// SPDX-License-Identifier: GPL-2.0+
/*
 * 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)
 */

/* #define VERBOSE_DEBUG */

#include <linux/kernel.h>
#include <linux/sched.h>
#include <linux/device.h>
#include <linux/delay.h>
#include <linux/tty.h>
#include <linux/tty_flip.h>
#include <linux/slab.h>
#include <linux/string_choices.h>
#include <linux/export.h>
#include <linux/module.h>
#include <linux/console.h>
#include <linux/kstrtox.h>
#include <linux/kthread.h>
#include <linux/workqueue.h>
#include <linux/kfifo.h>
#include <linux/serial.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 this module has been loaded, the individual TTY port can be requested
 * (gserial_alloc_line()) and it will stay available until they are removed
 * (gserial_free_line()). Each one may be connected to a USB function
 * (gserial_connect), or disconnected (with gserial_disconnect) when the USB
 * host issues a config change event. Data can only flow when the port is
 * connected to the host.
 *
 * A given TTY port can be made available in multiple configurations.
 * For example, each one might expose a ttyGS0 node which provides a
 * 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}().  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 */
#define GS_CONSOLE_BUF_SIZE     8192

/* Prevents race conditions while accessing gser->ioport */
static DEFINE_SPINLOCK(serial_port_lock);

/* console info */
struct gs_console {
        struct console          console;
        struct work_struct      work;
        spinlock_t              lock;
        struct usb_request      *req;
        struct kfifo            buf;
        size_t                  missed;
};

/*
 * 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;
#ifdef CONFIG_U_SERIAL_CONSOLE
        struct gs_console       *console;
#endif

        u8                      port_num;

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

        struct list_head        write_pool;
        int write_started;
        int write_allocated;
        struct kfifo            port_write_buf;
        wait_queue_head_t       drain_wait;     /* wait while writes drain */
        bool                    write_busy;
        wait_queue_head_t       close_wait;
        bool                    suspended;      /* port suspended */
        bool                    start_delayed;  /* delay start when suspended */
        struct async_icount     icount;

        /* REVISIT this state ... */
        struct usb_cdc_line_coding port_line_coding;    /* 8-N-1 etc */
};

static struct portmaster {
        struct mutex    lock;                   /* protect open/close */
        struct gs_port  *port;
} ports[MAX_U_SERIAL_PORTS];

#define GS_CLOSE_TIMEOUT                15              /* seconds */



#ifdef VERBOSE_DEBUG
#ifndef pr_vdebug
#define pr_vdebug(fmt, arg...) \
        pr_debug(fmt, ##arg)
#endif /* pr_vdebug */
#else
#ifndef pr_vdebug
#define pr_vdebug(fmt, arg...) \
        ({ if (0) pr_debug(fmt, ##arg); })
#endif /* pr_vdebug */
#endif

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

/* 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;
}
EXPORT_SYMBOL_GPL(gs_alloc_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);
}
EXPORT_SYMBOL_GPL(gs_free_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 = kfifo_len(&port->port_write_buf);
        if (len < size)
                size = len;
        if (size != 0)
                size = kfifo_out(&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;
        int                     status = 0;
        bool                    do_tty_wake = false;

        if (!port->port_usb)
                return status;

        in = port->port_usb->in;

        while (!port->write_busy && !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;
                port->icount.tx += len;

                req->length = len;
                list_del(&req->list);
                req->zero = kfifo_is_empty(&port->port_write_buf);

                pr_vdebug("ttyGS%d: tx len=%d, %3ph ...\n", port->port_num, len, req->buf);

                /* 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).
                 */
                port->write_busy = true;
                spin_unlock(&port->port_lock);
                status = usb_ep_queue(in, req, GFP_ATOMIC);
                spin_lock(&port->port_lock);
                port->write_busy = false;

                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)
                tty_port_tty_wakeup(&port->port);
        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 work 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(struct work_struct *work)
{
        struct delayed_work     *w = to_delayed_work(work);
        struct gs_port          *port = container_of(w, struct gs_port, push);
        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);

