usb gadget: split out serial core

This abstracts the "gadget serial" driver TTY glue into a separate
component, cleaning it up and disentangling it from connection state.

It also changed some behaviors for the better:

  - Stops using "experimental" major #127, and switches over to
    having the TTY layer allocate the dev_t numbers.
    
  - Provides /sys/class/tty/ttyGS* nodes, thus mdev/udev support.
    (Note "mdev" hotplug bug in Busybox v1.7.2: /dev/ttyGS0 will
    be a *block* device without CONFIG_SYSFS_DEPRECATED_V2.)

  - The tty nodes no longer reject opens when there's no host.
    Now they can support normal getty configs in /etc/inttab...

  - Now implements RX throttling.  When the line discipline says
    it doesn't want any more data, only packets in flight will be
    delivered (currently, max 1K/8K at full/high speeds) until it
    unthrottles the data.

  - Supports low_latency.  This is a good policy for all USB serial
    adapters, since it eliminates scheduler overhead on RX paths.

This also includes much cleanup including better comments, fixing
memory leaks and other bugs (including some locking fixes), messaging
cleanup, and an interface audit and tightening.  This added up to a
significant object code shrinkage, on the order of 20% (!) depending
on CPU and compiler.

A separate patch actually kicks in this new code, using the functions
declared in this new header, and removes the previous glue.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

1 files changed, 1243 insertions, 0 deletions

diff --git a/drivers/usb/gadget/u_serial.c b/drivers/usb/gadget/u_serial.c
new file mode 100644
index 000000000000..88d4f5452c49
--- /dev/null
+++ b/drivers/usb/gadget/u_serial.c
@@ -0,0 +1,1243 @@
+/*
+ * u_serial.c - utilities for USB gadget "serial port"/TTY support
+ *
+ * Copyright (C) 2003 Al Borchers (alborchers@steinerpoint.com)
+ * Copyright (C) 2008 David Brownell
+ * Copyright (C) 2008 by Nokia Corporation
+ *
+ * This code also borrows from usbserial.c, which is
+ * Copyright (C) 1999 - 2002 Greg Kroah-Hartman (greg@kroah.com)
+ * Copyright (C) 2000 Peter Berger (pberger@brimson.com)
+ * Copyright (C) 2000 Al Borchers (alborchers@steinerpoint.com)
+ *
+ * This software is distributed under the terms of the GNU General
+ * Public License ("GPL") as published by the Free Software Foundation,
+ * either version 2 of that License or (at your option) any later version.
+ */
+
+/* #define VERBOSE_DEBUG */
+
+#include <linux/kernel.h>
+#include <linux/interrupt.h>
+#include <linux/device.h>
+#include <linux/delay.h>
+#include <linux/tty.h>
+#include <linux/tty_flip.h>
+
+#include "u_serial.h"
+
+
+/*
+ * This component encapsulates the TTY layer glue needed to provide basic
+ * "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.
+ *
+ * A given TTY port can be made available in multiple configurations.
+ * For example, each one might expose a ttyGS0 node which provides a
+ * login application.  In one case that might use CDC ACM interface 0,
+ * while another configuration might use interface 3 for that.  The
+ * work to handle that (including descriptor management) is not part
+ * of this component.
+ *
+ * Configurations may expose more than one TTY port.  For example, if
+ * ttyGS0 provides login service, then ttyGS1 might provide dialer access
+ * for a telephone or fax link.  And ttyGS2 might be something that just
+ * needs a simple byte stream interface for some messaging protocol that
+ * is managed in userspace ... OBEX, PTP, and MTP have been mentioned.
+ */
+
+/*
+ * gserial is the lifecycle interface, used by USB functions
+ * gs_port is the I/O nexus, used by the tty driver
+ * tty_struct links to the tty/filesystem framework
+ *
+ * gserial <---> gs_port ... links will be null when the USB link is
+ * inactive; managed by gserial_{connect,disconnect}().
+ *      gserial->ioport == usb_ep->driver_data ... gs_port
+ *      gs_port->port_usb ... gserial
+ *
+ * gs_port <---> tty_struct ... links will be null when the TTY file
+ * isn't opened; managed by gs_open()/gs_close()
+ *      gserial->port_tty ... tty_struct
+ *      tty_struct->driver_data ... gserial
+ */
+
+/* RX and TX queues can buffer QUEUE_SIZE packets before they hit the
+ * next layer of buffering.  For TX that's a circular buffer; for RX
+ * consider it a NOP.  A third layer is provided by the TTY code.
