/*
 *  This program is free software; you can distribute it and/or modify it
 *  under the terms of the GNU General Public License (Version 2) as
 *  published by the Free Software Foundation.
 *
 *  This program is distributed in the hope it will be useful, but WITHOUT
 *  ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 *  FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 *  for more details.
 *
 *  You should have received a copy of the GNU General Public License along
 *  with this program; if not, see <http://www.gnu.org/licenses/>.
 *
 * Copyright (C) Hans Alblas PE1AYX <hans@esrac.ele.tue.nl>
 * Copyright (C) 2004, 05 Ralf Baechle DL5RB <ralf@linux-mips.org>
 * Copyright (C) 2004, 05 Thomas Osterried DL9SAU <thomas@x-berg.in-berlin.de>
 */
#include <linux/module.h>
#include <linux/bitops.h>
#include <asm/uaccess.h>
#include <linux/crc16.h>
#include <linux/string.h>
#include <linux/mm.h>
#include <linux/interrupt.h>
#include <linux/in.h>
#include <linux/inet.h>
#include <linux/slab.h>
#include <linux/tty.h>
#include <linux/errno.h>
#include <linux/netdevice.h>
#include <linux/major.h>
#include <linux/init.h>
#include <linux/rtnetlink.h>
#include <linux/etherdevice.h>
#include <linux/skbuff.h>
#include <linux/if_arp.h>
#include <linux/jiffies.h>
#include <linux/compat.h>

#include <net/ax25.h>

#define AX_MTU          236

/* SLIP/KISS protocol characters. */
#define END             0300            /* indicates end of frame       */
#define ESC             0333            /* indicates byte stuffing      */
#define ESC_END         0334            /* ESC ESC_END means END 'data' */
#define ESC_ESC         0335            /* ESC ESC_ESC means ESC 'data' */

struct mkiss {
        struct tty_struct       *tty;   /* ptr to TTY structure         */
        struct net_device       *dev;   /* easy for intr handling       */

        /* These are pointers to the malloc()ed frame buffers. */
        spinlock_t              buflock;/* lock for rbuf and xbuf */
        unsigned char           *rbuff; /* receiver buffer              */
        int                     rcount; /* received chars counter       */
        unsigned char           *xbuff; /* transmitter buffer           */
        unsigned char           *xhead; /* pointer to next byte to XMIT */
        int                     xleft;  /* bytes left in XMIT queue     */

        /* Detailed SLIP statistics. */
        int             mtu;            /* Our mtu (to spot changes!)   */
        int             buffsize;       /* Max buffers sizes            */

        unsigned long   flags;          /* Flag values/ mode etc        */
                                        /* long req'd: used by set_bit --RR */
#define AXF_INUSE       0               /* Channel in use               */
#define AXF_ESCAPE      1               /* ESC received                 */
#define AXF_ERROR       2               /* Parity, etc. error           */
#define AXF_KEEPTEST    3               /* Keepalive test flag          */
#define AXF_OUTWAIT     4               /* is outpacket was flag        */

        int             mode;
        int             crcmode;        /* MW: for FlexNet, SMACK etc.  */
        int             crcauto;        /* CRC auto mode */

#define CRC_MODE_NONE           0
#define CRC_MODE_FLEX           1
#define CRC_MODE_SMACK          2
#define CRC_MODE_FLEX_TEST      3
#define CRC_MODE_SMACK_TEST     4

        atomic_t                refcnt;
        struct semaphore        dead_sem;
};

