usb: gadget: allocate & giveback serial ports instead hard code them

This patch removes gserial_setup() and gserial_cleanup() and adds
gserial_alloc_line() and gserial_free_line() to replace them.

The initial setup of u_serial happens now on module load time. A
maximum of four TTY ports can be requested which is the current limit.
In theory we could extend this limit, the hard limit is the number of
available endpoints.
alloc_tty_driver() is now called at module init time with the max
available ports. The per-line footprint here is on 32bit is 3 * size of
pointer + 60 bytes (for cdevs).
The remaining memory (struct gs_port) is allocated once a port is
requested.

With this change it is possible to load g_multi and g_serial at the same
time. GS0 receives the module that is loaded first, GS1 is received by
the next module and so on. With the configfs interface the port number
can be exported and the device node is more predictable. Nothing changes
for g_serial and friends as long as one module is used.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>

11 files changed, 229 insertions, 209 deletions

diff --git a/drivers/usb/gadget/acm_ms.c b/drivers/usb/gadget/acm_ms.c
index 35cbe7283514..eb0fbdae5ccb 100644
--- a/drivers/usb/gadget/acm_ms.c
+++ b/drivers/usb/gadget/acm_ms.c
@@ -111,6 +111,7 @@ FSG_MODULE_PARAMETERS(/* no prefix */, fsg_mod_data);
 static struct fsg_common fsg_common;
 
 /*-------------------------------------------------------------------------*/
+static unsigned char tty_line;
 
 /*
  * We _always_ have both ACM and mass storage functions.
@@ -125,7 +126,7 @@ static int __init acm_ms_do_config(struct usb_configuration *c)
 	}
 
 
-	status = acm_bind_config(c, 0);
+	status = acm_bind_config(c, tty_line);
 	if (status < 0)
 		return status;
 
@@ -152,7 +153,7 @@ static int __init acm_ms_bind(struct usb_composite_dev *cdev)
 	void			*retp;
 
 	/* set up serial link layer */
-	status = gserial_setup(cdev->gadget, 1);
+	status = gserial_alloc_line(&tty_line);
 	if (status < 0)
 		return status;
 
@@ -188,14 +189,13 @@ static int __init acm_ms_bind(struct usb_composite_dev *cdev)
 fail1:
 	fsg_common_put(&fsg_common);
 fail0:
-	gserial_cleanup();
+	gserial_free_line(tty_line);
 	return status;
 }
 
 static int __exit acm_ms_unbind(struct usb_composite_dev *cdev)
 {
-	gserial_cleanup();
-
+	gserial_free_line(tty_line);
 	return 0;
 }
 
diff --git a/drivers/usb/gadget/cdc2.c b/drivers/usb/gadget/cdc2.c
index 379df679ef30..16911a5aeb03 100644
--- a/drivers/usb/gadget/cdc2.c
+++ b/drivers/usb/gadget/cdc2.c
@@ -108,6 +108,7 @@ static u8 hostaddr[ETH_ALEN];
 
 /*-------------------------------------------------------------------------*/
 
+static unsigned char tty_line;
 /*
  * We _always_ have both CDC ECM and CDC ACM functions.
  */
@@ -124,7 +125,7 @@ static int __init cdc_do_config(struct usb_configuration *c)
 	if (status < 0)
 		return status;
 
-	status = acm_bind_config(c, 0);
+	status = acm_bind_config(c, tty_line);
 	if (status < 0)
 		return status;
 
@@ -157,7 +158,7 @@ static int __init cdc_bind(struct usb_composite_dev *cdev)
 		return status;
 
 	/* set up serial link layer */
-	status = gserial_setup(cdev->gadget, 1);
+	status = gserial_alloc_line(&tty_line);
 	if (status < 0)
 		goto fail0;
 
