u_serial.c

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/*
 * u_serial.c - utilities for USB gadget "serial port"/TTY support
 *
 * Copyright (C) 2003 Al Borchers ([email protected])
 * 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 ([email protected])
 * Copyright (C) 2000 Peter Berger ([email protected])
 * Copyright (C) 2000 Al Borchers ([email protected])
 *
 * 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/sched.h>
#include <linux/interrupt.h>
#include <linux/device.h>
#include <linux/delay.h>
#include <linux/tty.h>
#include <linux/tty_flip.h>
#include <linux/slab.h>
#include <linux/export.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.
 */

#define PREFIX  "ttyGS"

/*
 * 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}().  each gserial
 * instance can wrap its own USB control protocol.
 *  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 {
    struct tty_port     port;
    spinlock_t      port_lock;  /* guard port_* access */

    struct gserial      *port_usb;

    bool            openclose;  /* open/close in progress */
    u8          port_num;

    struct list_head    read_pool;
    int read_started;
    int read_allocated;
    struct list_head    read_queue;
    unsigned        n_read;
    struct tasklet_struct   push;

    struct list_head    write_pool;
    int write_started;
    int write_allocated;
    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 written into 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.
 */
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.
 */
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;

        if (port->write_started >= QUEUE_SIZE)
            break;

        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);
        req->zero = (gs_buf_data_avail(&port->port_write_buf) == 0);

        pr_vdebug(PREFIX "%d: tx len=%d, 0x%02x 0x%02x 0x%02x ...\n",
                port->port_num, len, *((u8 *)req->buf),
                *((u8 *)req->buf+1), *((u8 *)req->buf+2));

        /* 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;
        }

        port->write_started++;

        /* abort immediately after disconnect */
        if (!port->port_usb)
            break;
    }

    if (do_tty_wake && port->port.tty)
        tty_wakeup(port->port.tty);
    return status;
}

/*
 * 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;

    while (!list_empty(pool)) {
        struct usb_request  *req;
        int         status;
        struct tty_struct   *tty;

        /* no more rx if closed */
        tty = port->port.tty;
        if (!tty)
            break;

        if (port->read_started >= QUEUE_SIZE)
            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;
        }
        port->read_started++;

        /* abort immediately after disconnect */
        if (!port->port_usb)
            break;
    }
    return port->read_started;
}

/*
 * RX tasklet takes data out of the RX queue and hands it up to the TTY
 * layer until it refuses to take any more data (or is throttled back).
 * Then it issues reads for any further data.
 *
 * If the RX queue becomes full enough that no usb_request is queued,
 * the OUT endpoint may begin NAKing as soon as its FIFO fills up.
 * So QUEUE_SIZE packets plus however many the FIFO holds (usually two)
 * can be buffered before the TTY layer's buffers (currently 64 KB).
 */
static void gs_rx_push(unsigned long _port)
{
    struct gs_port      *port = (void *)_port;
    struct tty_struct   *tty;
    struct list_head    *queue = &port->read_queue;
    bool            disconnect = false;
    bool            do_push = false;

    /* hand any queued data to the tty */
    spin_lock_irq(&port->port_lock);
    tty = port->port.tty;
    while (!list_empty(queue)) {
        struct usb_request  *req;

        req = list_first_entry(queue, struct usb_request, list);

        /* discard data if tty was closed */
        if (!tty)
            goto recycle;

        /* leave data queued if tty was rx throttled */
        if (test_bit(TTY_THROTTLED, &tty->flags))
            break;

        switch (req->status) {
        case -ESHUTDOWN:
            disconnect = true;
            pr_vdebug(PREFIX "%d: shutdown\n", port->port_num);
            break;

        default:
            /* presumably a transient fault */
            pr_warning(PREFIX "%d: unexpected RX status %d\n",
                    port->port_num, req->status);
            /* FALLTHROUGH */
        case 0:
            /* normal completion */
            break;
        }

        /* push data to (open) tty */
        if (req->actual) {
            char        *packet = req->buf;
            unsigned    size = req->actual;
            unsigned    n;
            int     count;

            /* we may have pushed part of this packet already... */
            n = port->n_read;
            if (n) {
                packet += n;
                size -= n;
            }

            count = tty_insert_flip_string(tty, packet, size);
            if (count)
                do_push = true;
            if (count != size) {
                /* stop pushing; TTY layer can't handle more */
                port->n_read += count;
                pr_vdebug(PREFIX "%d: rx block %d/%d\n",
                        port->port_num,
                        count, req->actual);
                break;
            }
            port->n_read = 0;
        }
recycle:
        list_move(&req->list, &port->read_pool);
        port->read_started--;
    }

    /* Push from tty to ldisc; without low_latency set this is handled by
     * a workqueue, so we won't get callbacks and can hold port_lock
     */
    if (tty && do_push)
        tty_flip_buffer_push(tty);


    /* We want our data queue to become empty ASAP, keeping data
     * in the tty and ldisc (not here).  If we couldn't push any
     * this time around, there may be trouble unless there's an
     * implicit tty_unthrottle() call on its way...
     *
     * REVISIT we should probably add a timer to keep the tasklet
     * from starving ... but it's not clear that case ever happens.
     */
    if (!list_empty(queue) && tty) {
        if (!test_bit(TTY_THROTTLED, &tty->flags)) {
            if (do_push)
                tasklet_schedule(&port->push);
            else
                pr_warning(PREFIX "%d: RX not scheduled?\n",
                    port->port_num);
        }
    }

    /* If we're still connected, refill the USB RX queue. */
    if (!disconnect && port->port_usb)
        gs_start_rx(port);

    spin_unlock_irq(&port->port_lock);
}

static void gs_read_complete(struct usb_ep *ep, struct usb_request *req)
{
    struct gs_port  *port = ep->driver_data;