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

static const unsigned short crc_flex_table[] = {
        0x0f87, 0x1e0e, 0x2c95, 0x3d1c, 0x49a3, 0x582a, 0x6ab1, 0x7b38,
        0x83cf, 0x9246, 0xa0dd, 0xb154, 0xc5eb, 0xd462, 0xe6f9, 0xf770,
        0x1f06, 0x0e8f, 0x3c14, 0x2d9d, 0x5922, 0x48ab, 0x7a30, 0x6bb9,
        0x934e, 0x82c7, 0xb05c, 0xa1d5, 0xd56a, 0xc4e3, 0xf678, 0xe7f1,
        0x2e85, 0x3f0c, 0x0d97, 0x1c1e, 0x68a1, 0x7928, 0x4bb3, 0x5a3a,
        0xa2cd, 0xb344, 0x81df, 0x9056, 0xe4e9, 0xf560, 0xc7fb, 0xd672,
        0x3e04, 0x2f8d, 0x1d16, 0x0c9f, 0x7820, 0x69a9, 0x5b32, 0x4abb,
        0xb24c, 0xa3c5, 0x915e, 0x80d7, 0xf468, 0xe5e1, 0xd77a, 0xc6f3,
        0x4d83, 0x5c0a, 0x6e91, 0x7f18, 0x0ba7, 0x1a2e, 0x28b5, 0x393c,
        0xc1cb, 0xd042, 0xe2d9, 0xf350, 0x87ef, 0x9666, 0xa4fd, 0xb574,
        0x5d02, 0x4c8b, 0x7e10, 0x6f99, 0x1b26, 0x0aaf, 0x3834, 0x29bd,
        0xd14a, 0xc0c3, 0xf258, 0xe3d1, 0x976e, 0x86e7, 0xb47c, 0xa5f5,
        0x6c81, 0x7d08, 0x4f93, 0x5e1a, 0x2aa5, 0x3b2c, 0x09b7, 0x183e,
        0xe0c9, 0xf140, 0xc3db, 0xd252, 0xa6ed, 0xb764, 0x85ff, 0x9476,
        0x7c00, 0x6d89, 0x5f12, 0x4e9b, 0x3a24, 0x2bad, 0x1936, 0x08bf,
        0xf048, 0xe1c1, 0xd35a, 0xc2d3, 0xb66c, 0xa7e5, 0x957e, 0x84f7,
        0x8b8f, 0x9a06, 0xa89d, 0xb914, 0xcdab, 0xdc22, 0xeeb9, 0xff30,
        0x07c7, 0x164e, 0x24d5, 0x355c, 0x41e3, 0x506a, 0x62f1, 0x7378,
        0x9b0e, 0x8a87, 0xb81c, 0xa995, 0xdd2a, 0xcca3, 0xfe38, 0xefb1,
        0x1746, 0x06cf, 0x3454, 0x25dd, 0x5162, 0x40eb, 0x7270, 0x63f9,
        0xaa8d, 0xbb04, 0x899f, 0x9816, 0xeca9, 0xfd20, 0xcfbb, 0xde32,
        0x26c5, 0x374c, 0x05d7, 0x145e, 0x60e1, 0x7168, 0x43f3, 0x527a,
        0xba0c, 0xab85, 0x991e, 0x8897, 0xfc28, 0xeda1, 0xdf3a, 0xceb3,
        0x3644, 0x27cd, 0x1556, 0x04df, 0x7060, 0x61e9, 0x5372, 0x42fb,
        0xc98b, 0xd802, 0xea99, 0xfb10, 0x8faf, 0x9e26, 0xacbd, 0xbd34,
        0x45c3, 0x544a, 0x66d1, 0x7758, 0x03e7, 0x126e, 0x20f5, 0x317c,
        0xd90a, 0xc883, 0xfa18, 0xeb91, 0x9f2e, 0x8ea7, 0xbc3c, 0xadb5,
        0x5542, 0x44cb, 0x7650, 0x67d9, 0x1366, 0x02ef, 0x3074, 0x21fd,
        0xe889, 0xf900, 0xcb9b, 0xda12, 0xaead, 0xbf24, 0x8dbf, 0x9c36,
        0x64c1, 0x7548, 0x47d3, 0x565a, 0x22e5, 0x336c, 0x01f7, 0x107e,
        0xf808, 0xe981, 0xdb1a, 0xca93, 0xbe2c, 0xafa5, 0x9d3e, 0x8cb7,
        0x7440, 0x65c9, 0x5752, 0x46db, 0x3264, 0x23ed, 0x1176, 0x00ff
};

static unsigned short calc_crc_flex(unsigned char *cp, int size)
{
        unsigned short crc = 0xffff;

        while (size--)
                crc = (crc << 8) ^ crc_flex_table[((crc >> 8) ^ *cp++) & 0xff];

        return crc;
}

static int check_crc_flex(unsigned char *cp, int size)
{
        unsigned short crc = 0xffff;

        if (size < 3)
                return -1;

        while (size--)
                crc = (crc << 8) ^ crc_flex_table[((crc >> 8) ^ *cp++) & 0xff];

        if ((crc & 0xffff) != 0x7070)
                return -1;

        return 0;
}

static int check_crc_16(unsigned char *cp, int size)
{
        unsigned short crc = 0x0000;

        if (size < 3)
                return -1;

        crc = crc16(0, cp, size);

        if (crc != 0x0000)
                return -1;

        return 0;
}

/*
 * Standard encapsulation
 */

static int kiss_esc(unsigned char *s, unsigned char *d, int len)
{
        unsigned char *ptr = d;
        unsigned char c;