@@ -183,7 +184,7 @@ static int __init cdc_bind(struct usb_composite_dev *cdev)
 	return 0;
 
 fail1:
-	gserial_cleanup();
+	gserial_free_line(tty_line);
 fail0:
 	gether_cleanup();
 	return status;
@@ -191,7 +192,7 @@ fail0:
 
 static int __exit cdc_unbind(struct usb_composite_dev *cdev)
 {
-	gserial_cleanup();
+	gserial_free_line(tty_line);
 	gether_cleanup();
 	return 0;
 }
diff --git a/drivers/usb/gadget/dbgp.c b/drivers/usb/gadget/dbgp.c
index 41eb98df5644..986fc511a2ed 100644
--- a/drivers/usb/gadget/dbgp.c
+++ b/drivers/usb/gadget/dbgp.c
@@ -231,6 +231,10 @@ static void dbgp_unbind(struct usb_gadget *gadget)
 	gadget->ep0->driver_data = NULL;
 }
 
+#ifdef CONFIG_USB_G_DBGP_SERIAL
+static unsigned char tty_line;
+#endif
+
 static int __init dbgp_configure_endpoints(struct usb_gadget *gadget)
 {
 	int stp;
@@ -268,7 +272,7 @@ static int __init dbgp_configure_endpoints(struct usb_gadget *gadget)
 	dbgp.serial->in->desc = &i_desc;
 	dbgp.serial->out->desc = &o_desc;
 
-	if (gserial_setup(gadget, 1) < 0) {
+	if (gserial_alloc_line(&tty_line)) {
 		stp = 3;
 		goto fail_3;
 	}
@@ -377,7 +381,7 @@ static int dbgp_setup(struct usb_gadget *gadget,
 #ifdef CONFIG_USB_G_DBGP_PRINTK
 		err = dbgp_enable_ep();
 #else
-		err = gserial_connect(dbgp.serial, 0);
+		err = gserial_connect(dbgp.serial, tty_line);
 #endif
 		if (err < 0)
 			goto fail;
@@ -420,7 +424,7 @@ static void __exit dbgp_exit(void)
 {
 	usb_gadget_unregister_driver(&dbgp_driver);
 #ifdef CONFIG_USB_G_DBGP_SERIAL
-	gserial_cleanup();
+	gserial_free_line(tty_line);
 #endif
 }
 
diff --git a/drivers/usb/gadget/f_acm.c b/drivers/usb/gadget/f_acm.c
index d4a7c1912105..3178fa70916e 100644
--- a/drivers/usb/gadget/f_acm.c
+++ b/drivers/usb/gadget/f_acm.c
@@ -719,9 +719,6 @@ acm_unbind(struct usb_configuration *c, struct usb_function *f)
  *
  * Returns zero on success, else negative errno.
  *
- * Caller must have called @gserial_setup() with enough ports to
- * handle all the ones it binds.  Caller is also responsible
- * for calling @gserial_cleanup() before module unload.
  */
 int acm_bind_config(struct usb_configuration *c, u8 port_num)
 {
diff --git a/drivers/usb/gadget/f_obex.c b/drivers/usb/gadget/f_obex.c
index d8dd8782768c..36a004563b82 100644
--- a/drivers/usb/gadget/f_obex.c
+++ b/drivers/usb/gadget/f_obex.c
@@ -406,10 +406,6 @@ static inline bool can_support_obex(struct usb_configuration *c)
  * Context: single threaded during gadget setup
  *
  * Returns zero on success, else negative errno.
- *
- * Caller must have called @gserial_setup() with enough ports to
- * handle all the ones it binds.  Caller is also responsible
- * for calling @gserial_cleanup() before module unload.
  */
 int __init obex_bind_config(struct usb_configuration *c, u8 port_num)
 {
diff --git a/drivers/usb/gadget/f_serial.c b/drivers/usb/gadget/f_serial.c
index 98fa7795df5f..da33cfb3031d 100644
--- a/drivers/usb/gadget/f_serial.c
+++ b/drivers/usb/gadget/f_serial.c
@@ -260,10 +260,6 @@ gser_unbind(struct usb_configuration *c, struct usb_function *f)
  * Context: single threaded during gadget setup
  *
  * Returns zero on success, else negative errno.
- *
- * Caller must have called @gserial_setup() with enough ports to
- * handle all the ones it binds.  Caller is also responsible
- * for calling @gserial_cleanup() before module unload.
  */
 int __init gser_bind_config(struct usb_configuration *c, u8 port_num)
 {
diff --git a/drivers/usb/gadget/multi.c b/drivers/usb/gadget/multi.c
index 00667911100d..9ca0ac0334e7 100644
--- a/drivers/usb/gadget/multi.c
+++ b/drivers/usb/gadget/multi.c
@@ -136,6 +136,7 @@ static struct fsg_common fsg_common;
 
 static u8 hostaddr[ETH_ALEN];
 