                /* leave data queued if tty was rx throttled */
                if (tty && tty_throttled(tty))
                        break;

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

                default:
                        /* presumably a transient fault */
                        pr_warn("ttyGS%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 && tty) {
                        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;
                        }

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

                list_move(&req->list, &port->read_pool);
                port->read_started--;
        }

        /* Push from tty to ldisc; this is handled by a workqueue,
         * so we won't get callbacks and can hold port_lock
         */
        if (do_push)
                tty_flip_buffer_push(&port->port);


        /* 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, RX may be starved, so wait until next jiffy.
         *
         * We may leave non-empty queue only when there is a tty, and
         * either it is throttled or there is no more room in flip buffer.
         */
        if (!list_empty(queue) && !tty_throttled(tty))
                schedule_delayed_work(&port->push, 1);

        /* 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);
        schedule_delayed_work(&port->push, 0);
        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_warn("%s: unexpected %s status %d\n",
                        __func__, ep->name, req->status);
                fallthrough;
        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;
}

/**
 * gs_start_io - start USB I/O streams
 * @port: port 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,
                &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);

        if (started) {
                gs_start_tx(port);
                /* Unblock any pending writes into our circular buffer, in case
                 * we didn't in gs_start_tx() */
                tty_port_tty_wakeup(&port->port);
        } else {
                /* Free reqs only if we are still connected */
                if (port->port_usb) {
                        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;
}

static int gserial_wakeup_host(struct gserial *gser)
{
        struct usb_function     *func = &gser->func;
        struct usb_gadget       *gadget = func->config->cdev->gadget;

        if (func->func_suspended)
                return usb_func_wakeup(func);
        else
                return usb_gadget_wakeup(gadget);
}

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

/* 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 = 0;

        mutex_lock(&ports[port_num].lock);
        port = ports[port_num].port;
        if (!port) {
                status = -ENODEV;
                goto out;
        }

        spin_lock_irq(&port->port_lock);

        /* allocate circular buffer on first open */
        if (!kfifo_initialized(&port->port_write_buf)) {

                spin_unlock_irq(&port->port_lock);

                /*
                 * portmaster's mutex still protects from simultaneous open(),
                 * and close() can't happen, yet.
                 */

                status = kfifo_alloc(&port->port_write_buf,
                                     WRITE_BUF_SIZE, GFP_KERNEL);
                if (status) {
                        pr_debug("gs_open: ttyGS%d (%p,%p) no buffer\n",
                                 port_num, tty, file);
                        goto out;
                }

                spin_lock_irq(&port->port_lock);
        }

        /* already open?  Great. */
        if (port->port.count++)
                goto exit_unlock_port;

        tty->driver_data = port;
        port->port.tty = tty;

        /* if connected, start the I/O stream */
        if (port->port_usb) {
                /* if port is suspended, wait resume to start I/0 stream */
                if (!port->suspended) {
                        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);
                } else {
                        pr_debug("delay start of ttyGS%d\n", port->port_num);
                        port->start_delayed = true;
                }
        }

        pr_debug("gs_open: ttyGS%d (%p,%p)\n", port->port_num, tty, file);

exit_unlock_port:
        spin_unlock_irq(&port->port_lock);
out:
        mutex_unlock(&ports[port_num].lock);
        return status;
}

static int gs_close_flush_done(struct gs_port *p)
{
        int cond;

        /* return true on disconnect or empty buffer or if raced with open() */
        spin_lock_irq(&p->port_lock);
        cond = p->port_usb == NULL || !kfifo_len(&p->port_write_buf) ||
                p->port.count > 1;
        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) {
raced_with_open:
                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);

        gser = port->port_usb;
        if (gser && !port->suspended && 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 (kfifo_len(&port->port_write_buf) > 0 && gser) {
                spin_unlock_irq(&port->port_lock);
                wait_event_interruptible_timeout(port->drain_wait,
                                        gs_close_flush_done(port),
                                        GS_CLOSE_TIMEOUT * HZ);
                spin_lock_irq(&port->port_lock);

                if (port->port.count != 1)
                        goto raced_with_open;

