[v3,2/2] serial: core: Add port port device to flush TX on runtime resume

diff --git a/drivers/tty/serial/Makefile b/drivers/tty/serial/Makefile
--- a/drivers/tty/serial/Makefile
+++ b/drivers/tty/serial/Makefile
@@ -3,7 +3,7 @@ 
 # Makefile for the kernel serial device drivers.
 #
 
-obj-$(CONFIG_SERIAL_CORE) += serial_core.o
+obj-$(CONFIG_SERIAL_CORE) += serial_core.o serial_port.o
 
 obj-$(CONFIG_SERIAL_EARLYCON) += earlycon.o
 obj-$(CONFIG_SERIAL_EARLYCON_ARM_SEMIHOST) += earlycon-arm-semihost.o
diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -16,6 +16,7 @@ 
 #include <linux/console.h>
 #include <linux/gpio/consumer.h>
 #include <linux/of.h>
+#include <linux/platform_device.h>
 #include <linux/pm_runtime.h>
 #include <linux/proc_fs.h>
 #include <linux/seq_file.h>
@@ -31,6 +32,8 @@ 
 #include <linux/irq.h>
 #include <linux/uaccess.h>
 
+#include "serial_port.h"
+
 /*
  * Serial port device specific data for serial core.
  *
@@ -151,9 +154,31 @@  static void __uart_start(struct tty_struct *tty)
 {
 	struct uart_state *state = tty->driver_data;
 	struct uart_port *port = state->uart_port;
+	struct device *port_dev;
+	int err;
+
+	if (!port || uart_tx_stopped(port))
+		return;
 
-	if (port && !uart_tx_stopped(port))
+	port_dev = port->state->port_dev;
+
+	err = pm_runtime_get(port_dev);
+	if (err < 0) {
+		/* Something went wrong, attempt to start TX anyways */
+		port->ops->start_tx(port);
+		pm_runtime_put_noidle(port_dev);
+		return;
+	}
+
+	/*
+	 * Start TX if enabled, and kick runtime PM. Otherwise we must
+	 * wait for a retry. See also serial_port.c for runtime PM
+	 * autosuspend timeout.
+	 */
+	if (pm_runtime_active(port_dev))
 		port->ops->start_tx(port);
+	pm_runtime_mark_last_busy(port_dev);
+	pm_runtime_put_autosuspend(port_dev);
 }
 
 static void uart_start(struct tty_struct *tty)
@@ -288,6 +313,37 @@  serial_core_find_controller(struct uart_driver *drv, struct device *dev)
 	return NULL;
 }
 
+static int serial_core_add_port_device(struct uart_port *port)
+{
+	struct serial_port_platdata pd;
+	struct platform_device *pdev;
+	int ret;
+
+	pdev = platform_device_alloc("serial-port", PLATFORM_DEVID_AUTO);
+	if (!pdev)
+		return -ENOMEM;
+
+	pdev->dev.parent = port->dev;
+	pd.state = port->state;
+
+	ret = platform_device_add_data(pdev, &pd, sizeof(pd));
+	if (ret)
+		goto err_put;
+
+	ret = platform_device_add(pdev);
+	if (ret)
+		goto err_put;
+
+	port->state->port_dev = &pdev->dev;
+
+	return 0;
+
+err_put:
+	platform_device_put(pdev);
+
+	return ret;
+}
+
 /*
  * Initialize a serial port device and serial port controller as
  * needed. Called from uart_add_one_port() with port_mutex held.
@@ -317,13 +373,21 @@  static int serial_core_register_port(struct uart_port *port,
 	port->state->controller = controller;
 	WARN_ON(port->supports_autosuspend != controller->supports_autosuspend);
 
+	ret = serial_core_add_port_device(port);
+	if (ret)
+		goto err_free;
+
 	ret = serial_core_pm_runtime_start(port);
 	if (ret < 0)
-		goto err_free;
+		goto err_del;
 
 	return 0;
 
+err_del:
+	platform_device_del(to_platform_device(port->state->port_dev));
+
 err_free:
+	port->state->port_dev = NULL;
 	port->state->controller = NULL;
 	if (allocated)
 		kfree(controller);
@@ -339,11 +403,14 @@  static int serial_core_register_port(struct uart_port *port,
 static void serial_core_unregister_port(struct uart_port *port)
 {
 	struct serial_controller *controller = to_controller(port);
+	struct device *dev = port->state->port_dev;
 
 	/* Check for a registered controller, no struct device early on */
 	if (!controller)
 		return;
 
+	platform_device_del(to_platform_device(dev));
+	port->state->port_dev = NULL;
 	port->state->controller = NULL;
 	kref_put(&controller->ref, serial_core_pm_runtime_cleanup);
 }
@@ -363,6 +430,10 @@  static int uart_port_startup(struct tty_struct *tty, struct uart_state *state,
 	if (uport->type == PORT_UNKNOWN)
 		return 1;
 