+static unsigned char tty_line;
 
 /********** RNDIS **********/
 
@@ -154,7 +155,7 @@ static __init int rndis_do_config(struct usb_configuration *c)
 	if (ret < 0)
 		return ret;
 
-	ret = acm_bind_config(c, 0);
+	ret = acm_bind_config(c, tty_line);
 	if (ret < 0)
 		return ret;
 
@@ -205,7 +206,7 @@ static __init int cdc_do_config(struct usb_configuration *c)
 	if (ret < 0)
 		return ret;
 
-	ret = acm_bind_config(c, 0);
+	ret = acm_bind_config(c, tty_line);
 	if (ret < 0)
 		return ret;
 
@@ -242,7 +243,6 @@ static int cdc_config_register(struct usb_composite_dev *cdev)
 
 /****************************** Gadget Bind ******************************/
 
-
 static int __ref multi_bind(struct usb_composite_dev *cdev)
 {
 	struct usb_gadget *gadget = cdev->gadget;
@@ -260,7 +260,7 @@ static int __ref multi_bind(struct usb_composite_dev *cdev)
 		return status;
 
 	/* set up serial link layer */
-	status = gserial_setup(cdev->gadget, 1);
+	status = gserial_alloc_line(&tty_line);
 	if (status < 0)
 		goto fail0;
 
@@ -300,7 +300,7 @@ static int __ref multi_bind(struct usb_composite_dev *cdev)
 fail2:
 	fsg_common_put(&fsg_common);
 fail1:
-	gserial_cleanup();
+	gserial_free_line(tty_line);
 fail0:
 	gether_cleanup();
 	return status;
@@ -308,7 +308,7 @@ fail0:
 
 static int __exit multi_unbind(struct usb_composite_dev *cdev)
 {
-	gserial_cleanup();
+	gserial_free_line(tty_line);
 	gether_cleanup();
 	return 0;
 }
diff --git a/drivers/usb/gadget/nokia.c b/drivers/usb/gadget/nokia.c
index 48f0d5c372a8..084d0d7cb8fa 100644
--- a/drivers/usb/gadget/nokia.c
+++ b/drivers/usb/gadget/nokia.c
@@ -100,6 +100,15 @@ MODULE_LICENSE("GPL");
 
 static u8 hostaddr[ETH_ALEN];
 
+enum {
+	TTY_PORT_OBEX0,
+	TTY_PORT_OBEX1,
+	TTY_PORT_ACM,
+	TTY_PORTS_MAX,
+};
+
+static unsigned char tty_lines[TTY_PORTS_MAX];
+
 static int __init nokia_bind_config(struct usb_configuration *c)
 {
 	int status = 0;
@@ -108,15 +117,15 @@ static int __init nokia_bind_config(struct usb_configuration *c)
 	if (status)
 		printk(KERN_DEBUG "could not bind phonet config\n");
 
-	status = obex_bind_config(c, 0);
+	status = obex_bind_config(c, tty_lines[TTY_PORT_OBEX0]);
 	if (status)
 		printk(KERN_DEBUG "could not bind obex config %d\n", 0);
 
-	status = obex_bind_config(c, 1);
+	status = obex_bind_config(c, tty_lines[TTY_PORT_OBEX1]);
 	if (status)
 		printk(KERN_DEBUG "could not bind obex config %d\n", 0);
 
-	status = acm_bind_config(c, 2);
+	status = acm_bind_config(c, tty_lines[TTY_PORT_ACM]);
 	if (status)
 		printk(KERN_DEBUG "could not bind acm config\n");
 
@@ -147,14 +156,17 @@ static int __init nokia_bind(struct usb_composite_dev *cdev)
 {
 	struct usb_gadget	*gadget = cdev->gadget;
 	int			status;
+	int			cur_line;
 
 	status = gphonet_setup(cdev->gadget);
 	if (status < 0)
 		goto err_phonet;
 