        /*
         * Send an initial END character to flush out any data that may have
         * accumulated in the receiver due to line noise.
         */

        *ptr++ = END;

        while (len-- > 0) {
                switch (c = *s++) {
                case END:
                        *ptr++ = ESC;
                        *ptr++ = ESC_END;
                        break;
                case ESC:
                        *ptr++ = ESC;
                        *ptr++ = ESC_ESC;
                        break;
                default:
                        *ptr++ = c;
                        break;
                }
        }

        *ptr++ = END;

        return ptr - d;
}

/*
 * MW:
 * OK its ugly, but tell me a better solution without copying the
 * packet to a temporary buffer :-)
 */
static int kiss_esc_crc(unsigned char *s, unsigned char *d, unsigned short crc,
        int len)
{
        unsigned char *ptr = d;
        unsigned char c=0;

        *ptr++ = END;
        while (len > 0) {
                if (len > 2)
                        c = *s++;
                else if (len > 1)
                        c = crc >> 8;
                else if (len > 0)
                        c = crc & 0xff;

                len--;

                switch (c) {
                case END:
                        *ptr++ = ESC;
                        *ptr++ = ESC_END;
                        break;
                case ESC:
                        *ptr++ = ESC;
                        *ptr++ = ESC_ESC;
                        break;
                default:
                        *ptr++ = c;
                        break;
                }
        }
        *ptr++ = END;

        return ptr - d;
}

/* Send one completely decapsulated AX.25 packet to the AX.25 layer. */
static void ax_bump(struct mkiss *ax)
{
        struct sk_buff *skb;
        int count;

        spin_lock_bh(&ax->buflock);
        if (ax->rbuff[0] > 0x0f) {
                if (ax->rbuff[0] & 0x80) {
                        if (check_crc_16(ax->rbuff, ax->rcount) < 0) {
                                ax->dev->stats.rx_errors++;
                                spin_unlock_bh(&ax->buflock);

                                return;
                        }
                        if (ax->crcmode != CRC_MODE_SMACK && ax->crcauto) {
                                printk(KERN_INFO
                                       "mkiss: %s: Switching to crc-smack\n",
                                       ax->dev->name);
                                ax->crcmode = CRC_MODE_SMACK;
                        }
                        ax->rcount -= 2;
                        *ax->rbuff &= ~0x80;
                } else if (ax->rbuff[0] & 0x20)  {
                        if (check_crc_flex(ax->rbuff, ax->rcount) < 0) {
                                ax->dev->stats.rx_errors++;
                                spin_unlock_bh(&ax->buflock);
                                return;
                        }
                        if (ax->crcmode != CRC_MODE_FLEX && ax->crcauto) {
                                printk(KERN_INFO
                                       "mkiss: %s: Switching to crc-flexnet\n",
                                       ax->dev->name);
                                ax->crcmode = CRC_MODE_FLEX;
                        }
                        ax->rcount -= 2;

                        /*
                         * dl9sau bugfix: the trailling two bytes flexnet crc
                         * will not be passed to the kernel. thus we have to
                         * correct the kissparm signature, because it indicates
                         * a crc but there's none
                         */
                        *ax->rbuff &= ~0x20;
                }
        }

        count = ax->rcount;

        if ((skb = dev_alloc_skb(count)) == NULL) {
                printk(KERN_ERR "mkiss: %s: memory squeeze, dropping packet.\n",
                       ax->dev->name);
                ax->dev->stats.rx_dropped++;
                spin_unlock_bh(&ax->buflock);
                return;
        }

        memcpy(skb_put(skb,count), ax->rbuff, count);
        skb->protocol = ax25_type_trans(skb, ax->dev);
        netif_rx(skb);
        ax->dev->stats.rx_packets++;
        ax->dev->stats.rx_bytes += count;
        spin_unlock_bh(&ax->buflock);
}

