[PATCH v3 13/14] serial: liteuart: add IRQ support for the TX path

[Gabriel Somlo <gsomlo@gmail.com>]

Modify the TX path to operate in an IRQ-compatible way, while
maintaining support for polling mode via the poll timer.

Signed-off-by: Gabriel Somlo <gsomlo@gmail.com>
---
 drivers/tty/serial/liteuart.c | 67 ++++++++++++++++++++++++-----------
 1 file changed, 47 insertions(+), 20 deletions(-)

diff --git a/drivers/tty/serial/liteuart.c b/drivers/tty/serial/liteuart.c
index e30adb30277f..307c27398e30 100644
--- a/drivers/tty/serial/liteuart.c
+++ b/drivers/tty/serial/liteuart.c
@@ -46,6 +46,7 @@ struct liteuart_port {
 	struct uart_port port;
 	struct timer_list timer;
 	u32 id;
+	bool poll_tx_started;
 };
 
 #define to_liteuart_port(port)	container_of(port, struct liteuart_port, port)
@@ -78,29 +79,24 @@ static void liteuart_putchar(struct uart_port *port, unsigned char ch)
 
 static void liteuart_stop_tx(struct uart_port *port)
 {
-	/* not used in LiteUART, but called unconditionally from serial_core */
+	if (port->irq) {
+		u8 irq_mask = litex_read8(port->membase + OFF_EV_ENABLE);
+		litex_write8(port->membase + OFF_EV_ENABLE, irq_mask & ~EV_TX);
+	} else {
+		struct liteuart_port *uart = to_liteuart_port(port);
+		uart->poll_tx_started = false;
+	}
 }
 
 static void liteuart_start_tx(struct uart_port *port)
 {
-	struct circ_buf *xmit = &port->state->xmit;
-	unsigned char ch;
-
-	if (unlikely(port->x_char)) {
-		litex_write8(port->membase + OFF_RXTX, port->x_char);
-		port->icount.tx++;
-		port->x_char = 0;
-	} else if (!uart_circ_empty(xmit)) {
-		while (xmit->head != xmit->tail) {
-			ch = xmit->buf[xmit->tail];
-			xmit->tail = (xmit->tail + 1) & (UART_XMIT_SIZE - 1);
-			port->icount.tx++;
-			liteuart_putchar(port, ch);
-		}
+	if (port->irq) {
+		u8 irq_mask = litex_read8(port->membase + OFF_EV_ENABLE);
+		litex_write8(port->membase + OFF_EV_ENABLE, irq_mask | EV_TX);
+	} else {
+		struct liteuart_port *uart = to_liteuart_port(port);
+		uart->poll_tx_started = true;
 	}
-
-	if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS)
-		uart_write_wakeup(port);
 }
 
 static void liteuart_stop_rx(struct uart_port *port)
@@ -131,18 +127,47 @@ static void liteuart_rx_chars(struct uart_port *port)
 	tty_flip_buffer_push(&port->state->port);
 }
 
+static void liteuart_tx_chars(struct uart_port *port)
+{
+	struct circ_buf *xmit = &port->state->xmit;
+
+	if (unlikely(port->x_char)) {
+		litex_write8(port->membase + OFF_RXTX, port->x_char);
+		port->x_char = 0;
+		port->icount.tx++;
+		return;
+	}
+
+	while (!litex_read8(port->membase + OFF_TXFULL)) {
+		if (xmit->head == xmit->tail)
+			break;
+		litex_write8(port->membase + OFF_RXTX, xmit->buf[xmit->tail]);
+		xmit->tail = (xmit->tail + 1) & (UART_XMIT_SIZE - 1);
+		port->icount.tx++;
+	}
+
+	if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS)
+		uart_write_wakeup(port);
+
+	if (uart_circ_empty(xmit))
+		liteuart_stop_tx(port);
+}
+
 static irqreturn_t liteuart_interrupt(int irq, void *data)
 {
 	struct liteuart_port *uart = data;
 	struct uart_port *port = &uart->port;
 	u8 isr = litex_read8(port->membase + OFF_EV_PENDING);
 
-	/* for now, only rx path triggers interrupts */
-	isr &= EV_RX;
+	if (!(port->irq || uart->poll_tx_started))
+		isr &= ~EV_TX;	/* polling mode with tx stopped */
 
 	spin_lock(&port->lock);
 	if (isr & EV_RX)
 		liteuart_rx_chars(port);
+	if (isr & EV_TX) {
+		liteuart_tx_chars(port);
+	}
 	spin_unlock(&port->lock);
 
 	return IRQ_RETVAL(isr);
@@ -196,6 +221,7 @@ static int liteuart_startup(struct uart_port *port)
 	}
 
 	if (!port->irq) {
+		uart->poll_tx_started = false;
 		timer_setup(&uart->timer, liteuart_timer, 0);
 		mod_timer(&uart->timer, jiffies + uart_poll_timeout(port));
 	}
@@ -210,6 +236,7 @@ static void liteuart_shutdown(struct uart_port *port)
 	struct liteuart_port *uart = to_liteuart_port(port);
 
 	litex_write8(port->membase + OFF_EV_ENABLE, 0);
+	uart->poll_tx_started = false;
 
 	if (port->irq)
 		free_irq(port->irq, port);
-- 
2.37.3