-	status = gserial_setup(cdev->gadget, 3);
-	if (status < 0)
-		goto err_serial;
+	for (cur_line = 0; cur_line < TTY_PORTS_MAX; cur_line++) {
+		status = gserial_alloc_line(&tty_lines[cur_line]);
+		if (status)
+			goto err_ether;
+	}
 
 	status = gether_setup(cdev->gadget, hostaddr);
 	if (status < 0)
@@ -191,8 +203,10 @@ static int __init nokia_bind(struct usb_composite_dev *cdev)
 err_usb:
 	gether_cleanup();
 err_ether:
-	gserial_cleanup();
-err_serial:
+	cur_line--;
+	while (cur_line >= 0)
+		gserial_free_line(tty_lines[cur_line--]);
+
 	gphonet_cleanup();
 err_phonet:
 	return status;
@@ -200,8 +214,13 @@ err_phonet:
 
 static int __exit nokia_unbind(struct usb_composite_dev *cdev)
 {
+	int i;
+
 	gphonet_cleanup();
-	gserial_cleanup();
+
+	for (i = 0; i < TTY_PORTS_MAX; i++)
+		gserial_free_line(tty_lines[i]);
+
 	gether_cleanup();
 
 	return 0;
diff --git a/drivers/usb/gadget/serial.c b/drivers/usb/gadget/serial.c
index 4816f494fc4d..a883562f6a89 100644
--- a/drivers/usb/gadget/serial.c
+++ b/drivers/usb/gadget/serial.c
@@ -127,6 +127,7 @@ module_param(n_ports, uint, 0);
 MODULE_PARM_DESC(n_ports, "number of ports to create, default=1");
 
 /*-------------------------------------------------------------------------*/
+static unsigned char tty_lines[MAX_U_SERIAL_PORTS];
 
 static int __init serial_bind_acm_config(struct usb_configuration *c)
 {
@@ -134,7 +135,7 @@ static int __init serial_bind_acm_config(struct usb_configuration *c)
 	int status = 0;
 
 	for (i = 0; i < n_ports && status == 0; i++)
-		status = acm_bind_config(c, i);
+		status = acm_bind_config(c, tty_lines[i]);
 	return status;
 }
 
@@ -144,7 +145,7 @@ static int __init serial_bind_obex_config(struct usb_configuration *c)
 	int status = 0;
 
 	for (i = 0; i < n_ports && status == 0; i++)
-		status = obex_bind_config(c, i);
+		status = obex_bind_config(c, tty_lines[i]);
 	return status;
 }
 
@@ -154,7 +155,7 @@ static int __init serial_bind_gser_config(struct usb_configuration *c)
 	int status = 0;
 
 	for (i = 0; i < n_ports && status == 0; i++)
-		status = gser_bind_config(c, i);
+		status = gser_bind_config(c, tty_lines[i]);
 	return status;
 }
 
@@ -165,13 +166,25 @@ static struct usb_configuration serial_config_driver = {
 	.bmAttributes	= USB_CONFIG_ATT_SELFPOWER,
 };
 
+static int gs_unbind(struct usb_composite_dev *cdev)
+{
+	int i;
+
+	for (i = 0; i < n_ports; i++)
+		gserial_free_line(tty_lines[i]);
+	return 0;
+}
+
 static int __init gs_bind(struct usb_composite_dev *cdev)
 {
 	int			status;
+	int			cur_line;
 
-	status = gserial_setup(cdev->gadget, n_ports);
-	if (status < 0)
-		return status;
+	for (cur_line = 0; cur_line < n_ports; cur_line++) {
+		status = gserial_alloc_line(&tty_lines[cur_line]);
+		if (status)
+			goto fail;
+	}
 
 	/* Allocate string descriptor numbers ... note that string
 	 * contents can be overridden by the composite_dev glue.
@@ -209,7 +222,9 @@ static int __init gs_bind(struct usb_composite_dev *cdev)
 	return 0;
 
 fail:
-	gserial_cleanup();
+	cur_line--;
+	while (cur_line >= 0)
+		gserial_free_line(tty_lines[cur_line--]);
 	return status;
 }
 