+ */
+#define QUEUE_SIZE              16
+#define WRITE_BUF_SIZE          8192            /* TX only */
+
+/* circular buffer */
+struct gs_buf {
+        unsigned                buf_size;
+        char                    *buf_buf;
+        char                    *buf_get;
+        char                    *buf_put;
+};
+
+/*
+ * The port structure holds info for each port, one for each minor number
+ * (and thus for each /dev/ node).
+ */
+struct gs_port {
+        spinlock_t              port_lock;      /* guard port_* access */
+
+        struct gserial          *port_usb;
+        struct tty_struct       *port_tty;
+
+        unsigned                open_count;
+        bool                    openclose;      /* open/close in progress */
+        u8                      port_num;
+
+        wait_queue_head_t       close_wait;     /* wait for last close */
+
+        struct list_head        read_pool;
+        struct tasklet_struct   push;
+
+        struct list_head        write_pool;
+        struct gs_buf           port_write_buf;
+        wait_queue_head_t       drain_wait;     /* wait while writes drain */
+
+        /* REVISIT this state ... */
+        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;
+
+#define GS_CLOSE_TIMEOUT                15              /* seconds */
+
+
+
+#ifdef VERBOSE_DEBUG
+#define pr_vdebug(fmt, arg...) \
+        pr_debug(fmt, ##arg)
+#else
+#define pr_vdebug(fmt, arg...) \
+        ({ if (0) pr_debug(fmt, ##arg); })
+#endif
+
+/*-------------------------------------------------------------------------*/
+
+/* Circular Buffer */
+
+/*
+ * gs_buf_alloc
+ *
+ * Allocate a circular buffer and all associated memory.
+ */
+static int gs_buf_alloc(struct gs_buf *gb, unsigned size)
+{
+        gb->buf_buf = kmalloc(size, GFP_KERNEL);
+        if (gb->buf_buf == NULL)
+                return -ENOMEM;
+
+        gb->buf_size = size;
+        gb->buf_put = gb->buf_buf;
+        gb->buf_get = gb->buf_buf;
+
+        return 0;
+}
+
+/*
+ * gs_buf_free
+ *
+ * Free the buffer and all associated memory.
+ */
+static void gs_buf_free(struct gs_buf *gb)
+{
+        kfree(gb->buf_buf);
+        gb->buf_buf = NULL;
+}
+
+/*
+ * gs_buf_clear
+ *
+ * Clear out all data in the circular buffer.
+ */
+static void gs_buf_clear(struct gs_buf *gb)
+{
+        gb->buf_get = gb->buf_put;
+        /* equivalent to a get of all data available */
+}
+
+/*
+ * gs_buf_data_avail
+ *
+ * Return the number of bytes of data available in the circular
+ * buffer.
+ */
+static unsigned gs_buf_data_avail(struct gs_buf *gb)
+{
+        return (gb->buf_size + gb->buf_put - gb->buf_get) % gb->buf_size;
+}
+
+/*
+ * gs_buf_space_avail
+ *
+ * Return the number of bytes of space available in the circular
+ * buffer.
+ */
+static unsigned gs_buf_space_avail(struct gs_buf *gb)
+{
+        return (gb->buf_size + gb->buf_get - gb->buf_put - 1) % gb->buf_size;
+}
+
+/*
+ * gs_buf_put
+ *
+ * Copy data data from a user buffer and put it into the circular buffer.
+ * Restrict to the amount of space available.
+ *
+ * Return the number of bytes copied.
+ */
+static unsigned
+gs_buf_put(struct gs_buf *gb, const char *buf, unsigned count)
+{
+        unsigned len;
+
+        len  = gs_buf_space_avail(gb);
+        if (count > len)
+                count = len;
+
+        if (count == 0)
+                return 0;
+
+        len = gb->buf_buf + gb->buf_size - gb->buf_put;
+        if (count > len) {
+                memcpy(gb->buf_put, buf, len);
+                memcpy(gb->buf_buf, buf+len, count - len);
+                gb->buf_put = gb->buf_buf + count - len;
+        } else {
+                memcpy(gb->buf_put, buf, count);
+                if (count < len)
+                        gb->buf_put += count;
+                else /* count == len */
+                        gb->buf_put = gb->buf_buf;
+        }
+
+        return count;
+}
+
+/*
+ * gs_buf_get
+ *
+ * Get data from the circular buffer and copy to the given buffer.
+ * Restrict to the amount of data available.
+ *
+ * Return the number of bytes copied.
