drivers/net/ppp_synctty.c

   1 /*
   2  * PPP synchronous tty channel driver for Linux.
   3  *
   4  * This is a ppp channel driver that can be used with tty device drivers
   5  * that are frame oriented, such as synchronous HDLC devices.
   6  *
   7  * Complete PPP frames without encoding/decoding are exchanged between
   8  * the channel driver and the device driver.
   9  *
  10  * The async map IOCTL codes are implemented to keep the user mode
  11  * applications happy if they call them. Synchronous PPP does not use
  12  * the async maps.
  13  *
  14  * Copyright 1999 Paul Mackerras.
  15  *
  16  * Also touched by the grubby hands of Paul Fulghum paulkf@microgate.com
  17  *
  18  *  This program is free software; you can redistribute it and/or
  19  *  modify it under the terms of the GNU General Public License
  20  *  as published by the Free Software Foundation; either version
  21  *  2 of the License, or (at your option) any later version.
  22  *
  23  * This driver provides the encapsulation and framing for sending
  24  * and receiving PPP frames over sync serial lines.  It relies on
  25  * the generic PPP layer to give it frames to send and to process
  26  * received frames.  It implements the PPP line discipline.
  27  *
  28  * Part of the code in this driver was inspired by the old async-only
  29  * PPP driver, written by Michael Callahan and Al Longyear, and
  30  * subsequently hacked by Paul Mackerras.
  31  *
  32  * ==FILEVERSION 20040616==
  33  */
  34
  35 #include <linux/module.h>
  36 #include <linux/kernel.h>
  37 #include <linux/skbuff.h>
  38 #include <linux/tty.h>
  39 #include <linux/netdevice.h>
  40 #include <linux/poll.h>
  41 #include <linux/ppp_defs.h>
  42 #include <linux/if_ppp.h>
  43 #include <linux/ppp_channel.h>
  44 #include <linux/spinlock.h>
  45 #include <linux/completion.h>
  46 #include <linux/init.h>
  47 #include <asm/uaccess.h>
  48
  49 #define PPP_VERSION     "2.4.2"
  50
  51 /* Structure for storing local state. */
  52 struct syncppp {
  53         struct tty_struct *tty;
  54         unsigned int    flags;
  55         unsigned int    rbits;
  56         int             mru;
  57         spinlock_t      xmit_lock;
  58         spinlock_t      recv_lock;
  59         unsigned long   xmit_flags;
  60         u32             xaccm[8];
  61         u32             raccm;
  62         unsigned int    bytes_sent;
  63         unsigned int    bytes_rcvd;
  64
  65         struct sk_buff  *tpkt;
  66         unsigned long   last_xmit;
  67
  68         struct sk_buff_head rqueue;
  69
  70         struct tasklet_struct tsk;
  71
  72         atomic_t        refcnt;
  73         struct completion dead_cmp;
  74         struct ppp_channel chan;        /* interface to generic ppp layer */
  75 };
  76
  77 /* Bit numbers in xmit_flags */
  78 #define XMIT_WAKEUP     0
  79 #define XMIT_FULL       1
  80
  81 /* Bits in rbits */
  82 #define SC_RCV_BITS     (SC_RCV_B7_1|SC_RCV_B7_0|SC_RCV_ODDP|SC_RCV_EVNP)
  83
  84 #define PPPSYNC_MAX_RQLEN       32      /* arbitrary */
  85
  86 /*
  87  * Prototypes.