@@ -219,6 +234,7 @@ static __refdata struct usb_composite_driver gserial_driver = {
 	.strings	= dev_strings,
 	.max_speed	= USB_SPEED_SUPER,
 	.bind		= gs_bind,
+	.unbind		= gs_unbind,
 };
 
 static int __init init(void)
@@ -254,6 +270,5 @@ module_init(init);
 static void __exit cleanup(void)
 {
 	usb_composite_unregister(&gserial_driver);
-	gserial_cleanup();
 }
 module_exit(cleanup);
diff --git a/drivers/usb/gadget/u_serial.c b/drivers/usb/gadget/u_serial.c
index 1662d839a1d6..ea360fda2e14 100644
--- a/drivers/usb/gadget/u_serial.c
+++ b/drivers/usb/gadget/u_serial.c
@@ -36,11 +36,12 @@
  * "serial port" functionality through the USB gadget stack.  Each such
  * port is exposed through a /dev/ttyGS* node.
  *
- * After initialization (gserial_setup), these TTY port devices stay
- * available until they are removed (gserial_cleanup).  Each one may be
- * connected to a USB function (gserial_connect), or disconnected (with
- * gserial_disconnect) when the USB host issues a config change event.
- * Data can only flow when the port is connected to the host.
+ * After this module has been loaded, the individual TTY port can be requested
+ * (gserial_alloc_line()) and it will stay available until they are removed
+ * (gserial_free_line()). Each one may be connected to a USB function
+ * (gserial_connect), or disconnected (with gserial_disconnect) when the USB
+ * host issues a config change event. Data can only flow when the port is
+ * connected to the host.
  *
  * A given TTY port can be made available in multiple configurations.
  * For example, each one might expose a ttyGS0 node which provides a
@@ -120,13 +121,10 @@ struct gs_port {
 	struct usb_cdc_line_coding port_line_coding;	/* 8-N-1 etc */
 };
 
-/* increase N_PORTS if you need more */
-#define N_PORTS		4
 static struct portmaster {
 	struct mutex	lock;			/* protect open/close */
 	struct gs_port	*port;
-} ports[N_PORTS];
-static unsigned	n_ports;
+} ports[MAX_U_SERIAL_PORTS];
 
 #define GS_CLOSE_TIMEOUT		15		/* seconds */
 
@@ -1033,10 +1031,19 @@ static int
 gs_port_alloc(unsigned port_num, struct usb_cdc_line_coding *coding)
 {
 	struct gs_port	*port;
+	int		ret = 0;
+
+	mutex_lock(&ports[port_num].lock);
+	if (ports[port_num].port) {
+		ret = -EBUSY;
+		goto out;
+	}
 
 	port = kzalloc(sizeof(struct gs_port), GFP_KERNEL);
-	if (port == NULL)
-		return -ENOMEM;
+	if (port == NULL) {
+		ret = -ENOMEM;
+		goto out;
+	}
 
 	tty_port_init(&port->port);
 	spin_lock_init(&port->port_lock);
@@ -1052,114 +1059,10 @@ gs_port_alloc(unsigned port_num, struct usb_cdc_line_coding *coding)
 	port->port_line_coding = *coding;
 