static void kiss_unesc(struct mkiss *ax, unsigned char s)
{
        switch (s) {
        case END:
                /* drop keeptest bit = VSV */
                if (test_bit(AXF_KEEPTEST, &ax->flags))
                        clear_bit(AXF_KEEPTEST, &ax->flags);

                if (!test_and_clear_bit(AXF_ERROR, &ax->flags) && (ax->rcount > 2))
                        ax_bump(ax);

                clear_bit(AXF_ESCAPE, &ax->flags);
                ax->rcount = 0;
                return;

        case ESC:
                set_bit(AXF_ESCAPE, &ax->flags);
                return;
        case ESC_ESC:
                if (test_and_clear_bit(AXF_ESCAPE, &ax->flags))
                        s = ESC;
                break;
        case ESC_END:
                if (test_and_clear_bit(AXF_ESCAPE, &ax->flags))
                        s = END;
                break;
        }

        spin_lock_bh(&ax->buflock);
        if (!test_bit(AXF_ERROR, &ax->flags)) {
                if (ax->rcount < ax->buffsize) {
                        ax->rbuff[ax->rcount++] = s;
                        spin_unlock_bh(&ax->buflock);
                        return;
                }

                ax->dev->stats.rx_over_errors++;
                set_bit(AXF_ERROR, &ax->flags);
        }
        spin_unlock_bh(&ax->buflock);
}

static int ax_set_mac_address(struct net_device *dev, void *addr)
{
        struct sockaddr_ax25 *sa = addr;

        netif_tx_lock_bh(dev);
        netif_addr_lock(dev);
        memcpy(dev->dev_addr, &sa->sax25_call, AX25_ADDR_LEN);
        netif_addr_unlock(dev);
        netif_tx_unlock_bh(dev);

        return 0;
}

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

static void ax_changedmtu(struct mkiss *ax)
{
        struct net_device *dev = ax->dev;
        unsigned char *xbuff, *rbuff, *oxbuff, *orbuff;
        int len;

        len = dev->mtu * 2;

        /*
         * allow for arrival of larger UDP packets, even if we say not to
         * also fixes a bug in which SunOS sends 512-byte packets even with
         * an MSS of 128
         */
        if (len < 576 * 2)
                len = 576 * 2;

        xbuff = kmalloc(len + 4, GFP_ATOMIC);
        rbuff = kmalloc(len + 4, GFP_ATOMIC);

        if (xbuff == NULL || rbuff == NULL)  {
                printk(KERN_ERR "mkiss: %s: unable to grow ax25 buffers, "
                       "MTU change cancelled.\n",
                       ax->dev->name);
                dev->mtu = ax->mtu;
                kfree(xbuff);
                kfree(rbuff);
                return;
        }

        spin_lock_bh(&ax->buflock);

        oxbuff    = ax->xbuff;
        ax->xbuff = xbuff;
        orbuff    = ax->rbuff;
        ax->rbuff = rbuff;

        if (ax->xleft) {
                if (ax->xleft <= len) {
                        memcpy(ax->xbuff, ax->xhead, ax->xleft);
                } else  {
                        ax->xleft = 0;
                        dev->stats.tx_dropped++;
                }
        }

        ax->xhead = ax->xbuff;

        if (ax->rcount) {
                if (ax->rcount <= len) {
                        memcpy(ax->rbuff, orbuff, ax->rcount);
                } else  {
                        ax->rcount = 0;
                        dev->stats.rx_over_errors++;
                        set_bit(AXF_ERROR, &ax->flags);
                }
        }

        ax->mtu      = dev->mtu + 73;
        ax->buffsize = len;

        spin_unlock_bh(&ax->buflock);

        kfree(oxbuff);
        kfree(orbuff);
}

/* Encapsulate one AX.25 packet and stuff into a TTY queue. */
static void ax_encaps(struct net_device *dev, unsigned char *icp, int len)
{
        struct mkiss *ax = netdev_priv(dev);
        unsigned char *p;
        int actual, count;

        if (ax->mtu != ax->dev->mtu + 73)       /* Someone has been ifconfigging */
                ax_changedmtu(ax);

        if (len > ax->mtu) {            /* Sigh, shouldn't occur BUT ... */
                len = ax->mtu;
                printk(KERN_ERR "mkiss: %s: truncating oversized transmit packet!\n", ax->dev->name);
                dev->stats.tx_dropped++;
                netif_start_queue(dev);
                return;
        }