  88  */
  89 static struct sk_buff* ppp_sync_txmunge(struct syncppp *ap, struct sk_buff *);
  90 static int ppp_sync_send(struct ppp_channel *chan, struct sk_buff *skb);
  91 static int ppp_sync_ioctl(struct ppp_channel *chan, unsigned int cmd,
  92                           unsigned long arg);
  93 static void ppp_sync_process(unsigned long arg);
  94 static int ppp_sync_push(struct syncppp *ap);
  95 static void ppp_sync_flush_output(struct syncppp *ap);
  96 static void ppp_sync_input(struct syncppp *ap, const unsigned char *buf,
  97                            char *flags, int count);
  98
  99 static struct ppp_channel_ops sync_ops = {
 100         ppp_sync_send,
 101         ppp_sync_ioctl
 102 };
 103
 104 /*
 105  * Utility procedures to print a buffer in hex/ascii
 106  */
 107 static void
 108 ppp_print_hex (register __u8 * out, const __u8 * in, int count)
 109 {
 110         register __u8 next_ch;
 111         static const char hex[] = "0123456789ABCDEF";
 112
 113         while (count-- > 0) {
 114                 next_ch = *in++;
 115                 *out++ = hex[(next_ch >> 4) & 0x0F];
 116                 *out++ = hex[next_ch & 0x0F];
 117                 ++out;
 118         }
 119 }
 120
 121 static void
 122 ppp_print_char (register __u8 * out, const __u8 * in, int count)
 123 {
 124         register __u8 next_ch;
 125
 126         while (count-- > 0) {
 127                 next_ch = *in++;
 128
 129                 if (next_ch < 0x20 || next_ch > 0x7e)
 130                         *out++ = '.';
 131                 else {
 132                         *out++ = next_ch;
 133                         if (next_ch == '%')   /* printk/syslogd has a bug !! */
 134                                 *out++ = '%';
 135                 }
 136         }
 137         *out = '\0';
 138 }
 139
 140 static void
 141 ppp_print_buffer (const char *name, const __u8 *buf, int count)
 142 {
 143         __u8 line[44];
 144
 145         if (name != NULL)
 146                 printk(KERN_DEBUG "ppp_synctty: %s, count = %d\n", name, count);
 147
 148         while (count > 8) {
 149                 memset (line, 32, 44);
 150                 ppp_print_hex (line, buf, 8);
 151                 ppp_print_char (&line[8 * 3], buf, 8);
 152                 printk(KERN_DEBUG "%s\n", line);
 153                 count -= 8;
 154                 buf += 8;
 155         }
 156
 157         if (count > 0) {
 158                 memset (line, 32, 44);
 159                 ppp_print_hex (line, buf, count);
 160                 ppp_print_char (&line[8 * 3], buf, count);
 161                 printk(KERN_DEBUG "%s\n", line);
 162         }
 163 }
 164
 165
 166 /*
 167  * Routines implementing the synchronous PPP line discipline.
 168  */
 169
 170 /*
 171  * We have a potential race on dereferencing tty->disc_data,
 172  * because the tty layer provides no locking at all - thus one
 173  * cpu could be running ppp_synctty_receive while another
 174  * calls ppp_synctty_close, which zeroes tty->disc_data and
 175  * frees the memory that ppp_synctty_receive is using.  The best
 176  * way to fix this is to use a rwlock in the tty struct, but for now
 177  * we use a single global rwlock for all ttys in ppp line discipline.
 178  *
 179  * FIXME: Fixed in tty_io nowdays.
 180  */
 181 static DEFINE_RWLOCK(disc_data_lock);
 182
 183 static struct syncppp *sp_get(struct tty_struct *tty)
 184 {
 185         struct syncppp *ap;
 186
 187         read_lock(&disc_data_lock);
 188         ap = tty->disc_data;
 189         if (ap != NULL)
 190                 atomic_inc(&ap->refcnt);
 191         read_unlock(&disc_data_lock);
 192         return ap;
 193 }
 194
 195 static void sp_put(struct syncppp *ap)
 196 {
 197         if (atomic_dec_and_test(&ap->refcnt))
 198                 complete(&ap->dead_cmp);
 199 }
 200
 201 /*
 202  * Called when a tty is put into sync-PPP line discipline.
 203  */
 204 static int
 205 ppp_sync_open(struct tty_struct *tty)
 206 {
 207         struct syncppp *ap;
 208         int err;
 209         int speed;
 210
 211         if (tty->ops->write == NULL)
 212                 return -EOPNOTSUPP;
 213
 214         ap = kzalloc(sizeof(*ap), GFP_KERNEL);
 215         err = -ENOMEM;
 216         if (!ap)
 217                 goto out;
 218
 219         /* initialize the syncppp structure */
 220         ap->tty = tty;
 221         ap->mru = PPP_MRU;
 222         spin_lock_init(&ap->xmit_lock);
 223         spin_lock_init(&ap->recv_lock);
 224         ap->xaccm[0] = ~0U;
 225         ap->xaccm[3] = 0x60000000U;
 226         ap->raccm = ~0U;
 227
 228         skb_queue_head_init(&ap->rqueue);
 229         tasklet_init(&ap->tsk, ppp_sync_process, (unsigned long) ap);
 230
 231         atomic_set(&ap->refcnt, 1);
 232         init_completion(&ap->dead_cmp);
 233
 234         ap->chan.private = ap;
 235         ap->chan.ops = &sync_ops;
 236         ap->chan.mtu = PPP_MRU;
 237         ap->chan.hdrlen = 2;    /* for A/C bytes */
 238         speed = tty_get_baud_rate(tty);
 239         ap->chan.speed = speed;
 240         err = ppp_register_channel(&ap->chan);
 241         if (err)
 242                 goto out_free;
 243
 244         tty->disc_data = ap;
 245         tty->receive_room = 65536;
 246         return 0;
 247
 248  out_free:
 249         kfree(ap);
 250  out:
 251         return err;
 252 }
 253
 254 /*
 255  * Called when the tty is put into another line discipline
 256  * or it hangs up.  We have to wait for any cpu currently
 257  * executing in any of the other ppp_synctty_* routines to
 258  * finish before we can call ppp_unregister_channel and free
 259  * the syncppp struct.  This routine must be called from
 260  * process context, not interrupt or softirq context.
