sc16is7xx: Set AUTOCTS and AUTORTS bits

Let serial core know that the chip automatically handles RTS/CTS signal.
This elimines completely unnecessary I2C/SPI bus traffic.

Cease reading from RX FIFO (by disabling RDI interrupt) when throttled.
Eventually the FIFO will fill up and the device will drive RTS output
inactive. Unthrottle by enabling back RDI interrupt.

Indirectly controlling RTS via RX FIFO state seems to be the only option
because RTS bit is ignored when hardware flow control is enabled.

Signed-off-by: Tomasz Moń <tomasz.mon@camlingroup.com>
Link: https://lore.kernel.org/r/20220301060332.2561851-4-tomasz.mon@camlingroup.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Tomasz Moń

drivers/tty/serial/sc16is7xx.c

@@ -957,6 +957,29 @@ static void sc16is7xx_start_tx(struct uart_port *port)
 	kthread_queue_work(&s->kworker, &one->tx_work);
 }
 
+static void sc16is7xx_throttle(struct uart_port *port)
+{
+	unsigned long flags;
+
+	/*
+	 * Hardware flow control is enabled and thus the device ignores RTS
+	 * value set in MCR register. Stop reading data from RX FIFO so the
+	 * AutoRTS feature will de-activate RTS output.
+	 */
+	spin_lock_irqsave(&port->lock, flags);
+	sc16is7xx_ier_clear(port, SC16IS7XX_IER_RDI_BIT);
+	spin_unlock_irqrestore(&port->lock, flags);
+}
+
+static void sc16is7xx_unthrottle(struct uart_port *port)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&port->lock, flags);
+	sc16is7xx_ier_set(port, SC16IS7XX_IER_RDI_BIT);
+	spin_unlock_irqrestore(&port->lock, flags);
+}
+
 static unsigned int sc16is7xx_tx_empty(struct uart_port *port)
 {
 	unsigned int lsr;
@@ -1062,9 +1085,13 @@ static void sc16is7xx_set_termios(struct uart_port *port,
 	regcache_cache_bypass(s->regmap, true);
 	sc16is7xx_port_write(port, SC16IS7XX_XON1_REG, termios->c_cc[VSTART]);
 	sc16is7xx_port_write(port, SC16IS7XX_XOFF1_REG, termios->c_cc[VSTOP]);
-	if (termios->c_cflag & CRTSCTS)
+
+	port->status &= ~(UPSTAT_AUTOCTS | UPSTAT_AUTORTS);
+	if (termios->c_cflag & CRTSCTS) {
 		flow |= SC16IS7XX_EFR_AUTOCTS_BIT |
 			SC16IS7XX_EFR_AUTORTS_BIT;
+		port->status |= UPSTAT_AUTOCTS | UPSTAT_AUTORTS;
+	}
 	if (termios->c_iflag & IXON)
 		flow |= SC16IS7XX_EFR_SWFLOW3_BIT;
 	if (termios->c_iflag & IXOFF)
@@ -1270,6 +1297,8 @@ static const struct uart_ops sc16is7xx_ops = {
 	.get_mctrl	= sc16is7xx_get_mctrl,
 	.stop_tx	= sc16is7xx_stop_tx,
 	.start_tx	= sc16is7xx_start_tx,
+	.throttle	= sc16is7xx_throttle,
+	.unthrottle	= sc16is7xx_unthrottle,
 	.stop_rx	= sc16is7xx_stop_rx,
 	.enable_ms	= sc16is7xx_enable_ms,
 	.break_ctl	= sc16is7xx_break_ctl,