        p = icp;

        spin_lock_bh(&ax->buflock);
        if ((*p & 0x0f) != 0) {
                /* Configuration Command (kissparms(1).
                 * Protocol spec says: never append CRC.
                 * This fixes a very old bug in the linux
                 * kiss driver. -- dl9sau */
                switch (*p & 0xff) {
                case 0x85:
                        /* command from userspace especially for us,
                         * not for delivery to the tnc */
                        if (len > 1) {
                                int cmd = (p[1] & 0xff);
                                switch(cmd) {
                                case 3:
                                  ax->crcmode = CRC_MODE_SMACK;
                                  break;
                                case 2:
                                  ax->crcmode = CRC_MODE_FLEX;
                                  break;
                                case 1:
                                  ax->crcmode = CRC_MODE_NONE;
                                  break;
                                case 0:
                                default:
                                  ax->crcmode = CRC_MODE_SMACK_TEST;
                                  cmd = 0;
                                }
                                ax->crcauto = (cmd ? 0 : 1);
                                printk(KERN_INFO "mkiss: %s: crc mode %s %d\n", ax->dev->name, (len) ? "set to" : "is", cmd);
                        }
                        spin_unlock_bh(&ax->buflock);
                        netif_start_queue(dev);

                        return;
                default:
                        count = kiss_esc(p, ax->xbuff, len);
                }
        } else {
                unsigned short crc;
                switch (ax->crcmode) {
                case CRC_MODE_SMACK_TEST:
                        ax->crcmode  = CRC_MODE_FLEX_TEST;
                        printk(KERN_INFO "mkiss: %s: Trying crc-smack\n", ax->dev->name);
                        // fall through
                case CRC_MODE_SMACK:
                        *p |= 0x80;
                        crc = swab16(crc16(0, p, len));
                        count = kiss_esc_crc(p, ax->xbuff, crc, len+2);
                        break;
                case CRC_MODE_FLEX_TEST:
                        ax->crcmode = CRC_MODE_NONE;
                        printk(KERN_INFO "mkiss: %s: Trying crc-flexnet\n", ax->dev->name);
                        // fall through
                case CRC_MODE_FLEX:
                        *p |= 0x20;
                        crc = calc_crc_flex(p, len);
                        count = kiss_esc_crc(p, ax->xbuff, crc, len+2);
                        break;

                default:
                        count = kiss_esc(p, ax->xbuff, len);
                }
        }
        spin_unlock_bh(&ax->buflock);

        set_bit(TTY_DO_WRITE_WAKEUP, &ax->tty->flags);
        actual = ax->tty->ops->write(ax->tty, ax->xbuff, count);
        dev->stats.tx_packets++;
        dev->stats.tx_bytes += actual;

        ax->dev->trans_start = jiffies;
        ax->xleft = count - actual;
        ax->xhead = ax->xbuff + actual;
}

/* Encapsulate an AX.25 packet and kick it into a TTY queue. */
static netdev_tx_t ax_xmit(struct sk_buff *skb, struct net_device *dev)
{
        struct mkiss *ax = netdev_priv(dev);

        if (skb->protocol == htons(ETH_P_IP))
                return ax25_ip_xmit(skb);

        if (!netif_running(dev))  {
                printk(KERN_ERR "mkiss: %s: xmit call when iface is down\n", dev->name);
                return NETDEV_TX_BUSY;
        }

        if (netif_queue_stopped(dev)) {
                /*
                 * May be we must check transmitter timeout here ?
                 *      14 Oct 1994 Dmitry Gorodchanin.
                 */
                if (time_before(jiffies, dev->trans_start + 20 * HZ)) {
                        /* 20 sec timeout not reached */
                        return NETDEV_TX_BUSY;
                }

                printk(KERN_ERR "mkiss: %s: transmit timed out, %s?\n", dev->name,
                       (tty_chars_in_buffer(ax->tty) || ax->xleft) ?
                       "bad line quality" : "driver error");

                ax->xleft = 0;
                clear_bit(TTY_DO_WRITE_WAKEUP, &ax->tty->flags);
                netif_start_queue(dev);
        }

