[PATCH 4/6] serial: 8250: Implement prep_tx for power management

[Tony Lindgren <tony@atomide.com>]

We can use the prep_tx() call to wake up an idle serial port. This allows
ust to remove the depedency to pm_runtime_irq_safe() for 8250_omap driver
in the following patches.

Signed-off-by: Tony Lindgren <tony@atomide.com>
---
 drivers/tty/serial/8250/8250_port.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -1650,6 +1650,29 @@ static enum hrtimer_restart serial8250_em485_handle_start_tx(struct hrtimer *t)
 	return HRTIMER_NORESTART;
 }
 
+static int serial8250_prep_tx(struct uart_port *port)
+{
+	struct uart_8250_port *up = up_to_u8250p(port);
+	struct device *dev = up->port.dev;
+	int err;
+
+	if (!(up->capabilities & UART_CAP_RPM))
+		return 0;
+
+	if (!pm_runtime_suspended(dev)) {
+		pm_runtime_mark_last_busy(dev);
+		return 0;
+	}
+
+	err = pm_request_resume(dev);
+	if (err < 0) {
+		dev_warn(dev, "prep_tx wakeup failed: %d\n", err);
+		return err;
+	}
+
+	return -EINPROGRESS;
+}
+
 static void serial8250_start_tx(struct uart_port *port)
 {
 	struct uart_8250_port *up = up_to_u8250p(port);
@@ -3227,6 +3250,7 @@ static const struct uart_ops serial8250_pops = {
 	.set_mctrl	= serial8250_set_mctrl,
 	.get_mctrl	= serial8250_get_mctrl,
 	.stop_tx	= serial8250_stop_tx,
+	.prep_tx	= serial8250_prep_tx,
 	.start_tx	= serial8250_start_tx,
 	.throttle	= serial8250_throttle,
 	.unthrottle	= serial8250_unthrottle,
-- 
2.33.0