+ */
+static unsigned
+gs_buf_get(struct gs_buf *gb, char *buf, unsigned count)
+{
+        unsigned len;
+
+        len = gs_buf_data_avail(gb);
+        if (count > len)
+                count = len;
+
+        if (count == 0)
+                return 0;
+
+        len = gb->buf_buf + gb->buf_size - gb->buf_get;
+        if (count > len) {
+                memcpy(buf, gb->buf_get, len);
+                memcpy(buf+len, gb->buf_buf, count - len);
+                gb->buf_get = gb->buf_buf + count - len;
+        } else {
+                memcpy(buf, gb->buf_get, count);
+                if (count < len)
+                        gb->buf_get += count;
+                else /* count == len */
+                        gb->buf_get = gb->buf_buf;
+        }
+
+        return count;
+}
+
+/*-------------------------------------------------------------------------*/
+
+/* I/O glue between TTY (upper) and USB function (lower) driver layers */
+
+/*
+ * gs_alloc_req
+ *
+ * Allocate a usb_request and its buffer.  Returns a pointer to the
+ * usb_request or NULL if there is an error.
+ */
+static struct usb_request *
+gs_alloc_req(struct usb_ep *ep, unsigned len, gfp_t kmalloc_flags)
+{
+        struct usb_request *req;
+
+        req = usb_ep_alloc_request(ep, kmalloc_flags);
+
+        if (req != NULL) {
+                req->length = len;
+                req->buf = kmalloc(len, kmalloc_flags);
+                if (req->buf == NULL) {
+                        usb_ep_free_request(ep, req);
+                        return NULL;
+                }
+        }
+
+        return req;
+}
+
+/*
+ * gs_free_req
+ *
+ * Free a usb_request and its buffer.
+ */
+static void gs_free_req(struct usb_ep *ep, struct usb_request *req)
+{
+        kfree(req->buf);
+        usb_ep_free_request(ep, req);
+}
+
+/*
+ * gs_send_packet
+ *
+ * If there is data to send, a packet is built in the given
+ * buffer and the size is returned.  If there is no data to
+ * send, 0 is returned.
+ *
+ * Called with port_lock held.
+ */
+static unsigned
+gs_send_packet(struct gs_port *port, char *packet, unsigned size)
+{
+        unsigned len;
+
+        len = gs_buf_data_avail(&port->port_write_buf);
+        if (len < size)
+                size = len;
+        if (size != 0)
+                size = gs_buf_get(&port->port_write_buf, packet, size);
+        return size;
+}
+
+/*
+ * gs_start_tx
+ *
+ * This function finds available write requests, calls
+ * gs_send_packet to fill these packets with data, and
+ * continues until either there are no more write requests
+ * available or no more data to send.  This function is
+ * run whenever data arrives or write requests are available.
+ *
+ * Context: caller owns port_lock; port_usb is non-null.
+ */
+static int gs_start_tx(struct gs_port *port)
+/*
+__releases(&port->port_lock)
+__acquires(&port->port_lock)
+*/
+{
+        struct list_head        *pool = &port->write_pool;
+        struct usb_ep           *in = port->port_usb->in;
+        int                     status = 0;
+        bool                    do_tty_wake = false;
+
+        while (!list_empty(pool)) {
+                struct usb_request      *req;
+                int                     len;
+
+                req = list_entry(pool->next, struct usb_request, list);
+                len = gs_send_packet(port, req->buf, in->maxpacket);
+                if (len == 0) {
+                        wake_up_interruptible(&port->drain_wait);
+                        break;
+                }
+                do_tty_wake = true;
+
+                req->length = len;
+                list_del(&req->list);
+
+#ifdef VERBOSE_DEBUG
+                pr_debug("%s: %s, len=%d, 0x%02x 0x%02x 0x%02x ...\n",
+                                __func__, in->name, len, *((u8 *)req->buf),
+                                *((u8 *)req->buf+1), *((u8 *)req->buf+2));
+#endif
+
+                /* Drop lock while we call out of driver; completions
+                 * could be issued while we do so.  Disconnection may
+                 * happen too; maybe immediately before we queue this!
+                 *
+                 * NOTE that we may keep sending data for a while after
+                 * the TTY closed (dev->ioport->port_tty is NULL).
+                 */
+                spin_unlock(&port->port_lock);
+                status = usb_ep_queue(in, req, GFP_ATOMIC);
+                spin_lock(&port->port_lock);
+
+                if (status) {
+                        pr_debug("%s: %s %s err %d\n",
+                                        __func__, "queue", in->name, status);
+                        list_add(&req->list, pool);
+                        break;
+                }
+
+                /* abort immediately after disconnect */
+                if (!port->port_usb)
+                        break;
+        }
+
+        if (do_tty_wake && port->port_tty)
+                tty_wakeup(port->port_tty);
+        return status;
+}
+
+static void gs_rx_push(unsigned long _port)
+{
+        struct gs_port          *port = (void *)_port;
+        struct tty_struct       *tty = port->port_tty;
+
+        /* With low_latency, tty_flip_buffer_push() doesn't put its
+         * real work through a workqueue, so the ldisc has a better
+         * chance to keep up with peak USB data rates.