        /* We were not busy, so we are now... :-) */
        netif_stop_queue(dev);
        ax_encaps(dev, skb->data, skb->len);
        kfree_skb(skb);

        return NETDEV_TX_OK;
}

static int ax_open_dev(struct net_device *dev)
{
        struct mkiss *ax = netdev_priv(dev);

        if (ax->tty == NULL)
                return -ENODEV;

        return 0;
}

/* Open the low-level part of the AX25 channel. Easy! */
static int ax_open(struct net_device *dev)
{
        struct mkiss *ax = netdev_priv(dev);
        unsigned long len;

        if (ax->tty == NULL)
                return -ENODEV;

        /*
         * Allocate the frame buffers:
         *
         * rbuff        Receive buffer.
         * xbuff        Transmit buffer.
         */
        len = dev->mtu * 2;

        /*
         * allow for arrival of larger UDP packets, even if we say not to
         * also fixes a bug in which SunOS sends 512-byte packets even with
         * an MSS of 128
         */
        if (len < 576 * 2)
                len = 576 * 2;

        if ((ax->rbuff = kmalloc(len + 4, GFP_KERNEL)) == NULL)
                goto norbuff;

        if ((ax->xbuff = kmalloc(len + 4, GFP_KERNEL)) == NULL)
                goto noxbuff;

        ax->mtu      = dev->mtu + 73;
        ax->buffsize = len;
        ax->rcount   = 0;
        ax->xleft    = 0;

        ax->flags   &= (1 << AXF_INUSE);      /* Clear ESCAPE & ERROR flags */

        spin_lock_init(&ax->buflock);

        return 0;

noxbuff:
        kfree(ax->rbuff);

norbuff:
        return -ENOMEM;
}


/* Close the low-level part of the AX25 channel. Easy! */
static int ax_close(struct net_device *dev)
{
        struct mkiss *ax = netdev_priv(dev);

        if (ax->tty)
                clear_bit(TTY_DO_WRITE_WAKEUP, &ax->tty->flags);

        netif_stop_queue(dev);

        return 0;
}

static const struct net_device_ops ax_netdev_ops = {
        .ndo_open            = ax_open_dev,
        .ndo_stop            = ax_close,
        .ndo_start_xmit      = ax_xmit,
        .ndo_set_mac_address = ax_set_mac_address,
};

static void ax_setup(struct net_device *dev)
{
        /* Finish setting up the DEVICE info. */
        dev->mtu             = AX_MTU;
        dev->hard_header_len = 0;
        dev->addr_len        = 0;
        dev->type            = ARPHRD_AX25;
        dev->tx_queue_len    = 10;
        dev->header_ops      = &ax25_header_ops;
        dev->netdev_ops      = &ax_netdev_ops;


        memcpy(dev->broadcast, &ax25_bcast, AX25_ADDR_LEN);
        memcpy(dev->dev_addr,  &ax25_defaddr,  AX25_ADDR_LEN);

        dev->flags      = IFF_BROADCAST | IFF_MULTICAST;
}

/*
 * We have a potential race on dereferencing tty->disc_data, because the tty
 * layer provides no locking at all - thus one cpu could be running
 * sixpack_receive_buf while another calls sixpack_close, which zeroes
 * tty->disc_data and frees the memory that sixpack_receive_buf is using.  The
 * best way to fix this is to use a rwlock in the tty struct, but for now we
 * use a single global rwlock for all ttys in ppp line discipline.
 */
static DEFINE_RWLOCK(disc_data_lock);

static struct mkiss *mkiss_get(struct tty_struct *tty)
{
        struct mkiss *ax;

        read_lock(&disc_data_lock);
        ax = tty->disc_data;
        if (ax)
                atomic_inc(&ax->refcnt);
        read_unlock(&disc_data_lock);

        return ax;
}

static void mkiss_put(struct mkiss *ax)
{
        if (atomic_dec_and_test(&ax->refcnt))
                up(&ax->dead_sem);
}

static int crc_force = 0;       /* Can be overridden with insmod */

static int mkiss_open(struct tty_struct *tty)
{
        struct net_device *dev;
        struct mkiss *ax;
        int err;

        if (!capable(CAP_NET_ADMIN))
                return -EPERM;
        if (tty->ops->write == NULL)
                return -EOPNOTSUPP;

        dev = alloc_netdev(sizeof(struct mkiss), "ax%d", NET_NAME_UNKNOWN,
                           ax_setup);
        if (!dev) {
                err = -ENOMEM;
                goto out;
        }

        ax = netdev_priv(dev);
        ax->dev = dev;

        spin_lock_init(&ax->buflock);
        atomic_set(&ax->refcnt, 1);
        sema_init(&ax->dead_sem, 0);

        ax->tty = tty;
        tty->disc_data = ax;
        tty->receive_room = 65535;

        tty_driver_flush_buffer(tty);

