tty: amba-pl011: Add un/throttle support

raspberrypi inclusion
category: feature
bugzilla: 50432

--------------------------------

The PL011 driver lacks throttle and unthrottle methods. As a result,
sending more data to the Pi than it can immediately sink while CRTSCTS
is enabled causes a NULL pointer to be followed.

Add a throttle handler that disables the RX interrupts, and an
unthrottle handler that reenables them.
Signed-off-by: NPhil Elwell <phil@raspberrypi.org>
Signed-off-by: NFang Yafen <yafen@iscas.ac.cn>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>

drivers/tty/serial/amba-pl011.c

@@ -1317,6 +1317,32 @@ static void pl011_start_tx(struct uart_port *port)
 		pl011_start_tx_pio(uap);
 }
 
+static void pl011_throttle(struct uart_port *port)
+{
+	struct uart_amba_port *uap =
+	    container_of(port, struct uart_amba_port, port);
+	unsigned long flags;
+
+	spin_lock_irqsave(&uap->port.lock, flags);
+	uap->im &= ~(UART011_RTIM | UART011_RXIM);
+	pl011_write(uap->im, uap, REG_IMSC);
+	spin_unlock_irqrestore(&uap->port.lock, flags);
+}
+
+static void pl011_unthrottle(struct uart_port *port)
+{
+	struct uart_amba_port *uap =
+	    container_of(port, struct uart_amba_port, port);
+	unsigned long flags;
+
+	spin_lock_irqsave(&uap->port.lock, flags);
+	uap->im |= UART011_RTIM;
+	if (!pl011_dma_rx_running(uap))
+	    uap->im |= UART011_RXIM;
+	pl011_write(uap->im, uap, REG_IMSC);
+	spin_unlock_irqrestore(&uap->port.lock, flags);
+}
+
 static void pl011_stop_rx(struct uart_port *port)
 {
 	struct uart_amba_port *uap =
@@ -2156,6 +2182,8 @@ static const struct uart_ops amba_pl011_pops = {
 	.stop_tx	= pl011_stop_tx,
 	.start_tx	= pl011_start_tx,
 	.stop_rx	= pl011_stop_rx,
+	.throttle	= pl011_throttle,
+	.unthrottle	= pl011_unthrottle,
 	.enable_ms	= pl011_enable_ms,
 	.break_ctl	= pl011_break_ctl,
 	.startup	= pl011_startup,