+         */
+        if (tty) {
+                tty_flip_buffer_push(tty);
+                wake_up_interruptible(&tty->read_wait);
+        }
+}
+
+/*
+ * gs_recv_packet
+ *
+ * Called for each USB packet received.  Reads the packet
+ * header and stuffs the data in the appropriate tty buffer.
+ * Returns 0 if successful, or a negative error number.
+ *
+ * Called during USB completion routine, on interrupt time.
+ * With port_lock.
+ */
+static int gs_recv_packet(struct gs_port *port, char *packet, unsigned size)
+{
+        unsigned                len;
+        struct tty_struct       *tty;
+
+        /* I/O completions can continue for a while after close(), until the
+         * request queue empties.  Just discard any data we receive, until
+         * something reopens this TTY ... as if there were no HW flow control.
+         */
+        tty = port->port_tty;
+        if (tty == NULL) {
+                pr_vdebug("%s: ttyGS%d, after close\n",
+                                __func__, port->port_num);
+                return -EIO;
+        }
+
+        len = tty_insert_flip_string(tty, packet, size);
+        if (len > 0)
+                tasklet_schedule(&port->push);
+        if (len < size)
+                pr_debug("%s: ttyGS%d, drop %d bytes\n",
+                                __func__, port->port_num, size - len);
+        return 0;
+}
+
+/*
+ * Context: caller owns port_lock, and port_usb is set
+ */
+static unsigned gs_start_rx(struct gs_port *port)
+/*
+__releases(&port->port_lock)
+__acquires(&port->port_lock)
+*/
+{
+        struct list_head        *pool = &port->read_pool;
+        struct usb_ep           *out = port->port_usb->out;
+        unsigned                started = 0;
+
+        while (!list_empty(pool)) {
+                struct usb_request      *req;
+                int                     status;
+                struct tty_struct       *tty;
+
+                /* no more rx if closed or throttled */
+                tty = port->port_tty;
+                if (!tty || test_bit(TTY_THROTTLED, &tty->flags))
+                        break;
+
+                req = list_entry(pool->next, struct usb_request, list);
+                list_del(&req->list);
+                req->length = out->maxpacket;
+
+                /* drop lock while we call out; the controller driver
+                 * may need to call us back (e.g. for disconnect)
+                 */
+                spin_unlock(&port->port_lock);
+                status = usb_ep_queue(out, req, GFP_ATOMIC);
+                spin_lock(&port->port_lock);
+
+                if (status) {
+                        pr_debug("%s: %s %s err %d\n",
+                                        __func__, "queue", out->name, status);
+                        list_add(&req->list, pool);
+                        break;
+                }
+                started++;
+
+                /* abort immediately after disconnect */
+                if (!port->port_usb)
+                        break;
+        }
+        return started;
+}
+
+static void gs_read_complete(struct usb_ep *ep, struct usb_request *req)
+{
+        int             status;
+        struct gs_port  *port = ep->driver_data;
+
+        spin_lock(&port->port_lock);
+        list_add(&req->list, &port->read_pool);
+
+        switch (req->status) {
+        case 0:
+                /* normal completion */
+                status = gs_recv_packet(port, req->buf, req->actual);
+                if (status && status != -EIO)
+                        pr_debug("%s: %s %s err %d\n",
+                                __func__, "recv", ep->name, status);
+                gs_start_rx(port);
+                break;
+
+        case -ESHUTDOWN:
+                /* disconnect */
+                pr_vdebug("%s: %s shutdown\n", __func__, ep->name);
+                break;
+
+        default:
+                /* presumably a transient fault */
+                pr_warning("%s: unexpected %s status %d\n",
+                                __func__, ep->name, req->status);
+                gs_start_rx(port);
+                break;
+        }
+        spin_unlock(&port->port_lock);
+}
+
+static void gs_write_complete(struct usb_ep *ep, struct usb_request *req)
+{
+        struct gs_port  *port = ep->driver_data;
+
+        spin_lock(&port->port_lock);
+        list_add(&req->list, &port->write_pool);
+
+        switch (req->status) {
+        default:
+                /* presumably a transient fault */
+                pr_warning("%s: unexpected %s status %d\n",
+                                __func__, ep->name, req->status);
+                /* FALL THROUGH */
+        case 0:
+                /* normal completion */
+                gs_start_tx(port);
+                break;
+
+        case -ESHUTDOWN:
+                /* disconnect */
+                pr_vdebug("%s: %s shutdown\n", __func__, ep->name);
+                break;
+        }
+
+        spin_unlock(&port->port_lock);
+}
+
+static void gs_free_requests(struct usb_ep *ep, struct list_head *head)
+{
+        struct usb_request      *req;
+
+        while (!list_empty(head)) {
+                req = list_entry(head->next, struct usb_request, list);
+                list_del(&req->list);
+                gs_free_req(ep, req);
+        }
+}
+
+static int gs_alloc_requests(struct usb_ep *ep, struct list_head *head,
+                void (*fn)(struct usb_ep *, struct usb_request *))
+{
+        int                     i;
+        struct usb_request      *req;
+
+        /* Pre-allocate up to QUEUE_SIZE transfers, but if we can't
+         * do quite that many this time, don't fail ... we just won't
+         * be as speedy as we might otherwise be.
