data->lock, key);
+		/*
+		 * The TX FIFO ready interrupt will be triggered if only if
+		 * when the pre-state is not empty. Thus, if the pre-state is
+		 * already empty, try to call the callback routine directly
+		 * to resolve it.
+		 */
+		int irq_lock_key = arch_irq_lock();
+
+		if (data->cb && (ns16550_inbyte(dev_cfg, LSR(dev)) & LSR_THRE)) {
+			data->sw_tx_irq = 1; /**< set tx ready */
+			data->cb(dev, data->cb_data);
+		}
+		arch_irq_unlock(irq_lock_key);
+		return;
+	}
+#endif
+
 	k_spin_unlock(&data->lock, key);
 }
 
@@ -1056,6 +1090,10 @@ static void uart_ns16550_irq_tx_disable(const struct device *dev)
 	const struct uart_ns16550_dev_config * const dev_cfg = dev->config;
 	k_spinlock_key_t key = k_spin_lock(&data->lock);
 
+#ifdef CONFIG_UART_NS16550_WA_TX_FIFO_EMPTY_INTERRUPT
+	data->sw_tx_irq = 0; /**< clean up */
+#endif
+
 	ns16550_outbyte(dev_cfg, IER(dev),
 			ns16550_inbyte(dev_cfg, IER(dev)) & (~IER_TBE));
 
@@ -1096,6 +1134,17 @@ static int uart_ns16550_irq_tx_ready(const struct device *dev)
 
 	int ret = ((IIRC(dev) & IIR_ID) == IIR_THRE) ? 1 : 0;
 
+#ifdef CONFIG_UART_NS16550_WA_TX_FIFO_EMPTY_INTERRUPT
+	if (ret == 0 && data->sw_tx_irq) {
+		/**< replace resoult when there is a software solution */
+		const struct uart_ns16550_dev_config * const dev_cfg = dev->config;
+
+		if (ns16550_inbyte(dev_cfg, IER(dev)) & IER_TBE) {
+			ret = 1;
+		}
+	}
+#endif
+
 	k_spin_unlock(&data->lock, key);
 
 	return ret;