    /* Queue all received data until the tty layer is ready for it. */
    spin_lock(&port->port_lock);
    list_add_tail(&req->list, &port->read_queue);
    tasklet_schedule(&port->push);
    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);
    port->write_started--;

    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,
                             int *allocated)
{
    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);
        if (allocated)
            (*allocated)--;
    }
}

static int gs_alloc_requests(struct usb_ep *ep, struct list_head *head,
        void (*fn)(struct usb_ep *, struct usb_request *),
        int *allocated)
{
    int         i;
    struct usb_request  *req;
    int n = allocated ? QUEUE_SIZE - *allocated : QUEUE_SIZE;

    /* 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 < n; 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);
        if (allocated)
            (*allocated)++;
    }
    return 0;
}

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,
        &port->read_allocated);
    if (status)
        return status;

    status = gs_alloc_requests(port->port_usb->in, &port->write_pool,
            gs_write_complete, &port->write_allocated);
    if (status) {
        gs_free_requests(ep, head, &port->read_allocated);
        return status;
    }

    /* queue read requests */
    port->n_read = 0;
    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, &port->read_allocated);
        gs_free_requests(port->port_usb->in, &port->write_pool,
            &port->write_allocated);
        status = -EIO;
    }

    return 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;

    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->port.count) {
                status = 0;
                port->port.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->port.count = 1;
    port->openclose = false;

    /* if connected, start the I/O stream */
    if (port->port_usb) {
        struct gserial  *gser = port->port_usb;

        pr_debug("gs_open: start ttyGS%d\n", port->port_num);
        gs_start_io(port);

        if (gser->connect)
            gser->connect(gser);
    }

    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;
    struct gserial  *gser;

    spin_lock_irq(&port->port_lock);

    if (port->port.count != 1) {
        if (port->port.count == 0)
            WARN_ON(1);
        else
            --port->port.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->port.count = 0;

    gser = port->port_usb;
    if (gser && gser->disconnect)
        gser->disconnect(gser);

    /* 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 && gser) {
        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);
        gser = port->port_usb;
    }

    /* 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 (gser == NULL)
        gs_buf_free(&port->port_write_buf);
    else
        gs_buf_clear(&port->port_write_buf);

    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->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 %pf\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;

    spin_lock_irqsave(&port->port_lock, flags);
    if (port->port_usb) {
        /* Kickstart read queue processing.  We don't do xon/xoff,
         * rts/cts, or other handshaking with the host, but if the
         * read queue backs up enough we'll be NAKing OUT packets.
         */
        tasklet_schedule(&port->push);
        pr_vdebug(PREFIX "%d: unthrottle\n", port->port_num);
    }
    spin_unlock_irqrestore(&port->port_lock, flags);
}

static int gs_break_ctl(struct tty_struct *tty, int duration)
{
    struct gs_port  *port = tty->driver_data;
    int     status = 0;
    struct gserial  *gser;

    pr_vdebug("gs_break_ctl: ttyGS%d, send break (%d) \n",
            port->port_num, duration);

    spin_lock_irq(&port->port_lock);
    gser = port->port_usb;
    if (gser && gser->send_break)
        status = gser->send_break(gser, duration);
    spin_unlock_irq(&port->port_lock);

    return status;
}

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,
    .break_ctl =        gs_break_ctl,
};

/*-------------------------------------------------------------------------*/

static struct tty_driver *gs_tty_driver;

static int
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;

    tty_port_init(&port->port);
    spin_lock_init(&port->port_lock);
    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->read_queue);
    INIT_LIST_HEAD(&port->write_pool);

    port->port_num = port_num;
    port->port_line_coding = *coding;

    ports[port_num].port = port;

    return 0;
}

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;

    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_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->port.count == 0) && !port->openclose;
    spin_unlock_irq(&port->port_lock);
    return cond;
}

void gserial_cleanup(void)
{
    unsigned    i;
    struct gs_port  *port;

    if (!gs_tty_driver)
        return;

    /* 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);

        /* wait for old opens to finish */
        wait_event(port->port.close_wait, gs_closed(port));

        WARN_ON(port->port_usb != NULL);

        kfree(port);
    }
    n_ports = 0;

    tty_unregister_driver(gs_tty_driver);
    put_tty_driver(gs_tty_driver);
    gs_tty_driver = NULL;

    pr_debug("%s: cleaned up ttyGS* support\n", __func__);
}

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);
    if (status < 0)
        return status;
    gser->in->driver_data = port;

    status = usb_ep_enable(gser->out);
    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. */

    /* if it's already open, start I/O ... and notify the serial
     * protocol about open/close status (connect/disconnect).
     */
    if (port->port.count) {
        pr_debug("gserial_connect: start ttyGS%d\n", port->port_num);
        gs_start_io(port);
        if (gser->connect)
            gser->connect(gser);
    } else {
        if (gser->disconnect)
            gser->disconnect(gser);
    }

    spin_unlock_irqrestore(&port->port_lock, flags);

    return status;

fail_out:
    usb_ep_disable(gser->in);
    gser->in->driver_data = NULL;
    return status;
}

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->port.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->port.count == 0 && !port->openclose)
        gs_buf_free(&port->port_write_buf);
    gs_free_requests(gser->out, &port->read_pool, NULL);
    gs_free_requests(gser->out, &port->read_queue, NULL);
    gs_free_requests(gser->in, &port->write_pool, NULL);

    port->read_allocated = port->read_started =
        port->write_allocated = port->write_started = 0;

    spin_unlock_irqrestore(&port->port_lock, flags);
}