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 <[e-mail]>
Suggested-by: Greg Kroah-Hartman <[e-mail]>
Suggested-by: Jiri Slaby <[e-mail]>
Suggested-by: Johan Hovold <[e-mail]>
Signed-off-by: Tony Lindgren <[e-mail]>
Reviewed-by: Andy Shevchenko <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/serial/8250/8250_core.c

···
 		if (uart->port.dev)
 			uart_remove_one_port(&serial8250_reg, &uart->port);
 
+		uart->port.ctrl_id	= up->port.ctrl_id;
 		uart->port.iobase       = up->port.iobase;
 		uart->port.membase      = up->port.membase;
 		uart->port.irq          = up->port.irq;

drivers/tty/serial/8250/8250_port.c

···
 	struct uart_port *port = &up->port;
 
 	spin_lock_init(&port->lock);
+	port->ctrl_id = 0;
 	port->ops = &serial8250_pops;
 	port->has_sysrq = IS_ENABLED(CONFIG_SERIAL_8250_CONSOLE);
 

drivers/tty/serial/Makefile

···
 # Makefile for the kernel serial device drivers.
 #
 
-obj-$(CONFIG_SERIAL_CORE) += serial_core.o
+obj-$(CONFIG_SERIAL_CORE) += serial_base.o
+serial_base-y := serial_core.o serial_base_bus.o serial_ctrl.o serial_port.o
 
 obj-$(CONFIG_SERIAL_EARLYCON) += earlycon.o
 obj-$(CONFIG_SERIAL_EARLYCON_SEMIHOST) += earlycon-semihost.o

drivers/tty/serial/serial_base.h

···
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * Serial core related functions, serial port device drivers do not need this.
+ *
+ * Copyright (C) 2023 Texas Instruments Incorporated - https://www.ti.com/
+ * Author: Tony Lindgren <[e-mail]>
+ */
+
+#define to_serial_base_ctrl_device(d) container_of((d), struct serial_ctrl_device, dev)
+#define to_serial_base_port_device(d) container_of((d), struct serial_port_device, dev)
+
+struct uart_driver;
+struct uart_port;
+struct device_driver;
+struct device;
+
+struct serial_ctrl_device {
+	struct device dev;
+};
+
+struct serial_port_device {
+	struct device dev;
+	struct uart_port *port;
+};
+
+int serial_base_ctrl_init(void);
+void serial_base_ctrl_exit(void);
+
+int serial_base_port_init(void);
+void serial_base_port_exit(void);
+
+int serial_base_driver_register(struct device_driver *driver);
+void serial_base_driver_unregister(struct device_driver *driver);
+
+struct serial_ctrl_device *serial_base_ctrl_add(struct uart_port *port,
+						struct device *parent);
+struct serial_port_device *serial_base_port_add(struct uart_port *port,
+						struct serial_ctrl_device *parent);
+void serial_base_ctrl_device_remove(struct serial_ctrl_device *ctrl_dev);
+void serial_base_port_device_remove(struct serial_port_device *port_dev);
+
+int serial_ctrl_register_port(struct uart_driver *drv, struct uart_port *port);
+void serial_ctrl_unregister_port(struct uart_driver *drv, struct uart_port *port);
+
+int serial_core_register_port(struct uart_driver *drv, struct uart_port *port);
+void serial_core_unregister_port(struct uart_driver *drv, struct uart_port *port);

