tty: serial: sh-sci: Add support for tx end interrupt handling

As per the RZ/G2L users hardware manual (Rev.1.20 Sep, 2022), section
23.3.7 Serial Data Transmission (Asynchronous Mode), it is mentioned
that, set the SCR.TIE bit to 0 and SCR.TEIE bit to 1, after the last
data to be transmitted are written to the TDR.

This will generate tx end interrupt and in the handler set SCR.TE and
SCR.TEIE to 0.

Signed-off-by: Biju Das <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/serial/sh-sci.c

···
 
 	if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS)
 		uart_write_wakeup(port);
-	if (uart_circ_empty(xmit))
-		sci_stop_tx(port);
+	if (uart_circ_empty(xmit)) {
+		if (port->type == PORT_SCI) {
+			ctrl = serial_port_in(port, SCSCR);
+			ctrl &= ~SCSCR_TIE;
+			ctrl |= SCSCR_TEIE;
+			serial_port_out(port, SCSCR, ctrl);
+		}
 
+		sci_stop_tx(port);
+	}
 }
 
 static void sci_receive_chars(struct uart_port *port)
···
 	return IRQ_HANDLED;
 }
 
+static irqreturn_t sci_tx_end_interrupt(int irq, void *ptr)
+{
+	struct uart_port *port = ptr;
+	unsigned long flags;
+	unsigned short ctrl;
+
+	if (port->type != PORT_SCI)
+		return sci_tx_interrupt(irq, ptr);
+
+	spin_lock_irqsave(&port->lock, flags);
+	ctrl = serial_port_in(port, SCSCR);
+	ctrl &= ~(SCSCR_TE | SCSCR_TEIE);
+	serial_port_out(port, SCSCR, ctrl);
+	spin_unlock_irqrestore(&port->lock, flags);
+
+	return IRQ_HANDLED;
+}
+
 static irqreturn_t sci_br_interrupt(int irq, void *ptr)
 {
 	struct uart_port *port = ptr;
···
 
 	[SCIx_TEI_IRQ] = {
 		.desc = "tx end",
-		.handler = sci_tx_interrupt,
+		.handler = sci_tx_end_interrupt,
 	},
 
 	/*

drivers/tty/serial/sh-sci.h

···
 #define SCSMR_SRC_19	0x0600	/* Sampling rate 1/19 */
 #define SCSMR_SRC_27	0x0700	/* Sampling rate 1/27 */
 
+/* Serial Control Register, SCI only bits */
+#define SCSCR_TEIE	BIT(2)  /* Transmit End Interrupt Enable */
+
 /* Serial Control Register, SCIFA/SCIFB only bits */
 #define SCSCR_TDRQE	BIT(15)	/* Tx Data Transfer Request Enable */
 #define SCSCR_RDRQE	BIT(14)	/* Rx Data Transfer Request Enable */