 261  */
 262 static void
 263 ppp_sync_close(struct tty_struct *tty)
 264 {
 265         struct syncppp *ap;
 266
 267         write_lock_irq(&disc_data_lock);
 268         ap = tty->disc_data;
 269         tty->disc_data = NULL;
 270         write_unlock_irq(&disc_data_lock);
 271         if (!ap)
 272                 return;
 273
 274         /*
 275          * We have now ensured that nobody can start using ap from now
 276          * on, but we have to wait for all existing users to finish.
 277          * Note that ppp_unregister_channel ensures that no calls to
 278          * our channel ops (i.e. ppp_sync_send/ioctl) are in progress
 279          * by the time it returns.
 280          */
 281         if (!atomic_dec_and_test(&ap->refcnt))
 282                 wait_for_completion(&ap->dead_cmp);
 283         tasklet_kill(&ap->tsk);
 284
 285         ppp_unregister_channel(&ap->chan);
 286         skb_queue_purge(&ap->rqueue);
 287         kfree_skb(ap->tpkt);
 288         kfree(ap);
 289 }
 290
 291 /*
 292  * Called on tty hangup in process context.
 293  *
 294  * Wait for I/O to driver to complete and unregister PPP channel.
 295  * This is already done by the close routine, so just call that.
 296  */
 297 static int ppp_sync_hangup(struct tty_struct *tty)
 298 {
 299         ppp_sync_close(tty);
 300         return 0;
 301 }
 302
 303 /*
 304  * Read does nothing - no data is ever available this way.
 305  * Pppd reads and writes packets via /dev/ppp instead.
 306  */
 307 static ssize_t
 308 ppp_sync_read(struct tty_struct *tty, struct file *file,
 309                unsigned char __user *buf, size_t count)
 310 {
 311         return -EAGAIN;
 312 }
 313
 314 /*
 315  * Write on the tty does nothing, the packets all come in
 316  * from the ppp generic stuff.
 317  */
 318 static ssize_t
 319 ppp_sync_write(struct tty_struct *tty, struct file *file,
 320                 const unsigned char *buf, size_t count)
 321 {
 322         return -EAGAIN;
 323 }
 324
 325 static int
 326 ppp_synctty_ioctl(struct tty_struct *tty, struct file *file,
 327                   unsigned int cmd, unsigned long arg)
 328 {
 329         struct syncppp *ap = sp_get(tty);
 330         int __user *p = (int __user *)arg;
 331         int err, val;
 332
 333         if (!ap)
 334                 return -ENXIO;
 335         err = -EFAULT;
 336         switch (cmd) {
 337         case PPPIOCGCHAN:
 338                 err = -EFAULT;
 339                 if (put_user(ppp_channel_index(&ap->chan), p))
 340                         break;
 341                 err = 0;
 342                 break;
 343
 344         case PPPIOCGUNIT:
 345                 err = -EFAULT;
 346                 if (put_user(ppp_unit_number(&ap->chan), p))
 347                         break;
 348                 err = 0;
 349                 break;
 350
 351         case TCFLSH:
 352                 /* flush our buffers and the serial port's buffer */
 353                 if (arg == TCIOFLUSH || arg == TCOFLUSH)
 354                         ppp_sync_flush_output(ap);
 355                 err = tty_perform_flush(tty, arg);
 356                 break;
 357
 358         case FIONREAD:
 359                 val = 0;
 360                 if (put_user(val, p))
 361                         break;
 362                 err = 0;
 363                 break;
 364
 365         default:
 366                 err = tty_mode_ioctl(tty, file, cmd, arg);
 367                 break;
 368         }
 369
 370         sp_put(ap);
 371         return err;
 372 }
 373
 374 /* No kernel lock - fine */
 375 static unsigned int
 376 ppp_sync_poll(struct tty_struct *tty, struct file *file, poll_table *wait)
 377 {
 378         return 0;
 379 }
 380
 381 /*
 382  * This can now be called from hard interrupt level as well
 383  * as soft interrupt level or mainline.