+	retval = serial_port_get(state);
+	if (retval)
+		return retval;
+
 	/*
 	 * Make sure the device is in D0 state.
 	 */
@@ -2030,6 +2101,8 @@  static void uart_port_shutdown(struct tty_port *port)
 		/* Ensure that the IRQ handler isn't running on another CPU. */
 		synchronize_irq(uport->irq);
 	}
+
+	serial_port_put(state);
 }
 
 static int uart_carrier_raised(struct tty_port *port)
diff --git a/drivers/tty/serial/serial_port.c b/drivers/tty/serial/serial_port.c
new file mode 100644
--- /dev/null
+++ b/drivers/tty/serial/serial_port.c
@@ -0,0 +1,111 @@ 
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+/*
+ * Serial port driver to provide port specific services for serial_core
+ */
+
+#include <linux/module.h>
+#include <linux/platform_device.h>
+#include <linux/pm_runtime.h>
+#include <linux/serial_core.h>
+
+#include "serial_port.h"
+
+#define SERIAL_PORT_AUTOSUSPEND_DELAY_MS	500
+
+struct serial_port_data {
+	struct uart_state *state;
+};
+
+int serial_port_get(struct uart_state *state)
+{
+	if (!state)
+		return -ENODEV;
+
+	/* Prevent uart_port from unloading */
+	if (!state->port_dev || !state->port_dev->driver ||
+	    !try_module_get(state->port_dev->driver->owner))
+		return -ENODEV;
+
+	return 0;
+}
+EXPORT_SYMBOL(serial_port_get);
+
+void serial_port_put(struct uart_state *state)
+{
+	if (!state || !state->port_dev || !state->port_dev->driver)
+		return;
+
+	module_put(state->port_dev->driver->owner);
+}
+EXPORT_SYMBOL(serial_port_put);
+
+/* Only considers TX for now. Caller must take care of locking */
+static int __serial_port_busy(struct uart_port *port)
+{
+	return (!uart_tx_stopped(port) &&
+		uart_circ_chars_pending(&port->state->xmit));
+}
+
+static int serial_port_runtime_resume(struct device *dev)
+{
+	struct serial_port_data *ddata = dev_get_drvdata(dev);
+	struct uart_port *port = ddata->state->uart_port;
+	unsigned long flags;
+
+	/* Flush any pending TX for the port */
+	spin_lock_irqsave(&port->lock, flags);
+	if (__serial_port_busy(port))
+		port->ops->start_tx(port);
+	spin_unlock_irqrestore(&port->lock, flags);
+	pm_runtime_mark_last_busy(dev);
+
+	return 0;
+}
+
+static const struct dev_pm_ops serial_port_pm = {
+	SET_RUNTIME_PM_OPS(NULL, serial_port_runtime_resume, NULL)
+};
+
+static int serial_port_probe(struct platform_device *pdev)
+{
+	struct serial_port_platdata *pd = dev_get_platdata(&pdev->dev);
+	struct serial_port_data *ddata;
+
+	ddata = devm_kzalloc(&pdev->dev, sizeof(*ddata), GFP_KERNEL);
+	if (!ddata)
+		return -ENOMEM;
+
+	ddata->state = pd->state;
+	platform_set_drvdata(pdev, ddata);
+
+	pm_runtime_enable(&pdev->dev);
+	pm_runtime_set_autosuspend_delay(&pdev->dev,
+					 SERIAL_PORT_AUTOSUSPEND_DELAY_MS);
+	pm_runtime_use_autosuspend(&pdev->dev);
+
+	return 0;
+}
+
+static int serial_port_remove(struct platform_device *pdev)
+{
+	pm_runtime_dont_use_autosuspend(&pdev->dev);
+	pm_runtime_disable(&pdev->dev);
+
+	return 0;
+}
+
+static struct platform_driver serial_port_driver = {
+	.driver = {
+		.name = "serial-port",
+		.pm = &serial_port_pm,
+	},
+	.probe = serial_port_probe,
+	.remove = serial_port_remove,
+};
+
+module_platform_driver(serial_port_driver);
+
+MODULE_AUTHOR("Tony Lindgren <tony@atomide.com>");
+MODULE_DESCRIPTION("Serial controller port driver");
+MODULE_LICENSE("GPL");
diff --git a/drivers/tty/serial/serial_port.h b/drivers/tty/serial/serial_port.h
new file mode 100644
--- /dev/null
+++ b/drivers/tty/serial/serial_port.h
@@ -0,0 +1,16 @@ 
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+
+/**
+ * struct serial_port_platdata - Serial port platform data
+ * @state: serial port state
+ *
+ * Used by serial_core and serial_port only. Allocated on uart_add_one_port(),
+ * and freed on uart_remove_one_port(). Note that the life cycle for uart_state
+ * is different from serial_port_device.
+ */
+struct serial_port_platdata {
+	struct uart_state *state;
+};
+
+extern int serial_port_get(struct uart_state *state);
+extern void serial_port_put(struct uart_state *state);
diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -614,6 +614,7 @@  struct uart_state {
 	struct tty_port		port;
 
 	struct serial_controller *controller;
+	struct device		*port_dev;
 
 	enum uart_pm_state	pm_state;
 	struct circ_buf		xmit;