USB: serial gadget: rx path data loss fixes

[linux-2.6] / drivers / usb / gadget / u_serial.c

/*
 * 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.
 */

#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}().
 *      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 list_head        read_queue;
        unsigned                n_read;
        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);

                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;
                }

                /* 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;
        unsigned                started = 0;

        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;

                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;
}

/*
 * 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);
        }

        /* Push from tty to ldisc; this is immediate with low_latency, and
         * may trigger callbacks to this driver ... so drop the spinlock.
         */
        if (tty && do_push) {
                spin_unlock_irq(&port->port_lock);
                tty_flip_buffer_push(tty);
                wake_up_interruptible(&tty->read_wait);
                spin_lock_irq(&port->port_lock);

                /* tty may have been closed */
                tty = port->port_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);

        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 */
        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);
                gs_free_requests(port->port_usb->in, &port->write_pool);
                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;

        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);

        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;

        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 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->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;
}

/**
 * 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 = 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 = __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;

        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->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->out, &port->read_queue);
        gs_free_requests(gser->in, &port->write_pool);
        spin_unlock_irqrestore(&port->port_lock, flags);
}