                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 async work fire again until we're re-opened.
         */
        if (gser == NULL)
                kfifo_free(&port->port_write_buf);
        else
                kfifo_reset(&port->port_write_buf);

        port->start_delayed = false;
        port->port.count = 0;
        port->port.tty = NULL;

        pr_debug("gs_close: ttyGS%d (%p,%p) done!\n",
                        port->port_num, tty, file);

        wake_up(&port->close_wait);
exit:
        spin_unlock_irq(&port->port_lock);
}

static ssize_t gs_write(struct tty_struct *tty, const u8 *buf, size_t count)
{
        struct gs_port  *port = tty->driver_data;
        unsigned long   flags;
        int ret = 0;
        struct gserial  *gser = port->port_usb;

        pr_vdebug("gs_write: ttyGS%d (%p) writing %zu bytes\n",
                        port->port_num, tty, count);

        spin_lock_irqsave(&port->port_lock, flags);
        if (count)
                count = kfifo_in(&port->port_write_buf, buf, count);

        if (port->suspended) {
                spin_unlock_irqrestore(&port->port_lock, flags);
                ret = gserial_wakeup_host(gser);
                if (ret) {
                        pr_debug("ttyGS%d: Remote wakeup failed:%d\n", port->port_num, ret);
                        return count;
                }
                spin_lock_irqsave(&port->port_lock, flags);
        }

        /* treat count == 0 as flush_chars() */
        if (port->port_usb)
                gs_start_tx(port);
        spin_unlock_irqrestore(&port->port_lock, flags);

        return count;
}

static int gs_put_char(struct tty_struct *tty, u8 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 %ps\n",
                port->port_num, tty, ch, __builtin_return_address(0));

        spin_lock_irqsave(&port->port_lock, flags);
        status = kfifo_put(&port->port_write_buf, ch);
        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;
        int ret = 0;
        struct gserial  *gser = port->port_usb;

        pr_vdebug("gs_flush_chars: (%d,%p)\n", port->port_num, tty);

        spin_lock_irqsave(&port->port_lock, flags);
        if (port->suspended) {
                spin_unlock_irqrestore(&port->port_lock, flags);
                ret = gserial_wakeup_host(gser);
                if (ret) {
                        pr_debug("ttyGS%d: Remote wakeup failed:%d\n", port->port_num, ret);
                        return;
                }
                spin_lock_irqsave(&port->port_lock, flags);
        }

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

static unsigned int gs_write_room(struct tty_struct *tty)
{
        struct gs_port  *port = tty->driver_data;
        unsigned long   flags;
        unsigned int room = 0;

        spin_lock_irqsave(&port->port_lock, flags);
        if (port->port_usb)
                room = kfifo_avail(&port->port_write_buf);
        spin_unlock_irqrestore(&port->port_lock, flags);

        pr_vdebug("gs_write_room: (%d,%p) room=%u\n",
                port->port_num, tty, room);

        return room;
}

static unsigned int gs_chars_in_buffer(struct tty_struct *tty)
{
        struct gs_port  *port = tty->driver_data;
        unsigned long   flags;
        unsigned int    chars;

        spin_lock_irqsave(&port->port_lock, flags);
        chars = kfifo_len(&port->port_write_buf);
        spin_unlock_irqrestore(&port->port_lock, flags);

        pr_vdebug("gs_chars_in_buffer: (%d,%p) chars=%u\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.
                 */
                pr_vdebug("ttyGS%d: unthrottle\n", port->port_num);
                schedule_delayed_work(&port->push, 0);
        }
        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 int gs_get_icount(struct tty_struct *tty,
                         struct serial_icounter_struct *icount)
{
        struct gs_port *port = tty->driver_data;
        struct async_icount cnow;
        unsigned long flags;

        spin_lock_irqsave(&port->port_lock, flags);
        cnow = port->icount;
        spin_unlock_irqrestore(&port->port_lock, flags);

        icount->rx = cnow.rx;
        icount->tx = cnow.tx;

        return 0;
}

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,
        .get_icount =           gs_get_icount,
};

