[PATCH 4/7] serial: 8250: Implement wakeup for TX and use it for 8250_omap

[Tony Lindgren <tony@atomide.com>]

We can use the wakeup() and uart_start_pending_tx() calls to wake up an
idle serial port and send out the pending TX buffer on runtime PM resume.
This allows us to remove the dependency to pm_runtime_irq_safe() for
8250_omap driver in the following patches.

We manage the port runtime_suspended flag in the serial port driver as
only the driver knows when the hardware is runtime PM suspended. Note that
The current flag for rpm_tx_active cannot be used as it is TX specific
for 8250_port.

We already have serial8250_start_tx() call serial8250_rpm_get_tx(), and
serial8250_stop_tx() call serial8250_rpm_put_tx() to take care of the
runtime PM usage count for TX. To have the serial port driver call
uart_start_pending_tx() on runtime resume, we must now use just
pm_runtime_get() for serial8250_start_tx() instead of the sync version.

With these changes we must now also flip the 8250_omap driver over to
call uart_start_pending_tx(). That's currently the only user of
UART_CAP_RPM.

Signed-off-by: Tony Lindgren <tony@atomide.com>
---
 drivers/tty/serial/8250/8250_omap.c | 19 ++++++++++++++
 drivers/tty/serial/8250/8250_port.c | 39 ++++++++++++++++++++++++++++-
 2 files changed, 57 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/serial/8250/8250_omap.c b/drivers/tty/serial/8250/8250_omap.c
--- a/drivers/tty/serial/8250/8250_omap.c
+++ b/drivers/tty/serial/8250/8250_omap.c
@@ -1593,12 +1593,16 @@ static int omap8250_runtime_suspend(struct device *dev)
 {
 	struct omap8250_priv *priv = dev_get_drvdata(dev);
 	struct uart_8250_port *up;
+	struct uart_port *port;
+	unsigned long flags;
 
 	/* In case runtime-pm tries this before we are setup */
 	if (!priv)
 		return 0;
 
 	up = serial8250_get_port(priv->line);
+	port = &up->port;
+
 	/*
 	 * When using 'no_console_suspend', the console UART must not be
 	 * suspended. Since driver suspend is managed by runtime suspend,
@@ -1610,6 +1614,10 @@ static int omap8250_runtime_suspend(struct device *dev)
 			return -EBUSY;
 	}
 
+	spin_lock_irqsave(&port->lock, flags);
+	port->runtime_suspended = 1;
+	spin_unlock_irqrestore(&port->lock, flags);
+
 	if (priv->habit & UART_ERRATA_CLOCK_DISABLE) {
 		int ret;
 
@@ -1636,13 +1644,18 @@ static int omap8250_runtime_resume(struct device *dev)
 {
 	struct omap8250_priv *priv = dev_get_drvdata(dev);
 	struct uart_8250_port *up;
+	struct uart_port *port;
+	unsigned long flags;
 
 	/* In case runtime-pm tries this before we are setup */
 	if (!priv)
 		return 0;
 
 	up = serial8250_get_port(priv->line);
+	port = &up->port;
 
+	/* Restore state with interrupts disabled */
+	spin_lock_irqsave(&port->lock, flags);
 	if (omap8250_lost_context(up))
 		omap8250_restore_regs(up);
 
@@ -1651,6 +1664,12 @@ static int omap8250_runtime_resume(struct device *dev)
 
 	priv->latency = priv->calc_latency;
 	schedule_work(&priv->qos_work);
+
+	port->runtime_suspended = 0;
+	spin_unlock_irqrestore(&port->lock, flags);
+
+	uart_start_pending_tx(port);
+
 	return 0;
 }
 #endif
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
@@ -724,7 +724,7 @@ void serial8250_rpm_get_tx(struct uart_8250_port *p)
 	rpm_active = xchg(&p->rpm_tx_active, 1);
 	if (rpm_active)
 		return;
-	pm_runtime_get_sync(p->port.dev);
+	pm_runtime_get(p->port.dev);
 }
 EXPORT_SYMBOL_GPL(serial8250_rpm_get_tx);
 
@@ -2509,6 +2509,42 @@ static void serial8250_shutdown(struct uart_port *port)
 		serial8250_do_shutdown(port);
 }
 
+/*
+ * Wakes up the serial port if it has been runtime PM suspended.
+ *
+ * Note that we rely on the serial8250_rpm functions to manage the
+ * runtime PM usage count. We also currently depend on the runtime
+ * PM autosuspend timeout to keep the port awake until start_tx().
+ * Eventually we should just use runtime PM functions and not rely
+ * on the autosuspend timeout.
+ *
+ * Caller must hold port->lock for port->runtime_suspended status.
+ * Also the port drivers must hold port->lock when changing the
+ * state for port->runtime_suspended in runtime PM functions.
+ */
+static int serial8250_wakeup(struct uart_port *port)
+{
+	struct uart_8250_port *up = up_to_u8250p(port);
+	struct device *dev = up->port.dev;
+	int err;
+
+	if (!pm_runtime_enabled(dev))
+		return 0;
+
+	if (!port->runtime_suspended) {
+		pm_runtime_mark_last_busy(dev);
+		return 0;
+	}
+
+	err = pm_request_resume(dev);
+	if (err < 0) {
+		dev_warn(dev, "wakeup failed: %d\n", err);
+		return err;
+	}
+
+	return -EINPROGRESS;
+}
+
 /* Nuvoton NPCM UARTs have a custom divisor calculation */
 static unsigned int npcm_get_divisor(struct uart_8250_port *up,
 		unsigned int baud)
@@ -3237,6 +3273,7 @@ static const struct uart_ops serial8250_pops = {
 	.break_ctl	= serial8250_break_ctl,
 	.startup	= serial8250_startup,
 	.shutdown	= serial8250_shutdown,
+	.wakeup		= serial8250_wakeup,
 	.set_termios	= serial8250_set_termios,
 	.set_ldisc	= serial8250_set_ldisc,
 	.pm		= serial8250_pm,
-- 
2.33.1