drivers/tty/serial/serial_base_bus.c

···
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Serial base bus layer for controllers
+ *
+ * Copyright (C) 2023 Texas Instruments Incorporated - https://www.ti.com/
+ * Author: Tony Lindgren <[e-mail]>
+ *
+ * The serial core bus manages the serial core controller instances.
+ */
+
+#include <linux/container_of.h>
+#include <linux/device.h>
+#include <linux/module.h>
+#include <linux/serial_core.h>
+#include <linux/slab.h>
+#include <linux/spinlock.h>
+
+#include "serial_base.h"
+
+static int serial_base_match(struct device *dev, struct device_driver *drv)
+{
+	int len = strlen(drv->name);
+
+	return !strncmp(dev_name(dev), drv->name, len);
+}
+
+static struct bus_type serial_base_bus_type = {
+	.name = "serial-base",
+	.match = serial_base_match,
+};
+
+int serial_base_driver_register(struct device_driver *driver)
+{
+	driver->bus = &serial_base_bus_type;
+
+	return driver_register(driver);
+}
+
+void serial_base_driver_unregister(struct device_driver *driver)
+{
+	driver_unregister(driver);
+}
+
+static int serial_base_device_init(struct uart_port *port,
+				   struct device *dev,
+				   struct device *parent_dev,
+				   const struct device_type *type,
+				   void (*release)(struct device *dev),
+				   int id)
+{
+	device_initialize(dev);
+	dev->type = type;
+	dev->parent = parent_dev;
+	dev->bus = &serial_base_bus_type;
+	dev->release = release;
+
+	return dev_set_name(dev, "%s.%s.%d", type->name, dev_name(port->dev), id);
+}
+
+static const struct device_type serial_ctrl_type = {
+	.name = "ctrl",
+};
+
+static void serial_base_ctrl_release(struct device *dev)
+{
+	struct serial_ctrl_device *ctrl_dev = to_serial_base_ctrl_device(dev);
+
+	kfree(ctrl_dev);
+}
+
+void serial_base_ctrl_device_remove(struct serial_ctrl_device *ctrl_dev)
+{
+	if (!ctrl_dev)
+		return;
+
+	device_del(&ctrl_dev->dev);
+}
+
+struct serial_ctrl_device *serial_base_ctrl_add(struct uart_port *port,
+						struct device *parent)
+{
+	struct serial_ctrl_device *ctrl_dev;
+	int err;
+
+	ctrl_dev = kzalloc(sizeof(*ctrl_dev), GFP_KERNEL);
+	if (!ctrl_dev)
+		return ERR_PTR(-ENOMEM);
+
+	err = serial_base_device_init(port, &ctrl_dev->dev,
+				      parent, &serial_ctrl_type,
+				      serial_base_ctrl_release,
+				      port->ctrl_id);
+	if (err)
+		goto err_free_ctrl_dev;
+
+	err = device_add(&ctrl_dev->dev);
+	if (err)
+		goto err_put_device;
+
+	return ctrl_dev;
+
+err_put_device:
+	put_device(&ctrl_dev->dev);
+err_free_ctrl_dev:
+	kfree(ctrl_dev);
+
+	return ERR_PTR(err);
+}
+
+static const struct device_type serial_port_type = {
+	.name = "port",
+};
+
+static void serial_base_port_release(struct device *dev)
+{
+	struct serial_port_device *port_dev = to_serial_base_port_device(dev);
+
+	kfree(port_dev);
+}
+
+struct serial_port_device *serial_base_port_add(struct uart_port *port,
+						struct serial_ctrl_device *ctrl_dev)
+{
+	struct serial_port_device *port_dev;
+	int err;
+
+	port_dev = kzalloc(sizeof(*port_dev), GFP_KERNEL);
+	if (!port_dev)
+		return ERR_PTR(-ENOMEM);
+
+	err = serial_base_device_init(port, &port_dev->dev,
+				      &ctrl_dev->dev, &serial_port_type,
+				      serial_base_port_release,
+				      port->line);
+	if (err)
+		goto err_free_port_dev;
+
+	port_dev->port = port;
+
+	err = device_add(&port_dev->dev);
+	if (err)
+		goto err_put_device;
+
+	return port_dev;
+
+err_put_device:
+	put_device(&port_dev->dev);
+err_free_port_dev:
+	kfree(port_dev);
+
+	return ERR_PTR(err);
+}
+
+void serial_base_port_device_remove(struct serial_port_device *port_dev)
+{
+	if (!port_dev)
+		return;
+
+	device_del(&port_dev->dev);
+}
+
+static int serial_base_init(void)
+{
+	int ret;
+
+	ret = bus_register(&serial_base_bus_type);
+	if (ret)
+		return ret;
+
+	ret = serial_base_ctrl_init();
+	if (ret)
+		goto err_bus_unregister;
+
+	ret = serial_base_port_init();
+	if (ret)
+		goto err_ctrl_exit;
+
+	return 0;
+
+err_ctrl_exit:
+	serial_base_ctrl_exit();
+
+err_bus_unregister:
+	bus_unregister(&serial_base_bus_type);
+
+	return ret;
+}
+module_init(serial_base_init);
+
+static void serial_base_exit(void)
+{
+	serial_base_port_exit();
+	serial_base_ctrl_exit();
+	bus_unregister(&serial_base_bus_type);
+}
+module_exit(serial_base_exit);
+
+MODULE_AUTHOR("Tony Lindgren <[e-mail]>");
+MODULE_DESCRIPTION("Serial core bus");
+MODULE_LICENSE("GPL");