+         */
+        for (i = 0; i < QUEUE_SIZE; i++) {
+                req = gs_alloc_req(ep, ep->maxpacket, GFP_ATOMIC);
+                if (!req)
+                        return list_empty(head) ? -ENOMEM : 0;
+                req->complete = fn;
+                list_add_tail(&req->list, head);
+        }
+        return 0;
+}
+
+/**
+ * gs_start_io - start USB I/O streams
+ * @dev: encapsulates endpoints to use
+ * Context: holding port_lock; port_tty and port_usb are non-null
+ *
+ * We only start I/O when something is connected to both sides of
+ * this port.  If nothing is listening on the host side, we may
+ * be pointlessly filling up our TX buffers and FIFO.
+ */
+static int gs_start_io(struct gs_port *port)
+{
+        struct list_head        *head = &port->read_pool;
+        struct usb_ep           *ep = port->port_usb->out;
+        int                     status;
+        unsigned                started;
+
+        /* Allocate RX and TX I/O buffers.  We can't easily do this much
+         * earlier (with GFP_KERNEL) because the requests are coupled to
+         * endpoints, as are the packet sizes we'll be using.  Different
+         * configurations may use different endpoints with a given port;
+         * and high speed vs full speed changes packet sizes too.
+         */
+        status = gs_alloc_requests(ep, head, gs_read_complete);
+        if (status)
+                return status;
+
+        status = gs_alloc_requests(port->port_usb->in, &port->write_pool,
+                        gs_write_complete);
+        if (status) {
+                gs_free_requests(ep, head);
+                return status;
+        }
+
+        /* queue read requests */
+        started = gs_start_rx(port);
+
+        /* unblock any pending writes into our circular buffer */
+        if (started) {
+                tty_wakeup(port->port_tty);
+        } else {
+                gs_free_requests(ep, head);
+                gs_free_requests(port->port_usb->in, &port->write_pool);
+        }
+
+        return started ? 0 : status;
+}
+
+/*-------------------------------------------------------------------------*/
+
+/* TTY Driver */
+
+/*
+ * gs_open sets up the link between a gs_port and its associated TTY.
+ * That link is broken *only* by TTY close(), and all driver methods
+ * know that.
+ */
+static int gs_open(struct tty_struct *tty, struct file *file)
+{
+        int             port_num = tty->index;
+        struct gs_port  *port;
+        int             status;
+
+        if (port_num < 0 || port_num >= n_ports)
+                return -ENXIO;
+
+        do {
+                mutex_lock(&ports[port_num].lock);
+                port = ports[port_num].port;
+                if (!port)
+                        status = -ENODEV;
+                else {
+                        spin_lock_irq(&port->port_lock);
+
+                        /* already open?  Great. */
+                        if (port->open_count) {
+                                status = 0;
+                                port->open_count++;
+
+                        /* currently opening/closing? wait ... */
+                        } else if (port->openclose) {
+                                status = -EBUSY;
+
+                        /* ... else we do the work */
+                        } else {
+                                status = -EAGAIN;
+                                port->openclose = true;
+                        }
+                        spin_unlock_irq(&port->port_lock);
+                }
+                mutex_unlock(&ports[port_num].lock);
+
+                switch (status) {
+                default:
+                        /* fully handled */
+                        return status;
+                case -EAGAIN:
+                        /* must do the work */
+                        break;
+                case -EBUSY:
+                        /* wait for EAGAIN task to finish */
+                        msleep(1);
+                        /* REVISIT could have a waitchannel here, if
+                         * concurrent open performance is important
+                         */
+                        break;
+                }
+        } while (status != -EAGAIN);
+
+        /* Do the "real open" */
+        spin_lock_irq(&port->port_lock);
+
+        /* allocate circular buffer on first open */
+        if (port->port_write_buf.buf_buf == NULL) {
+
+                spin_unlock_irq(&port->port_lock);
+                status = gs_buf_alloc(&port->port_write_buf, WRITE_BUF_SIZE);
+                spin_lock_irq(&port->port_lock);
+
+                if (status) {
+                        pr_debug("gs_open: ttyGS%d (%p,%p) no buffer\n",
+                                port->port_num, tty, file);
+                        port->openclose = false;
+                        goto exit_unlock_port;
+                }
+        }
+
+        /* REVISIT if REMOVED (ports[].port NULL), abort the open
+         * to let rmmod work faster (but this way isn't wrong).
