serial: core: Start managing serial controllers to enable runtime PM

We want to enable runtime PM for serial port device drivers in a generic
way. To do this, we want to have the serial core layer manage the
registered physical serial controller devices.

To manage serial controllers, let's set up a struct bus and struct device
for the serial core controller as suggested by Greg and Jiri. The serial
core controller devices are children of the physical serial port device.
The serial core controller device is needed to support multiple different
kind of ports connected to single physical serial port device.

Let's also set up a struct device for the serial core port. The serial
core port instances are children of the serial core controller device.

With the serial core port device we can now flush pending TX on the
runtime PM resume as suggested by Johan.

Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Suggested-by: Jiri Slaby <jirislaby@kernel.org>
Suggested-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20230525113034.46880-1-tony@atomide.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 105 insertions, 0 deletions

diff --git a/drivers/tty/serial/serial_port.c b/drivers/tty/serial/serial_port.c
new file mode 100644
index 000000000000..862423237007
--- /dev/null
+++ b/drivers/tty/serial/serial_port.c
@@ -0,0 +1,105 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Serial core port device driver
+ *
+ * Copyright (C) 2023 Texas Instruments Incorporated - https://www.ti.com/
+ * Author: Tony Lindgren <tony@atomide.com>
+ */
+
+#include <linux/device.h>
+#include <linux/module.h>
+#include <linux/pm_runtime.h>
+#include <linux/serial_core.h>
+#include <linux/spinlock.h>
+
+#include "serial_base.h"
+
+#define SERIAL_PORT_AUTOSUSPEND_DELAY_MS	500
+
+/* 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) &&
+		uart_circ_chars_pending(&port->state->xmit);
+}
+
+static int serial_port_runtime_resume(struct device *dev)
+{
+	struct serial_port_device *port_dev = to_serial_base_port_device(dev);
+	struct uart_port *port;
+	unsigned long flags;
+
+	port = port_dev->port;
+
+	if (port->flags & UPF_DEAD)
+		goto out;
+
+	/* 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);
+
+out:
+	pm_runtime_mark_last_busy(dev);
+
+	return 0;
+}
+
+static DEFINE_RUNTIME_DEV_PM_OPS(serial_port_pm,
+				 NULL, serial_port_runtime_resume, NULL);
+
+static int serial_port_probe(struct device *dev)
+{
+	pm_runtime_enable(dev);
+	pm_runtime_set_autosuspend_delay(dev, SERIAL_PORT_AUTOSUSPEND_DELAY_MS);
+	pm_runtime_use_autosuspend(dev);
+
+	return 0;
+}
+
+static int serial_port_remove(struct device *dev)
+{
+	pm_runtime_dont_use_autosuspend(dev);
+	pm_runtime_disable(dev);
+
+	return 0;
+}
+
+/*
+ * Serial core port device init functions. Note that the physical serial
+ * port device driver may not have completed probe at this point.
+ */
+int uart_add_one_port(struct uart_driver *drv, struct uart_port *port)
+{
+	return serial_ctrl_register_port(drv, port);
+}
+EXPORT_SYMBOL(uart_add_one_port);
+
+void uart_remove_one_port(struct uart_driver *drv, struct uart_port *port)
+{
+	serial_ctrl_unregister_port(drv, port);
+}
+EXPORT_SYMBOL(uart_remove_one_port);
+
+static struct device_driver serial_port_driver = {
+	.name = "port",
+	.suppress_bind_attrs = true,
+	.probe = serial_port_probe,
+	.remove = serial_port_remove,
+	.pm = pm_ptr(&serial_port_pm),
+};
+
+int serial_base_port_init(void)
+{
+	return serial_base_driver_register(&serial_port_driver);
+}
+
+void serial_base_port_exit(void)
+{
+	serial_base_driver_unregister(&serial_port_driver);
+}
+
+MODULE_AUTHOR("Tony Lindgren <tony@atomide.com>");
+MODULE_DESCRIPTION("Serial controller port driver");
+MODULE_LICENSE("GPL");