drivers/tty/serial/serial_core.c

···
 #include <linux/gpio/consumer.h>
 #include <linux/kernel.h>
 #include <linux/of.h>
+#include <linux/pm_runtime.h>
 #include <linux/proc_fs.h>
 #include <linux/seq_file.h>
 #include <linux/device.h>
···
 
 #include <linux/irq.h>
 #include <linux/uaccess.h>
+
+#include "serial_base.h"
 
 /*
  * This is used to lock changes in serial line configuration.
···
 {
 	struct uart_state *state = tty->driver_data;
 	struct uart_port *port = state->uart_port;
+	struct serial_port_device *port_dev;
+	int err;
 
-	if (port && !(port->flags & UPF_DEAD) && !uart_tx_stopped(port))
+	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) {
+		pm_runtime_put_noidle(&port_dev->dev);
+		return;
+	}
+
+	/*
+	 * Start TX if enabled, and kick runtime PM. If the device is not
+	 * enabled, serial_port_runtime_resume() calls start_tx() again
+	 * after enabling the device.
+	 */
+	if (pm_runtime_active(&port_dev->dev))
 		port->ops->start_tx(port);
+	pm_runtime_mark_last_busy(&port_dev->dev);
+	pm_runtime_put_autosuspend(&port_dev->dev);
 }
 
 static void uart_start(struct tty_struct *tty)
···
 };
 
 /**
- * uart_add_one_port - attach a driver-defined port structure
+ * serial_core_add_one_port - attach a driver-defined port structure
  * @drv: pointer to the uart low level driver structure for this port
  * @uport: uart port structure to use for this port.
  *
···
  * This allows the driver @drv to register its own uart_port structure with the
  * core driver. The main purpose is to allow the low level uart drivers to
  * expand uart_port, rather than having yet more levels of structures.
+ * Caller must hold port_mutex.
  */
-int uart_add_one_port(struct uart_driver *drv, struct uart_port *uport)
+static int serial_core_add_one_port(struct uart_driver *drv, struct uart_port *uport)
 {
 	struct uart_state *state;
 	struct tty_port *port;
···
 	state = drv->state + uport->line;
 	port = &state->port;
 
-	mutex_lock(&port_mutex);
 	mutex_lock(&port->mutex);
 	if (state->uart_port) {
 		ret = -EINVAL;
···
 		       uport->line);
 	}
 
-	/*
-	 * Ensure UPF_DEAD is not set.
-	 */
-	uport->flags &= ~UPF_DEAD;
-
  out:
 	mutex_unlock(&port->mutex);
-	mutex_unlock(&port_mutex);
 
 	return ret;
 }
-EXPORT_SYMBOL(uart_add_one_port);
 
 /**
- * uart_remove_one_port - detach a driver defined port structure
+ * serial_core_remove_one_port - detach a driver defined port structure
  * @drv: pointer to the uart low level driver structure for this port
  * @uport: uart port structure for this port
  *
···
  *
  * This unhooks (and hangs up) the specified port structure from the core
  * driver. No further calls will be made to the low-level code for this port.
+ * Caller must hold port_mutex.
  */
-void uart_remove_one_port(struct uart_driver *drv, struct uart_port *uport)
+static void serial_core_remove_one_port(struct uart_driver *drv,
+					struct uart_port *uport)
 {
 	struct uart_state *state = drv->state + uport->line;
 	struct tty_port *port = &state->port;
 	struct uart_port *uart_port;
 	struct tty_struct *tty;
 
-	mutex_lock(&port_mutex);
-
-	/*
-	 * Mark the port "dead" - this prevents any opens from
-	 * succeeding while we shut down the port.
-	 */
 	mutex_lock(&port->mutex);
 	uart_port = uart_port_check(state);
 	if (uart_port != uport)
···
 		mutex_unlock(&port->mutex);
 		goto out;
 	}
-	uport->flags |= UPF_DEAD;
 	mutex_unlock(&port->mutex);
 