        /* Restore default settings */
        dev->type = ARPHRD_AX25;

        /* Perform the low-level AX25 initialization. */
        err = ax_open(ax->dev);
        if (err)
                goto out_free_netdev;

        err = register_netdev(dev);
        if (err)
                goto out_free_buffers;

        /* after register_netdev() - because else printk smashes the kernel */
        switch (crc_force) {
        case 3:
                ax->crcmode  = CRC_MODE_SMACK;
                printk(KERN_INFO "mkiss: %s: crc mode smack forced.\n",
                       ax->dev->name);
                break;
        case 2:
                ax->crcmode  = CRC_MODE_FLEX;
                printk(KERN_INFO "mkiss: %s: crc mode flexnet forced.\n",
                       ax->dev->name);
                break;
        case 1:
                ax->crcmode  = CRC_MODE_NONE;
                printk(KERN_INFO "mkiss: %s: crc mode disabled.\n",
                       ax->dev->name);
                break;
        case 0:
                /* fall through */
        default:
                crc_force = 0;
                printk(KERN_INFO "mkiss: %s: crc mode is auto.\n",
                       ax->dev->name);
                ax->crcmode  = CRC_MODE_SMACK_TEST;
        }
        ax->crcauto = (crc_force ? 0 : 1);

        netif_start_queue(dev);

        /* Done.  We have linked the TTY line to a channel. */
        return 0;

out_free_buffers:
        kfree(ax->rbuff);
        kfree(ax->xbuff);

out_free_netdev:
        free_netdev(dev);

out:
        return err;
}

static void mkiss_close(struct tty_struct *tty)
{
        struct mkiss *ax;

        write_lock_bh(&disc_data_lock);
        ax = tty->disc_data;
        tty->disc_data = NULL;
        write_unlock_bh(&disc_data_lock);

        if (!ax)
                return;

        /*
         * We have now ensured that nobody can start using ap from now on, but
         * we have to wait for all existing users to finish.
         */
        if (!atomic_dec_and_test(&ax->refcnt))
                down(&ax->dead_sem);
        /*
         * Halt the transmit queue so that a new transmit cannot scribble
         * on our buffers
         */
        netif_stop_queue(ax->dev);

        /* Free all AX25 frame buffers. */
        kfree(ax->rbuff);
        kfree(ax->xbuff);

        ax->tty = NULL;

        unregister_netdev(ax->dev);
}

/* Perform I/O control on an active ax25 channel. */
static int mkiss_ioctl(struct tty_struct *tty, struct file *file,
        unsigned int cmd, unsigned long arg)
{
        struct mkiss *ax = mkiss_get(tty);
        struct net_device *dev;
        unsigned int tmp, err;

        /* First make sure we're connected. */
        if (ax == NULL)
                return -ENXIO;
        dev = ax->dev;

        switch (cmd) {
        case SIOCGIFNAME:
                err = copy_to_user((void __user *) arg, ax->dev->name,
                                   strlen(ax->dev->name) + 1) ? -EFAULT : 0;
                break;

        case SIOCGIFENCAP:
                err = put_user(4, (int __user *) arg);
                break;

        case SIOCSIFENCAP:
                if (get_user(tmp, (int __user *) arg)) {
                        err = -EFAULT;
                        break;
                }

                ax->mode = tmp;
                dev->addr_len        = AX25_ADDR_LEN;
                dev->hard_header_len = AX25_KISS_HEADER_LEN +
                                       AX25_MAX_HEADER_LEN + 3;
                dev->type            = ARPHRD_AX25;

                err = 0;
                break;

        case SIOCSIFHWADDR: {
                char addr[AX25_ADDR_LEN];

                if (copy_from_user(&addr,
                                   (void __user *) arg, AX25_ADDR_LEN)) {
                        err = -EFAULT;
                        break;
                }

