[PATCH 02/24] serial: core: respect runtime PM when sending x_char

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Josef Bacik <josef@toxicpanda.com>

__uart_start() calls the start_tx operation only if the port is runtime
active.  Otherwise it asks for a resume, and
serial_port_runtime_resume() starts the transmitter.

uart_send_xchar() calls start_tx at once, under the port lock, whatever
the state of the device.  With 8250_omap, serial8250_start_tx() then
calls pm_runtime_get_sync() for a suspended device: that sleeps, and
omap8250_runtime_resume() takes the port lock again.  A tcflow(TCIOFF)
on an idle port is enough, if autosuspend has been enabled.  It was
allowed as long as that driver used pm_runtime_irq_safe().

Share the part of __uart_start() that deals with runtime PM.  A stopped
transmission still does not hold up an XON or XOFF character.  Let a
pending x_char count as work for serial_port_runtime_resume(), and keep
the port from being suspended with it.

Fixes: 8700a7ea5519 ("serial: 8250_omap: Drop pm_runtime_irq_safe()")
Cc: stable <stable@kernel.org>
Cc: Jiri Slaby <jirislaby@kernel.org>
Cc: Hugo Villeneuve <hvilleneuve@dimonoff.com>
Cc: Johan Hovold <johan@kernel.org>
Cc: Karl Mehltretter <kmehltretter@gmail.com>
Cc: John Ogness <john.ogness@linutronix.de>
Cc: "Uwe Kleine-König (The Capable Hub)" <u.kleine-koenig@baylibre.com>
Cc: Tony Lindgren <tony.lindgren@linux.intel.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/tty/serial/serial_core.c | 25 +++++++++++++++----------
 drivers/tty/serial/serial_port.c |  4 ++--
 2 files changed, 17 insertions(+), 12 deletions(-)

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index c0deec0e6af4..b06d2649be34 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -135,17 +135,11 @@ static void uart_stop(struct tty_struct *tty)
 	uart_port_unlock_deref(port, flags);
 }
 
-static void __uart_start(struct uart_state *state)
+static void uart_port_start_tx(struct uart_port *port)
 {
-	struct uart_port *port = state->uart_port;
-	struct serial_port_device *port_dev;
+	struct serial_port_device *port_dev = port->port_dev;
 	int err;
 
-	if (!port || port->flags & UPF_DEAD || uart_tx_stopped(port))
-		return;
-
-	port_dev = port->port_dev;
-
 	/* Increment the runtime PM usage count for the active check below */
 	err = pm_runtime_get(&port_dev->dev);
 	if (err < 0 && err != -EINPROGRESS) {
@@ -164,6 +158,16 @@ static void __uart_start(struct uart_state *state)
 	pm_runtime_put_autosuspend(&port_dev->dev);
 }
 
+static void __uart_start(struct uart_state *state)
+{
+	struct uart_port *port = state->uart_port;
+
+	if (!port || port->flags & UPF_DEAD || uart_tx_stopped(port))
+		return;
+
+	uart_port_start_tx(port);
+}
+
 static void uart_start(struct tty_struct *tty)
 {
 	struct uart_state *state = tty->driver_data;
@@ -718,8 +722,9 @@ static void uart_send_xchar(struct tty_struct *tty, u8 ch)
 	else {
 		guard(uart_port_lock_irqsave)(port);
 		port->x_char = ch;
-		if (ch)
-			port->ops->start_tx(port);
+		/* It goes out even if the transmission is stopped */
+		if (ch && !(port->flags & UPF_DEAD))
+			uart_port_start_tx(port);
 	}
 	uart_port_deref(port);
 }
diff --git a/drivers/tty/serial/serial_port.c b/drivers/tty/serial/serial_port.c
index 2fc48cd63f6c..f2f7cdd3a54c 100644
--- a/drivers/tty/serial/serial_port.c
+++ b/drivers/tty/serial/serial_port.c
@@ -23,8 +23,8 @@
 /* Only considers pending TX for now. Caller must take care of locking */
 static int __serial_port_busy(struct uart_port *port)
 {
-	return !uart_tx_stopped(port) &&
-		!kfifo_is_empty(&port->state->port.xmit_fifo);
+	return port->x_char || (!uart_tx_stopped(port) &&
+		!kfifo_is_empty(&port->state->port.xmit_fifo));
 }
 
 static int serial_port_runtime_resume(struct device *dev)

-- 
2.56.0