 	/*
···
 	 * Indicate that there isn't a port here anymore.
 	 */
 	uport->type = PORT_UNKNOWN;
+	uport->port_dev = NULL;
 
 	mutex_lock(&port->mutex);
 	WARN_ON(atomic_dec_return(&state->refcount) < 0);
···
 out:
 	mutex_unlock(&port_mutex);
 }
-EXPORT_SYMBOL(uart_remove_one_port);
 
 /**
  * uart_match_port - are the two ports equivalent?
···
 	return false;
 }
 EXPORT_SYMBOL(uart_match_port);
+
+static struct serial_ctrl_device *
+serial_core_get_ctrl_dev(struct serial_port_device *port_dev)
+{
+	struct device *dev = &port_dev->dev;
+
+	return to_serial_base_ctrl_device(dev->parent);
+}
+
+/*
+ * Find a registered serial core controller device if one exists. Returns
+ * the first device matching the ctrl_id. Caller must hold port_mutex.
+ */
+static struct serial_ctrl_device *serial_core_ctrl_find(struct uart_driver *drv,
+							struct device *phys_dev,
+							int ctrl_id)
+{
+	struct uart_state *state;
+	int i;
+
+	lockdep_assert_held(&port_mutex);
+
+	for (i = 0; i < drv->nr; i++) {
+		state = drv->state + i;
+		if (!state->uart_port || !state->uart_port->port_dev)
+			continue;
+
+		if (state->uart_port->dev == phys_dev &&
+		    state->uart_port->ctrl_id == ctrl_id)
+			return serial_core_get_ctrl_dev(state->uart_port->port_dev);
+	}
+
+	return NULL;
+}
+
+static struct serial_ctrl_device *serial_core_ctrl_device_add(struct uart_port *port)
+{
+	return serial_base_ctrl_add(port, port->dev);
+}
+
+static int serial_core_port_device_add(struct serial_ctrl_device *ctrl_dev,
+				       struct uart_port *port)
+{
+	struct serial_port_device *port_dev;
+
+	port_dev = serial_base_port_add(port, ctrl_dev);
+	if (IS_ERR(port_dev))
+		return PTR_ERR(port_dev);
+
+	port->port_dev = port_dev;
+
+	return 0;
+}
+
+/*
+ * Initialize a serial core port device, and a controller device if needed.
+ */
+int serial_core_register_port(struct uart_driver *drv, struct uart_port *port)
+{
+	struct serial_ctrl_device *ctrl_dev, *new_ctrl_dev = NULL;
+	int ret;
+
+	mutex_lock(&port_mutex);
+
+	/*
+	 * Prevent serial_port_runtime_resume() from trying to use the port
+	 * until serial_core_add_one_port() has completed
+	 */
+	port->flags |= UPF_DEAD;
+
+	/* Inititalize a serial core controller device if needed */
+	ctrl_dev = serial_core_ctrl_find(drv, port->dev, port->ctrl_id);
+	if (!ctrl_dev) {
+		new_ctrl_dev = serial_core_ctrl_device_add(port);
+		if (!new_ctrl_dev) {
+			ret = -ENODEV;
+			goto err_unlock;
+		}
+		ctrl_dev = new_ctrl_dev;
+	}
+
+	/*
+	 * Initialize a serial core port device. Tag the port dead to prevent
+	 * serial_port_runtime_resume() trying to do anything until port has
+	 * been registered. It gets cleared by serial_core_add_one_port().
+	 */
+	ret = serial_core_port_device_add(ctrl_dev, port);
+	if (ret)
+		goto err_unregister_ctrl_dev;
+
+	ret = serial_core_add_one_port(drv, port);
+	if (ret)
+		goto err_unregister_port_dev;
+
+	port->flags &= ~UPF_DEAD;
+
+	mutex_unlock(&port_mutex);
+
+	return 0;
+
+err_unregister_port_dev:
+	serial_base_port_device_remove(port->port_dev);
+
+err_unregister_ctrl_dev:
+	serial_base_ctrl_device_remove(new_ctrl_dev);
+
+err_unlock:
+	mutex_unlock(&port_mutex);
+
+	return ret;
+}
+
+/*
+ * Removes a serial core port device, and the related serial core controller
+ * device if the last instance.
+ */
+void serial_core_unregister_port(struct uart_driver *drv, struct uart_port *port)
+{
+	struct device *phys_dev = port->dev;
+	struct serial_port_device *port_dev = port->port_dev;
+	struct serial_ctrl_device *ctrl_dev = serial_core_get_ctrl_dev(port_dev);
+	int ctrl_id = port->ctrl_id;
+
+	mutex_lock(&port_mutex);
+
+	port->flags |= UPF_DEAD;
+
+	serial_core_remove_one_port(drv, port);
+
+	/* Note that struct uart_port *port is no longer valid at this point */
+	serial_base_port_device_remove(port_dev);
+
+	/* Drop the serial core controller device if no ports are using it */
+	if (!serial_core_ctrl_find(drv, phys_dev, ctrl_id))
+		serial_base_ctrl_device_remove(ctrl_dev);
+
+	mutex_unlock(&port_mutex);
+}
 