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

static struct tty_driver *gs_tty_driver;

#ifdef CONFIG_U_SERIAL_CONSOLE

static void gs_console_complete_out(struct usb_ep *ep, struct usb_request *req)
{
        struct gs_console *cons = req->context;

        switch (req->status) {
        default:
                pr_warn("%s: unexpected %s status %d\n",
                        __func__, ep->name, req->status);
                fallthrough;
        case 0:
                /* normal completion */
                spin_lock(&cons->lock);
                req->length = 0;
                schedule_work(&cons->work);
                spin_unlock(&cons->lock);
                break;
        case -ECONNRESET:
        case -ESHUTDOWN:
                /* disconnect */
                pr_vdebug("%s: %s shutdown\n", __func__, ep->name);
                break;
        }
}

static void __gs_console_push(struct gs_console *cons)
{
        struct usb_request *req = cons->req;
        struct usb_ep *ep;
        size_t size;

        if (!req)
                return; /* disconnected */

        if (req->length)
                return; /* busy */

        ep = cons->console.data;
        size = kfifo_out(&cons->buf, req->buf, ep->maxpacket);
        if (!size)
                return;

        if (cons->missed && ep->maxpacket >= 64) {
                char buf[64];
                size_t len;

                len = sprintf(buf, "\n[missed %zu bytes]\n", cons->missed);
                kfifo_in(&cons->buf, buf, len);
                cons->missed = 0;
        }

        req->length = size;

        spin_unlock_irq(&cons->lock);
        if (usb_ep_queue(ep, req, GFP_ATOMIC))
                req->length = 0;
        spin_lock_irq(&cons->lock);
}

static void gs_console_work(struct work_struct *work)
{
        struct gs_console *cons = container_of(work, struct gs_console, work);

        spin_lock_irq(&cons->lock);

        __gs_console_push(cons);

        spin_unlock_irq(&cons->lock);
}

static void gs_console_write(struct console *co,
                             const char *buf, unsigned count)
{
        struct gs_console *cons = container_of(co, struct gs_console, console);
        unsigned long flags;
        size_t n;

        spin_lock_irqsave(&cons->lock, flags);

        n = kfifo_in(&cons->buf, buf, count);
        if (n < count)
                cons->missed += count - n;

        if (cons->req && !cons->req->length)
                schedule_work(&cons->work);

        spin_unlock_irqrestore(&cons->lock, flags);
}

static struct tty_driver *gs_console_device(struct console *co, int *index)
{
        *index = co->index;
        return gs_tty_driver;
}

static int gs_console_connect(struct gs_port *port)
{
        struct gs_console *cons = port->console;
        struct usb_request *req;
        struct usb_ep *ep;

        if (!cons)
                return 0;

        ep = port->port_usb->in;
        req = gs_alloc_req(ep, ep->maxpacket, GFP_ATOMIC);
        if (!req)
                return -ENOMEM;
        req->complete = gs_console_complete_out;
        req->context = cons;
        req->length = 0;

        spin_lock(&cons->lock);
        cons->req = req;
        cons->console.data = ep;
        spin_unlock(&cons->lock);

        pr_debug("ttyGS%d: console connected!\n", port->port_num);

        schedule_work(&cons->work);

        return 0;
}

static void gs_console_disconnect(struct gs_port *port)
{
        struct gs_console *cons = port->console;
        struct usb_request *req;
        struct usb_ep *ep;

        if (!cons)
                return;

        spin_lock(&cons->lock);

        req = cons->req;
        ep = cons->console.data;
        cons->req = NULL;

        spin_unlock(&cons->lock);

        if (!req)
                return;

        usb_ep_dequeue(ep, req);
        gs_free_req(ep, req);
}

static int gs_console_init(struct gs_port *port)
{
        struct gs_console *cons;
        int err;

        if (port->console)
                return 0;

        cons = kzalloc_obj(*port->console);
        if (!cons)
                return -ENOMEM;

        strcpy(cons->console.name, "ttyGS");
        cons->console.write = gs_console_write;
        cons->console.device = gs_console_device;
        cons->console.flags = CON_PRINTBUFFER;
        cons->console.index = port->port_num;