+         */
+
+        /* REVISIT maybe wait for "carrier detect" */
+
+        tty->driver_data = port;
+        port->port_tty = tty;
+
+        port->open_count = 1;
+        port->openclose = false;
+
+        /* low_latency means ldiscs work in tasklet context, without
+         * needing a workqueue schedule ... easier to keep up.
+         */
+        tty->low_latency = 1;
+
+        /* if connected, start the I/O stream */
+        if (port->port_usb) {
+                pr_debug("gs_open: start ttyGS%d\n", port->port_num);
+                gs_start_io(port);
+
+                /* REVISIT for ACM, issue "network connected" event */
+        }
+
+        pr_debug("gs_open: ttyGS%d (%p,%p)\n", port->port_num, tty, file);
+
+        status = 0;
+
+exit_unlock_port:
+        spin_unlock_irq(&port->port_lock);
+        return status;
+}
+
+static int gs_writes_finished(struct gs_port *p)
+{
+        int cond;
+
+        /* return true on disconnect or empty buffer */
+        spin_lock_irq(&p->port_lock);
+        cond = (p->port_usb == NULL) || !gs_buf_data_avail(&p->port_write_buf);
+        spin_unlock_irq(&p->port_lock);
+
+        return cond;
+}
+
+static void gs_close(struct tty_struct *tty, struct file *file)
+{
+        struct gs_port *port = tty->driver_data;
+
+        spin_lock_irq(&port->port_lock);
+
+        if (port->open_count != 1) {
+                if (port->open_count == 0)
+                        WARN_ON(1);
+                else
+                        --port->open_count;
+                goto exit;
+        }
+
+        pr_debug("gs_close: ttyGS%d (%p,%p) ...\n", port->port_num, tty, file);
+
+        /* mark port as closing but in use; we can drop port lock
+         * and sleep if necessary
+         */
+        port->openclose = true;
+        port->open_count = 0;
+
+        if (port->port_usb)
+                /* REVISIT for ACM, issue "network disconnected" event */;
+
+        /* wait for circular write buffer to drain, disconnect, or at
+         * most GS_CLOSE_TIMEOUT seconds; then discard the rest
+         */
+        if (gs_buf_data_avail(&port->port_write_buf) > 0
+                        && port->port_usb) {
+                spin_unlock_irq(&port->port_lock);
+                wait_event_interruptible_timeout(port->drain_wait,
+                                        gs_writes_finished(port),
+                                        GS_CLOSE_TIMEOUT * HZ);
+                spin_lock_irq(&port->port_lock);
+        }
+
+        /* Iff we're disconnected, there can be no I/O in flight so it's
+         * ok to free the circular buffer; else just scrub it.  And don't
+         * let the push tasklet fire again until we're re-opened.
+         */
+        if (port->port_usb == NULL)
+                gs_buf_free(&port->port_write_buf);
+        else
+                gs_buf_clear(&port->port_write_buf);
+
+        tasklet_kill(&port->push);
+
+        tty->driver_data = NULL;
+        port->port_tty = NULL;
+
+        port->openclose = false;
+
+        pr_debug("gs_close: ttyGS%d (%p,%p) done!\n",
+                        port->port_num, tty, file);
+
+        wake_up_interruptible(&port->close_wait);
+exit:
+        spin_unlock_irq(&port->port_lock);
+}
+
+static int gs_write(struct tty_struct *tty, const unsigned char *buf, int count)
+{
+        struct gs_port  *port = tty->driver_data;
+        unsigned long   flags;
+        int             status;
+
+        pr_vdebug("gs_write: ttyGS%d (%p) writing %d bytes\n",
+                        port->port_num, tty, count);
+
+        spin_lock_irqsave(&port->port_lock, flags);
+        if (count)
+                count = gs_buf_put(&port->port_write_buf, buf, count);
+        /* treat count == 0 as flush_chars() */
+        if (port->port_usb)
+                status = gs_start_tx(port);
+        spin_unlock_irqrestore(&port->port_lock, flags);
+
+        return count;
+}
+
+static int gs_put_char(struct tty_struct *tty, unsigned char ch)
+{
+        struct gs_port  *port = tty->driver_data;
+        unsigned long   flags;
+        int             status;
+
+        pr_vdebug("gs_put_char: (%d,%p) char=0x%x, called from %p\n",
+                port->port_num, tty, ch, __builtin_return_address(0));
+
+        spin_lock_irqsave(&port->port_lock, flags);
+        status = gs_buf_put(&port->port_write_buf, &ch, 1);
+        spin_unlock_irqrestore(&port->port_lock, flags);
+
+        return status;
+}
+
+static void gs_flush_chars(struct tty_struct *tty)
+{
+        struct gs_port  *port = tty->driver_data;
+        unsigned long   flags;
+
+        pr_vdebug("gs_flush_chars: (%d,%p)\n", port->port_num, tty);
+
+        spin_lock_irqsave(&port->port_lock, flags);
+        if (port->port_usb)
+                gs_start_tx(port);
+        spin_unlock_irqrestore(&port->port_lock, flags);
+}
+
+static int gs_write_room(struct tty_struct *tty)