 /**
  * uart_handle_dcd_change - handle a change of carrier detect state

drivers/tty/serial/serial_ctrl.c

···
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Serial core controller driver
+ *
+ * Copyright (C) 2023 Texas Instruments Incorporated - https://www.ti.com/
+ * Author: Tony Lindgren <[e-mail]>
+ *
+ * This driver manages the serial core controller struct device instances.
+ * The serial core controller devices are children of the physical serial
+ * port device.
+ */
+
+#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"
+
+static int serial_ctrl_probe(struct device *dev)
+{
+	pm_runtime_enable(dev);
+
+	return 0;
+}
+
+static int serial_ctrl_remove(struct device *dev)
+{
+	pm_runtime_disable(dev);
+
+	return 0;
+}
+
+/*
+ * Serial core controller device init functions. Note that the physical
+ * serial port device driver may not have completed probe at this point.
+ */
+int serial_ctrl_register_port(struct uart_driver *drv, struct uart_port *port)
+{
+	return serial_core_register_port(drv, port);
+}
+
+void serial_ctrl_unregister_port(struct uart_driver *drv, struct uart_port *port)
+{
+	serial_core_unregister_port(drv, port);
+}
+
+static struct device_driver serial_ctrl_driver = {
+	.name = "ctrl",
+	.suppress_bind_attrs = true,
+	.probe = serial_ctrl_probe,
+	.remove = serial_ctrl_remove,
+};
+
+int serial_base_ctrl_init(void)
+{
+	return serial_base_driver_register(&serial_ctrl_driver);
+}
+
+void serial_base_ctrl_exit(void)
+{
+	serial_base_driver_unregister(&serial_ctrl_driver);
+}
+
+MODULE_AUTHOR("Tony Lindgren <[e-mail]>");
+MODULE_DESCRIPTION("Serial core controller driver");
+MODULE_LICENSE("GPL");

drivers/tty/serial/serial_port.c

···
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Serial core port device driver
+ *
+ * Copyright (C) 2023 Texas Instruments Incorporated - https://www.ti.com/
+ * Author: Tony Lindgren <[e-mail]>
+ */
+
+#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 <[e-mail]>");
+MODULE_DESCRIPTION("Serial controller port driver");
+MODULE_LICENSE("GPL");

include/linux/serial_core.h

···
 
 struct uart_port;
 struct serial_struct;
+struct serial_port_device;
 struct device;
 struct gpio_desc;
 
···
 						struct serial_rs485 *rs485);
 	int			(*iso7816_config)(struct uart_port *,
 						  struct serial_iso7816 *iso7816);
+	int			ctrl_id;		/* optional serial core controller id */
 	unsigned int		irq;			/* irq number */
 	unsigned long		irqflags;		/* irq flags  */
 	unsigned int		uartclk;		/* base uart clock */
···
 	unsigned int		minor;
 	resource_size_t		mapbase;		/* for ioremap */
 	resource_size_t		mapsize;
-	struct device		*dev;			/* parent device */
+	struct device		*dev;			/* serial port physical parent device */
+	struct serial_port_device *port_dev;		/* serial core port device */
 
 	unsigned long		sysrq;			/* sysrq timeout */
 	unsigned int		sysrq_ch;		/* char for sysrq */