        INIT_WORK(&cons->work, gs_console_work);
        spin_lock_init(&cons->lock);

        err = kfifo_alloc(&cons->buf, GS_CONSOLE_BUF_SIZE, GFP_KERNEL);
        if (err) {
                pr_err("ttyGS%d: allocate console buffer failed\n", port->port_num);
                kfree(cons);
                return err;
        }

        port->console = cons;
        register_console(&cons->console);

        spin_lock_irq(&port->port_lock);
        if (port->port_usb)
                gs_console_connect(port);
        spin_unlock_irq(&port->port_lock);

        return 0;
}

static void gs_console_exit(struct gs_port *port)
{
        struct gs_console *cons = port->console;

        if (!cons)
                return;

        unregister_console(&cons->console);

        spin_lock_irq(&port->port_lock);
        if (cons->req)
                gs_console_disconnect(port);
        spin_unlock_irq(&port->port_lock);

        cancel_work_sync(&cons->work);
        kfifo_free(&cons->buf);
        kfree(cons);
        port->console = NULL;
}

ssize_t gserial_set_console(unsigned char port_num, const char *page, size_t count)
{
        struct gs_port *port;
        bool enable;
        int ret;

        ret = kstrtobool(page, &enable);
        if (ret)
                return ret;

        mutex_lock(&ports[port_num].lock);
        port = ports[port_num].port;

        if (WARN_ON(port == NULL)) {
                ret = -ENXIO;
                goto out;
        }

        if (enable)
                ret = gs_console_init(port);
        else
                gs_console_exit(port);
out:
        mutex_unlock(&ports[port_num].lock);

        return ret < 0 ? ret : count;
}
EXPORT_SYMBOL_GPL(gserial_set_console);

ssize_t gserial_get_console(unsigned char port_num, char *page)
{
        struct gs_port *port;
        ssize_t ret;

        mutex_lock(&ports[port_num].lock);
        port = ports[port_num].port;

        if (WARN_ON(port == NULL))
                ret = -ENXIO;
        else
                ret = sprintf(page, "%u\n", !!port->console);

        mutex_unlock(&ports[port_num].lock);

        return ret;
}
EXPORT_SYMBOL_GPL(gserial_get_console);

#else

static int gs_console_connect(struct gs_port *port)
{
        return 0;
}

static void gs_console_disconnect(struct gs_port *port)
{
}

static int gs_console_init(struct gs_port *port)
{
        return -ENOSYS;
}

static void gs_console_exit(struct gs_port *port)
{
}

#endif

static int
gs_port_alloc(unsigned port_num, struct usb_cdc_line_coding *coding)
{
        struct gs_port  *port;
        int             ret = 0;

        mutex_lock(&ports[port_num].lock);
        if (ports[port_num].port) {
                ret = -EBUSY;
                goto out;
        }

        port = kzalloc_obj(struct gs_port);
        if (port == NULL) {
                ret = -ENOMEM;
                goto out;
        }

        tty_port_init(&port->port);
        spin_lock_init(&port->port_lock);
        init_waitqueue_head(&port->drain_wait);
        init_waitqueue_head(&port->close_wait);

        INIT_DELAYED_WORK(&port->push, gs_rx_push);