 	ports[port_num].port = port;
-
-	return 0;
-}
-
-/**
- * gserial_setup - initialize TTY driver for one or more ports
- * @g: gadget to associate with these ports
- * @count: how many ports to support
- * Context: may sleep
- *
- * The TTY stack needs to know in advance how many devices it should
- * plan to manage.  Use this call to set up the ports you will be
- * exporting through USB.  Later, connect them to functions based
- * on what configuration is activated by the USB host; and disconnect
- * them as appropriate.
- *
- * An example would be a two-configuration device in which both
- * configurations expose port 0, but through different functions.
- * One configuration could even expose port 1 while the other
- * one doesn't.
- *
- * Returns negative errno or zero.
- */
-int gserial_setup(struct usb_gadget *g, unsigned count)
-{
-	unsigned			i;
-	struct usb_cdc_line_coding	coding;
-	int				status;
-
-	if (count == 0 || count > N_PORTS)
-		return -EINVAL;
-
-	if (gs_tty_driver)
-		return -EBUSY;
-
-	gs_tty_driver = alloc_tty_driver(count);
-	if (!gs_tty_driver)
-		return -ENOMEM;
-
-	gs_tty_driver->driver_name = "g_serial";
-	gs_tty_driver->name = PREFIX;
-	/* uses dynamically assigned dev_t values */
-
-	gs_tty_driver->type = TTY_DRIVER_TYPE_SERIAL;
-	gs_tty_driver->subtype = SERIAL_TYPE_NORMAL;
-	gs_tty_driver->flags = TTY_DRIVER_REAL_RAW | TTY_DRIVER_DYNAMIC_DEV;
-	gs_tty_driver->init_termios = tty_std_termios;
-
-	/* 9600-8-N-1 ... matches defaults expected by "usbser.sys" on
-	 * MS-Windows.  Otherwise, most of these flags shouldn't affect
-	 * anything unless we were to actually hook up to a serial line.
-	 */
-	gs_tty_driver->init_termios.c_cflag =
-			B9600 | CS8 | CREAD | HUPCL | CLOCAL;
-	gs_tty_driver->init_termios.c_ispeed = 9600;
-	gs_tty_driver->init_termios.c_ospeed = 9600;
-
-	coding.dwDTERate = cpu_to_le32(9600);
-	coding.bCharFormat = 8;
-	coding.bParityType = USB_CDC_NO_PARITY;
-	coding.bDataBits = USB_CDC_1_STOP_BITS;
-
-	tty_set_operations(gs_tty_driver, &gs_tty_ops);
-
-	/* make devices be openable */
-	for (i = 0; i < count; i++) {
-		mutex_init(&ports[i].lock);
-		status = gs_port_alloc(i, &coding);
-		if (status) {
-			count = i;
-			goto fail;
-		}
-	}
-	n_ports = count;
-
-	/* export the driver ... */
-	status = tty_register_driver(gs_tty_driver);
-	if (status) {
-		pr_err("%s: cannot register, err %d\n",
-				__func__, status);
-		goto fail;
-	}
-
-	/* ... and sysfs class devices, so mdev/udev make /dev/ttyGS* */
-	for (i = 0; i < count; i++) {
-		struct device	*tty_dev;
-
-		tty_dev = tty_port_register_device(&ports[i].port->port,
-				gs_tty_driver, i, &g->dev);
-		if (IS_ERR(tty_dev))
-			pr_warning("%s: no classdev for port %d, err %ld\n",
-				__func__, i, PTR_ERR(tty_dev));
-	}
-
-	pr_debug("%s: registered %d ttyGS* device%s\n", __func__,
-			count, (count == 1) ? "" : "s");
-
-	return status;
-fail:
-	while (count--) {
-		tty_port_destroy(&ports[count].port->port);
-		kfree(ports[count].port);
-	}
-	put_tty_driver(gs_tty_driver);
-	gs_tty_driver = NULL;
-	return status;
+out:
+	mutex_unlock(&ports[port_num].lock);
+	return ret;
 }
-EXPORT_SYMBOL_GPL(gserial_setup);
 
 static int gs_closed(struct gs_port *port)
 {
@@ -1171,56 +1074,77 @@ static int gs_closed(struct gs_port *port)
 	return cond;
 }
 
-/**
- * gserial_cleanup - remove TTY-over-USB driver and devices
- * Context: may sleep
- *
- * This is called to free all resources allocated by @gserial_setup().
- * Accordingly, it may need to wait until some open /dev/ files have
- * closed.
- *
- * The caller must have issued @gserial_disconnect() for any ports
- * that had previously been connected, so that there is never any
- * I/O pending when it's called.
- */
-void gserial_cleanup(void)
+static void gserial_free_port(struct gs_port *port)
+{
+	tasklet_kill(&port->push);
+	/* wait for old opens to finish */
+	wait_event(port->port.close_wait, gs_closed(port));
+	WARN_ON(port->port_usb != NULL);
+	tty_port_destroy(&port->port);
+	kfree(port);
+}
+
+void gserial_free_line(unsigned char port_num)
 {
-	unsigned	i;
 	struct gs_port	*port;
 