                netif_tx_lock_bh(dev);
                memcpy(dev->dev_addr, addr, AX25_ADDR_LEN);
                netif_tx_unlock_bh(dev);

                err = 0;
                break;
        }
        default:
                err = -ENOIOCTLCMD;
        }

        mkiss_put(ax);

        return err;
}

#ifdef CONFIG_COMPAT
static long mkiss_compat_ioctl(struct tty_struct *tty, struct file *file,
        unsigned int cmd, unsigned long arg)
{
        switch (cmd) {
        case SIOCGIFNAME:
        case SIOCGIFENCAP:
        case SIOCSIFENCAP:
        case SIOCSIFHWADDR:
                return mkiss_ioctl(tty, file, cmd,
                                   (unsigned long)compat_ptr(arg));
        }

        return -ENOIOCTLCMD;
}
#endif

/*
 * Handle the 'receiver data ready' interrupt.
 * This function is called by the 'tty_io' module in the kernel when
 * a block of data has been received, which can now be decapsulated
 * and sent on to the AX.25 layer for further processing.
 */
static void mkiss_receive_buf(struct tty_struct *tty, const unsigned char *cp,
        char *fp, int count)
{
        struct mkiss *ax = mkiss_get(tty);

        if (!ax)
                return;

        /*
         * Argh! mtu change time! - costs us the packet part received
         * at the change
         */
        if (ax->mtu != ax->dev->mtu + 73)
                ax_changedmtu(ax);

        /* Read the characters out of the buffer */
        while (count--) {
                if (fp != NULL && *fp++) {
                        if (!test_and_set_bit(AXF_ERROR, &ax->flags))
                                ax->dev->stats.rx_errors++;
                        cp++;
                        continue;
                }

                kiss_unesc(ax, *cp++);
        }

        mkiss_put(ax);
        tty_unthrottle(tty);
}

/*
 * Called by the driver when there's room for more data.  If we have
 * more packets to send, we send them here.
 */
static void mkiss_write_wakeup(struct tty_struct *tty)
{
        struct mkiss *ax = mkiss_get(tty);
        int actual;

        if (!ax)
                return;

        if (ax->xleft <= 0)  {
                /* Now serial buffer is almost free & we can start
                 * transmission of another packet
                 */
                clear_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);

                netif_wake_queue(ax->dev);
                goto out;
        }

        actual = tty->ops->write(tty, ax->xhead, ax->xleft);
        ax->xleft -= actual;
        ax->xhead += actual;

out:
        mkiss_put(ax);
}

static struct tty_ldisc_ops ax_ldisc = {
        .owner          = THIS_MODULE,
        .magic          = TTY_LDISC_MAGIC,
        .name           = "mkiss",
        .open           = mkiss_open,
        .close          = mkiss_close,
        .ioctl          = mkiss_ioctl,
#ifdef CONFIG_COMPAT
        .compat_ioctl   = mkiss_compat_ioctl,
#endif
        .receive_buf    = mkiss_receive_buf,
        .write_wakeup   = mkiss_write_wakeup
};

static const char banner[] __initconst = KERN_INFO \
        "mkiss: AX.25 Multikiss, Hans Albas PE1AYX\n";
static const char msg_regfail[] __initconst = KERN_ERR \
        "mkiss: can't register line discipline (err = %d)\n";

static int __init mkiss_init_driver(void)
{
        int status;

        printk(banner);

        status = tty_register_ldisc(N_AX25, &ax_ldisc);
        if (status != 0)
                printk(msg_regfail, status);

        return status;
}

static const char msg_unregfail[] = KERN_ERR \
        "mkiss: can't unregister line discipline (err = %d)\n";

static void __exit mkiss_exit_driver(void)
{
        int ret;

        if ((ret = tty_unregister_ldisc(N_AX25)))
                printk(msg_unregfail, ret);
}

MODULE_AUTHOR("Ralf Baechle DL5RB <ralf@linux-mips.org>");
MODULE_DESCRIPTION("KISS driver for AX.25 over TTYs");
module_param(crc_force, int, 0);
MODULE_PARM_DESC(crc_force, "crc [0 = auto | 1 = none | 2 = flexnet | 3 = smack]");
MODULE_LICENSE("GPL");
MODULE_ALIAS_LDISC(N_AX25);

module_init(mkiss_init_driver);
module_exit(mkiss_exit_driver);