+{
+        struct gs_port  *port = tty->driver_data;
+        unsigned long   flags;
+        int             room = 0;
+
+        spin_lock_irqsave(&port->port_lock, flags);
+        if (port->port_usb)
+                room = gs_buf_space_avail(&port->port_write_buf);
+        spin_unlock_irqrestore(&port->port_lock, flags);
+
+        pr_vdebug("gs_write_room: (%d,%p) room=%d\n",
+                port->port_num, tty, room);
+
+        return room;
+}
+
+static int gs_chars_in_buffer(struct tty_struct *tty)
+{
+        struct gs_port  *port = tty->driver_data;
+        unsigned long   flags;
+        int             chars = 0;
+
+        spin_lock_irqsave(&port->port_lock, flags);
+        chars = gs_buf_data_avail(&port->port_write_buf);
+        spin_unlock_irqrestore(&port->port_lock, flags);
+
+        pr_vdebug("gs_chars_in_buffer: (%d,%p) chars=%d\n",
+                port->port_num, tty, chars);
+
+        return chars;
+}
+
+/* undo side effects of setting TTY_THROTTLED */
+static void gs_unthrottle(struct tty_struct *tty)
+{
+        struct gs_port          *port = tty->driver_data;
+        unsigned long           flags;
+        unsigned                started = 0;
+
+        spin_lock_irqsave(&port->port_lock, flags);
+        if (port->port_usb)
+                started = gs_start_rx(port);
+        spin_unlock_irqrestore(&port->port_lock, flags);
+
+        pr_vdebug("gs_unthrottle: ttyGS%d, %d packets\n",
+                        port->port_num, started);
+}
+
+static const struct tty_operations gs_tty_ops = {
+        .open =                 gs_open,
+        .close =                gs_close,
+        .write =                gs_write,
+        .put_char =             gs_put_char,
+        .flush_chars =          gs_flush_chars,
+        .write_room =           gs_write_room,
+        .chars_in_buffer =      gs_chars_in_buffer,
+        .unthrottle =           gs_unthrottle,
+};
+
+/*-------------------------------------------------------------------------*/
+
+static struct tty_driver *gs_tty_driver;
+
+static int __init
+gs_port_alloc(unsigned port_num, struct usb_cdc_line_coding *coding)
+{
+        struct gs_port  *port;
+
+        port = kzalloc(sizeof(struct gs_port), GFP_KERNEL);
+        if (port == NULL)
+                return -ENOMEM;
+
+        spin_lock_init(&port->port_lock);
+        init_waitqueue_head(&port->close_wait);
+        init_waitqueue_head(&port->drain_wait);
+
+        tasklet_init(&port->push, gs_rx_push, (unsigned long) port);
+
+        INIT_LIST_HEAD(&port->read_pool);
+        INIT_LIST_HEAD(&port->write_pool);
+
+        port->port_num = port_num;
+        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 __init 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;
+
+        gs_tty_driver = alloc_tty_driver(count);
+        if (!gs_tty_driver)
+                return -ENOMEM;
+
+        gs_tty_driver->owner = THIS_MODULE;
+        gs_tty_driver->driver_name = "g_serial";
+        gs_tty_driver->name = "ttyGS";
+        /* 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 = __constant_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) {
+                put_tty_driver(gs_tty_driver);
+                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_register_device(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--)
+                kfree(ports[count].port);
+        put_tty_driver(gs_tty_driver);
+        gs_tty_driver = NULL;
+        return status;
+}
+
+static int gs_closed(struct gs_port *port)
+{
+        int cond;
+
+        spin_lock_irq(&port->port_lock);
+        cond = (port->open_count == 0) && !port->openclose;
+        spin_unlock_irq(&port->port_lock);
+        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)
+{
+        unsigned        i;
+        struct gs_port  *port;
+
+        /* 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);
+
+                /* wait for old opens to finish */
+                wait_event(port->close_wait, gs_closed(port));
+
+                WARN_ON(port->port_usb != NULL);
+
+                kfree(port);
+        }
+        n_ports = 0;
+
+        tty_unregister_driver(gs_tty_driver);
+        gs_tty_driver = NULL;
+
+        pr_debug("%s: cleaned up ttyGS* support\n", __func__);
+}
+
+/**
+ * gserial_connect - notify TTY I/O glue that USB link is active
+ * @gser: the function, set up with endpoints and descriptors
+ * @port_num: which port is active
+ * Context: any (usually from irq)
+ *
+ * This is called activate endpoints and let the TTY layer know that
+ * the connection is active ... not unlike "carrier detect".  It won't
+ * necessarily start I/O queues; unless the TTY is held open by any
+ * task, there would be no point.  However, the endpoints will be
+ * activated so the USB host can perform I/O, subject to basic USB
+ * hardware flow control.