-	if (!gs_tty_driver)
+	mutex_lock(&ports[port_num].lock);
+	if (WARN_ON(!ports[port_num].port)) {
+		mutex_unlock(&ports[port_num].lock);
 		return;
+	}
+	port = ports[port_num].port;
+	ports[port_num].port = NULL;
+	mutex_unlock(&ports[port_num].lock);
 
-	/* start sysfs and /dev/ttyGS* node removal */
-	for (i = 0; i < n_ports; i++)
-		tty_unregister_device(gs_tty_driver, i);
-
-	for (i = 0; i < n_ports; i++) {
-		/* prevent new opens */
-		mutex_lock(&ports[i].lock);
-		port = ports[i].port;
-		ports[i].port = NULL;
-		mutex_unlock(&ports[i].lock);
-
-		tasklet_kill(&port->push);
+	gserial_free_port(port);
+	tty_unregister_device(gs_tty_driver, port_num);
+}
+EXPORT_SYMBOL_GPL(gserial_free_line);
 
-		/* wait for old opens to finish */
-		wait_event(port->port.close_wait, gs_closed(port));
+int gserial_alloc_line(unsigned char *line_num)
+{
+	struct usb_cdc_line_coding	coding;
+	struct device			*tty_dev;
+	int				ret;
+	int				port_num;
 
-		WARN_ON(port->port_usb != NULL);
+	coding.dwDTERate = cpu_to_le32(9600);
+	coding.bCharFormat = 8;
+	coding.bParityType = USB_CDC_NO_PARITY;
+	coding.bDataBits = USB_CDC_1_STOP_BITS;
 
-		tty_port_destroy(&port->port);
-		kfree(port);
+	for (port_num = 0; port_num < MAX_U_SERIAL_PORTS; port_num++) {
+		ret = gs_port_alloc(port_num, &coding);
+		if (ret == -EBUSY)
+			continue;
+		if (ret)
+			return ret;
+		break;
 	}
-	n_ports = 0;
+	if (ret)
+		return ret;
 
-	tty_unregister_driver(gs_tty_driver);
-	put_tty_driver(gs_tty_driver);
-	gs_tty_driver = NULL;
+	/* ... and sysfs class devices, so mdev/udev make /dev/ttyGS* */
 
-	pr_debug("%s: cleaned up ttyGS* support\n", __func__);
+	tty_dev = tty_port_register_device(&ports[port_num].port->port,
+			gs_tty_driver, port_num, NULL);
+	if (IS_ERR(tty_dev)) {
+		struct gs_port	*port;
+		pr_err("%s: failed to register tty for port %d, err %ld\n",
+				__func__, port_num, PTR_ERR(tty_dev));
+
+		ret = PTR_ERR(tty_dev);
+		port = ports[port_num].port;
+		ports[port_num].port = NULL;
+		gserial_free_port(port);
+		goto err;
+	}
+	*line_num = port_num;
+err:
+	return ret;
 }
-EXPORT_SYMBOL_GPL(gserial_cleanup);
+EXPORT_SYMBOL_GPL(gserial_alloc_line);
 
 /**
  * gserial_connect - notify TTY I/O glue that USB link is active
@@ -1237,8 +1161,8 @@ EXPORT_SYMBOL_GPL(gserial_cleanup);
  *
  * Caller needs to have set up the endpoints and USB function in @dev
  * before calling this, as well as the appropriate (speed-specific)
- * endpoint descriptors, and also have set up the TTY driver by calling
- * @gserial_setup().
+ * endpoint descriptors, and also have allocate @port_num by calling
+ * @gserial_alloc_line().
  *
  * Returns negative errno or zero.
  * On success, ep->driver_data will be overwritten.
@@ -1249,11 +1173,18 @@ int gserial_connect(struct gserial *gser, u8 port_num)
 	unsigned long	flags;
 	int		status;
 
-	if (!gs_tty_driver || port_num >= n_ports)
+	if (port_num >= MAX_U_SERIAL_PORTS)
 		return -ENXIO;
 