        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;
out:
        mutex_unlock(&ports[port_num].lock);
        return ret;
}

static int gs_closed(struct gs_port *port)
{
        int cond;

        spin_lock_irq(&port->port_lock);
        cond = port->port.count == 0;
        spin_unlock_irq(&port->port_lock);

        return cond;
}

static void gserial_free_port(struct gs_port *port)
{
        cancel_delayed_work_sync(&port->push);
        /* wait for old opens to finish */
        wait_event(port->close_wait, gs_closed(port));
        WARN_ON(port->port_usb != NULL);
        tty_port_destroy(&port->port);
        kfree(port);
}

void gserial_free_line(unsigned char port_num)
{
        struct gs_port  *port;

        mutex_lock(&ports[port_num].lock);
        if (!ports[port_num].port) {
                mutex_unlock(&ports[port_num].lock);
                return;
        }
        port = ports[port_num].port;
        gs_console_exit(port);
        ports[port_num].port = NULL;
        mutex_unlock(&ports[port_num].lock);

        gserial_free_port(port);
        tty_unregister_device(gs_tty_driver, port_num);
}
EXPORT_SYMBOL_GPL(gserial_free_line);

int gserial_alloc_line_no_console(unsigned char *line_num)
{
        struct usb_cdc_line_coding      coding;
        struct gs_port                  *port;
        struct device                   *tty_dev;
        int                             ret;
        int                             port_num;

        coding.dwDTERate = cpu_to_le32(9600);
        coding.bCharFormat = 8;
        coding.bParityType = USB_CDC_NO_PARITY;
        coding.bDataBits = USB_CDC_1_STOP_BITS;

        for (port_num = 0; port_num < MAX_U_SERIAL_PORTS; port_num++) {
                ret = gs_port_alloc(port_num, &coding);
                if (ret == -EBUSY)
                        continue;
                if (ret)
                        return ret;
                break;
        }
        if (ret)
                return ret;

        /* ... and sysfs class devices, so mdev/udev make /dev/ttyGS* */

        port = ports[port_num].port;
        tty_dev = tty_port_register_device(&port->port,
                        gs_tty_driver, port_num, NULL);
        if (IS_ERR(tty_dev)) {
                pr_err("%s: failed to register tty for port %d, err %ld\n",
                                __func__, port_num, PTR_ERR(tty_dev));

                ret = PTR_ERR(tty_dev);
                mutex_lock(&ports[port_num].lock);
                ports[port_num].port = NULL;
                mutex_unlock(&ports[port_num].lock);
                gserial_free_port(port);
                goto err;
        }
        *line_num = port_num;
err:
        return ret;
}
EXPORT_SYMBOL_GPL(gserial_alloc_line_no_console);

int gserial_alloc_line(unsigned char *line_num)
{
        int ret = gserial_alloc_line_no_console(line_num);

        if (!ret && !*line_num)
                gs_console_init(ports[*line_num].port);

        return ret;
}
EXPORT_SYMBOL_GPL(gserial_alloc_line);

/**
 * 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 allocate @port_num by calling
 * @gserial_alloc_line().
 *
 * 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 (port_num >= MAX_U_SERIAL_PORTS)
                return -ENXIO;

        port = ports[port_num].port;
        if (!port) {
                pr_err("serial line %d not allocated.\n", port_num);
                return -EINVAL;
        }
        if (port->port_usb) {
                pr_err("serial line %d is in use.\n", port_num);
                return -EBUSY;
        }

        /* activate the endpoints */
        status = usb_ep_enable(gser->in);
        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);
        }

        status = gs_console_connect(port);
        spin_unlock_irqrestore(&port->port_lock, flags);

        return status;

fail_out:
        usb_ep_disable(gser->in);
        return status;
}
EXPORT_SYMBOL_GPL(gserial_connect);
/**
 * 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;

        spin_lock_irqsave(&serial_port_lock, flags);

        /* tell the TTY glue not to do I/O here any more */
        spin_lock(&port->port_lock);

        gs_console_disconnect(port);

        /* 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) {
                wake_up_interruptible(&port->drain_wait);
                if (port->port.tty)
                        tty_hangup(port->port.tty);
        }
        port->suspended = false;
        spin_unlock(&port->port_lock);
        spin_unlock_irqrestore(&serial_port_lock, flags);