+ *
+ * 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().
+ *
+ * Returns negative errno or zero.
+ * On success, ep->driver_data will be overwritten.
+ */
+int gserial_connect(struct gserial *gser, u8 port_num)
+{
+        struct gs_port  *port;
+        unsigned long   flags;
+        int             status;
+
+        if (!gs_tty_driver || port_num >= n_ports)
+                return -ENXIO;
+
+        /* we "know" gserial_cleanup() hasn't been called */
+        port = ports[port_num].port;
+
+        /* activate the endpoints */
+        status = usb_ep_enable(gser->in, gser->in_desc);
+        if (status < 0)
+                return status;
+        gser->in->driver_data = port;
+
+        status = usb_ep_enable(gser->out, gser->out_desc);
+        if (status < 0)
+                goto fail_out;
+        gser->out->driver_data = port;
+
+        /* then tell the tty glue that I/O can work */
+        spin_lock_irqsave(&port->port_lock, flags);
+        gser->ioport = port;
+        port->port_usb = gser;
+
+        /* REVISIT unclear how best to handle this state...
+         * we don't really couple it with the Linux TTY.
+         */
+        gser->port_line_coding = port->port_line_coding;
+
+        /* REVISIT if waiting on "carrier detect", signal. */
+
+        /* REVISIT for ACM, issue "network connection" status notification:
+         * connected if open_count, else disconnected.
+         */
+
+        /* if it's already open, start I/O */
+        if (port->open_count) {
+                pr_debug("gserial_connect: start ttyGS%d\n", port->port_num);
+                gs_start_io(port);
+        }
+
+        spin_unlock_irqrestore(&port->port_lock, flags);
+
+        return status;
+
+fail_out:
+        usb_ep_disable(gser->in);
+        gser->in->driver_data = NULL;
+        return status;
+}
+
+/**
+ * gserial_disconnect - notify TTY I/O glue that USB link is inactive
+ * @gser: the function, on which gserial_connect() was called
+ * Context: any (usually from irq)
+ *
+ * This is called to deactivate endpoints and let the TTY layer know
+ * that the connection went inactive ... not unlike "hangup".
+ *
+ * On return, the state is as if gserial_connect() had never been called;
+ * there is no active USB I/O on these endpoints.
+ */
+void gserial_disconnect(struct gserial *gser)
+{
+        struct gs_port  *port = gser->ioport;
+        unsigned long   flags;
+
+        if (!port)
+                return;
+
+        /* tell the TTY glue not to do I/O here any more */
+        spin_lock_irqsave(&port->port_lock, flags);
+
+        /* REVISIT as above: how best to track this? */
+        port->port_line_coding = gser->port_line_coding;
+
+        port->port_usb = NULL;
+        gser->ioport = NULL;
+        if (port->open_count > 0 || port->openclose) {
+                wake_up_interruptible(&port->drain_wait);
+                if (port->port_tty)
+                        tty_hangup(port->port_tty);
+        }
+        spin_unlock_irqrestore(&port->port_lock, flags);
+
+        /* disable endpoints, aborting down any active I/O */
+        usb_ep_disable(gser->out);
+        gser->out->driver_data = NULL;
+
+        usb_ep_disable(gser->in);
+        gser->in->driver_data = NULL;
+
+        /* finally, free any unused/unusable I/O buffers */
+        spin_lock_irqsave(&port->port_lock, flags);
+        if (port->open_count == 0 && !port->openclose)
+                gs_buf_free(&port->port_write_buf);
+        gs_free_requests(gser->out, &port->read_pool);
+        gs_free_requests(gser->in, &port->write_pool);
+        spin_unlock_irqrestore(&port->port_lock, flags);
+}