-	/* we "know" gserial_cleanup() hasn't been called */
 	port = ports[port_num].port;
+	if (!port) {
+		pr_err("serial line %d not allocated.\n", port_num);
+		return -EINVAL;
+	}
+	if (port->port_usb) {
+		pr_err("serial line %d is in use.\n", port_num);
+		return -EBUSY;
+	}
 
 	/* activate the endpoints */
 	status = usb_ep_enable(gser->in);
@@ -1357,4 +1288,63 @@ void gserial_disconnect(struct gserial *gser)
 }
 EXPORT_SYMBOL_GPL(gserial_disconnect);
 
+int userial_init(void)
+{
+	unsigned			i;
+	int				status;
+
+	gs_tty_driver = alloc_tty_driver(MAX_U_SERIAL_PORTS);
+	if (!gs_tty_driver)
+		return -ENOMEM;
+
+	gs_tty_driver->driver_name = "g_serial";
+	gs_tty_driver->name = PREFIX;
+	/* uses dynamically assigned dev_t values */
+
+	gs_tty_driver->type = TTY_DRIVER_TYPE_SERIAL;
+	gs_tty_driver->subtype = SERIAL_TYPE_NORMAL;
+	gs_tty_driver->flags = TTY_DRIVER_REAL_RAW | TTY_DRIVER_DYNAMIC_DEV;
+	gs_tty_driver->init_termios = tty_std_termios;
+
+	/* 9600-8-N-1 ... matches defaults expected by "usbser.sys" on
+	 * MS-Windows.  Otherwise, most of these flags shouldn't affect
+	 * anything unless we were to actually hook up to a serial line.
+	 */
+	gs_tty_driver->init_termios.c_cflag =
+			B9600 | CS8 | CREAD | HUPCL | CLOCAL;
+	gs_tty_driver->init_termios.c_ispeed = 9600;
+	gs_tty_driver->init_termios.c_ospeed = 9600;
+
+	tty_set_operations(gs_tty_driver, &gs_tty_ops);
+	for (i = 0; i < MAX_U_SERIAL_PORTS; i++)
+		mutex_init(&ports[i].lock);
+
+	/* export the driver ... */
+	status = tty_register_driver(gs_tty_driver);
+	if (status) {
+		pr_err("%s: cannot register, err %d\n",
+				__func__, status);
+		goto fail;
+	}
+
+	pr_debug("%s: registered %d ttyGS* device%s\n", __func__,
+			MAX_U_SERIAL_PORTS,
+			(MAX_U_SERIAL_PORTS == 1) ? "" : "s");
+
+	return status;
+fail:
+	put_tty_driver(gs_tty_driver);
+	gs_tty_driver = NULL;
+	return status;
+}
+module_init(userial_init);
+
+static void userial_cleanup(void)
+{
+	tty_unregister_driver(gs_tty_driver);
+	put_tty_driver(gs_tty_driver);
+	gs_tty_driver = NULL;
+}
+module_exit(userial_cleanup);
+
 MODULE_LICENSE("GPL");
diff --git a/drivers/usb/gadget/u_serial.h b/drivers/usb/gadget/u_serial.h
index 9b0fe6450fbf..f06ee9a5db51 100644
--- a/drivers/usb/gadget/u_serial.h
+++ b/drivers/usb/gadget/u_serial.h
@@ -15,6 +15,8 @@
 #include <linux/usb/composite.h>
 #include <linux/usb/cdc.h>
 
+#define MAX_U_SERIAL_PORTS	4
+
 /*
  * One non-multiplexed "serial" I/O port ... there can be several of these
  * on any given USB peripheral device, if it provides enough endpoints.
@@ -49,9 +51,9 @@ struct gserial {
 struct usb_request *gs_alloc_req(struct usb_ep *ep, unsigned len, gfp_t flags);
 void gs_free_req(struct usb_ep *, struct usb_request *req);
 
-/* port setup/teardown is handled by gadget driver */
-int gserial_setup(struct usb_gadget *g, unsigned n_ports);
-void gserial_cleanup(void);
+/* management of individual TTY ports */
+int gserial_alloc_line(unsigned char *port_line);
+void gserial_free_line(unsigned char port_line);
 
 /* connect/disconnect is handled by individual functions */
 int gserial_connect(struct gserial *, u8 port_num);