        /* disable endpoints, aborting down any active I/O */
        usb_ep_disable(gser->out);
        usb_ep_disable(gser->in);

        /* finally, free any unused/unusable I/O buffers */
        spin_lock_irqsave(&port->port_lock, flags);
        if (port->port.count == 0)
                kfifo_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);
}
EXPORT_SYMBOL_GPL(gserial_disconnect);

void gserial_suspend(struct gserial *gser)
{
        struct gs_port  *port;
        unsigned long   flags;

        spin_lock_irqsave(&serial_port_lock, flags);
        port = gser->ioport;

        if (!port) {
                spin_unlock_irqrestore(&serial_port_lock, flags);
                return;
        }

        if (port->write_busy || port->write_started) {
                /* Wakeup to host if there are ongoing transfers */
                spin_unlock_irqrestore(&serial_port_lock, flags);
                if (!gserial_wakeup_host(gser))
                        return;
                spin_lock_irqsave(&serial_port_lock, flags);
        }

        spin_lock(&port->port_lock);
        spin_unlock(&serial_port_lock);
        port->suspended = true;
        port->start_delayed = true;
        spin_unlock_irqrestore(&port->port_lock, flags);
}
EXPORT_SYMBOL_GPL(gserial_suspend);

void gserial_resume(struct gserial *gser)
{
        struct gs_port *port;
        unsigned long   flags;

        spin_lock_irqsave(&serial_port_lock, flags);
        port = gser->ioport;

        if (!port) {
                spin_unlock_irqrestore(&serial_port_lock, flags);
                return;
        }

        spin_lock(&port->port_lock);
        spin_unlock(&serial_port_lock);
        port->suspended = false;
        if (!port->start_delayed) {
                spin_unlock_irqrestore(&port->port_lock, flags);
                return;
        }

        pr_debug("delayed start ttyGS%d\n", port->port_num);
        gs_start_io(port);
        if (gser->connect)
                gser->connect(gser);
        port->start_delayed = false;
        spin_unlock_irqrestore(&port->port_lock, flags);
}
EXPORT_SYMBOL_GPL(gserial_resume);

static int __init userial_init(void)
{
        struct tty_driver *driver;
        unsigned                        i;
        int                             status;

        driver = tty_alloc_driver(MAX_U_SERIAL_PORTS, TTY_DRIVER_REAL_RAW |
                        TTY_DRIVER_DYNAMIC_DEV);
        if (IS_ERR(driver))
                return PTR_ERR(driver);

        driver->driver_name = "g_serial";
        driver->name = "ttyGS";
        /* uses dynamically assigned dev_t values */

        driver->type = TTY_DRIVER_TYPE_SERIAL;
        driver->subtype = SERIAL_TYPE_NORMAL;
        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.
         */
        driver->init_termios.c_cflag =
                        B9600 | CS8 | CREAD | HUPCL | CLOCAL;
        driver->init_termios.c_ispeed = 9600;
        driver->init_termios.c_ospeed = 9600;

        tty_set_operations(driver, &gs_tty_ops);
        for (i = 0; i < MAX_U_SERIAL_PORTS; i++)
                mutex_init(&ports[i].lock);

        /* export the driver ... */
        status = tty_register_driver(driver);
        if (status) {
                pr_err("%s: cannot register, err %d\n",
                                __func__, status);
                goto fail;
        }

        gs_tty_driver = driver;

        pr_debug("%s: registered %d ttyGS* device%s\n", __func__,
                        MAX_U_SERIAL_PORTS,
                        str_plural(MAX_U_SERIAL_PORTS));

        return status;
fail:
        tty_driver_kref_put(driver);
        return status;
}
module_init(userial_init);

static void __exit userial_cleanup(void)
{
        tty_unregister_driver(gs_tty_driver);
        tty_driver_kref_put(gs_tty_driver);
        gs_tty_driver = NULL;
}
module_exit(userial_cleanup);

MODULE_DESCRIPTION("utilities for USB gadget \"serial port\"/TTY support");
MODULE_LICENSE("GPL");