 384  */
 385 static void
 386 ppp_sync_receive(struct tty_struct *tty, const unsigned char *buf,
 387                   char *cflags, int count)
 388 {
 389         struct syncppp *ap = sp_get(tty);
 390         unsigned long flags;
 391
 392         if (!ap)
 393                 return;
 394         spin_lock_irqsave(&ap->recv_lock, flags);
 395         ppp_sync_input(ap, buf, cflags, count);
 396         spin_unlock_irqrestore(&ap->recv_lock, flags);
 397         if (!skb_queue_empty(&ap->rqueue))
 398                 tasklet_schedule(&ap->tsk);
 399         sp_put(ap);
 400         tty_unthrottle(tty);
 401 }
 402
 403 static void
 404 ppp_sync_wakeup(struct tty_struct *tty)
 405 {
 406         struct syncppp *ap = sp_get(tty);
 407
 408         clear_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
 409         if (!ap)
 410                 return;
 411         set_bit(XMIT_WAKEUP, &ap->xmit_flags);
 412         tasklet_schedule(&ap->tsk);
 413         sp_put(ap);
 414 }
 415
 416
 417 static struct tty_ldisc_ops ppp_sync_ldisc = {
 418         .owner  = THIS_MODULE,
 419         .magic  = TTY_LDISC_MAGIC,
 420         .name   = "pppsync",
 421         .open   = ppp_sync_open,
 422         .close  = ppp_sync_close,
 423         .hangup = ppp_sync_hangup,
 424         .read   = ppp_sync_read,
 425         .write  = ppp_sync_write,
 426         .ioctl  = ppp_synctty_ioctl,
 427         .poll   = ppp_sync_poll,
 428         .receive_buf = ppp_sync_receive,
 429         .write_wakeup = ppp_sync_wakeup,
 430 };
 431
 432 static int __init
 433 ppp_sync_init(void)
 434 {
 435         int err;
 436
 437         err = tty_register_ldisc(N_SYNC_PPP, &ppp_sync_ldisc);
 438         if (err != 0)
 439                 printk(KERN_ERR "PPP_sync: error %d registering line disc.\n",
 440                        err);
 441         return err;
 442 }
 443
 444 /*
 445  * The following routines provide the PPP channel interface.
 446  */
 447 static int
 448 ppp_sync_ioctl(struct ppp_channel *chan, unsigned int cmd, unsigned long arg)
 449 {
 450         struct syncppp *ap = chan->private;
 451         int err, val;
 452         u32 accm[8];
 453         void __user *argp = (void __user *)arg;
 454         u32 __user *p = argp;
 455
 456         err = -EFAULT;
 457         switch (cmd) {
 458         case PPPIOCGFLAGS:
 459                 val = ap->flags | ap->rbits;
 460                 if (put_user(val, (int __user *) argp))
 461                         break;
 462                 err = 0;
 463                 break;
 464         case PPPIOCSFLAGS:
 465                 if (get_user(val, (int __user *) argp))
 466                         break;
 467                 ap->flags = val & ~SC_RCV_BITS;
 468                 spin_lock_irq(&ap->recv_lock);
 469                 ap->rbits = val & SC_RCV_BITS;
 470                 spin_unlock_irq(&ap->recv_lock);
 471                 err = 0;
 472                 break;
 473
 474         case PPPIOCGASYNCMAP:
 475                 if (put_user(ap->xaccm[0], p))
 476                         break;
 477                 err = 0;
 478                 break;
 479         case PPPIOCSASYNCMAP:
 480                 if (get_user(ap->xaccm[0], p))
 481                         break;
 482                 err = 0;
 483                 break;
 484
 485         case PPPIOCGRASYNCMAP:
 486                 if (put_user(ap->raccm, p))
 487                         break;
 488                 err = 0;
 489                 break;
 490         case PPPIOCSRASYNCMAP:
 491                 if (get_user(ap->raccm, p))
 492                         break;
 493                 err = 0;
 494                 break;
 495
 496         case PPPIOCGXASYNCMAP:
 497                 if (copy_to_user(argp, ap->xaccm, sizeof(ap->xaccm)))
 498                         break;
 499                 err = 0;
 500                 break;
 501         case PPPIOCSXASYNCMAP:
 502                 if (copy_from_user(accm, argp, sizeof(accm)))
 503                         break;
 504                 accm[2] &= ~0x40000000U;        /* can't escape 0x5e */
 505                 accm[3] |= 0x60000000U;         /* must escape 0x7d, 0x7e */
 506                 memcpy(ap->xaccm, accm, sizeof(ap->xaccm));
 507                 err = 0;
 508                 break;
 509
 510         case PPPIOCGMRU:
 511                 if (put_user(ap->mru, (int __user *) argp))
 512                         break;
 513                 err = 0;
 514                 break;
 515         case PPPIOCSMRU:
 516                 if (get_user(val, (int __user *) argp))
 517                         break;
 518                 if (val < PPP_MRU)
 519                         val = PPP_MRU;
 520                 ap->mru = val;
 521                 err = 0;
 522                 break;
 523
 524         default:
 525                 err = -ENOTTY;
 526         }
 527         return err;
 528 }
 529
 530 /*
 531  * This is called at softirq level to deliver received packets
 532  * to the ppp_generic code, and to tell the ppp_generic code
 533  * if we can accept more output now.
 534  */
 535 static void ppp_sync_process(unsigned long arg)
 536 {
 537         struct syncppp *ap = (struct syncppp *) arg;
 538         struct sk_buff *skb;
 539
 540         /* process received packets */
 541         while ((skb = skb_dequeue(&ap->rqueue)) != NULL) {
 542                 if (skb->len == 0) {
 543                         /* zero length buffers indicate error */
 544                         ppp_input_error(&ap->chan, 0);
 545                         kfree_skb(skb);
 546                 }
 547                 else
 548                         ppp_input(&ap->chan, skb);
 549         }
 550
 551         /* try to push more stuff out */
 552         if (test_bit(XMIT_WAKEUP, &ap->xmit_flags) && ppp_sync_push(ap))
 553                 ppp_output_wakeup(&ap->chan);
 554 }
 555
 556 /*
 557  * Procedures for encapsulation and framing.
 558  */
 559
 560 static struct sk_buff*
 561 ppp_sync_txmunge(struct syncppp *ap, struct sk_buff *skb)
 562 {
 563         int proto;
 564         unsigned char *data;
 565         int islcp;
 566
 567         data  = skb->data;
 568         proto = (data[0] << 8) + data[1];
 569
 570         /* LCP packets with codes between 1 (configure-request)
 571          * and 7 (code-reject) must be sent as though no options
 572          * have been negotiated.
 573          */
 574         islcp = proto == PPP_LCP && 1 <= data[2] && data[2] <= 7;
 575
 576         /* compress protocol field if option enabled */
 577         if (data[0] == 0 && (ap->flags & SC_COMP_PROT) && !islcp)
 578                 skb_pull(skb,1);
 579
 580         /* prepend address/control fields if necessary */
 581         if ((ap->flags & SC_COMP_AC) == 0 || islcp) {
 582                 if (skb_headroom(skb) < 2) {
 583                         struct sk_buff *npkt = dev_alloc_skb(skb->len + 2);
 584                         if (npkt == NULL) {
 585                                 kfree_skb(skb);
 586                                 return NULL;
 587                         }
 588                         skb_reserve(npkt,2);
 589                         skb_copy_from_linear_data(skb,
 590                                       skb_put(npkt, skb->len), skb->len);
 591                         kfree_skb(skb);
 592                         skb = npkt;
 593                 }
 594                 skb_push(skb,2);
 595                 skb->data[0] = PPP_ALLSTATIONS;
 596                 skb->data[1] = PPP_UI;
 597         }
 598
 599         ap->last_xmit = jiffies;
 600
 601         if (skb && ap->flags & SC_LOG_OUTPKT)
 602                 ppp_print_buffer ("send buffer", skb->data, skb->len);
 603
 604         return skb;
 605 }
 606
 607 /*
 608  * Transmit-side routines.
 609  */
 610
 611 /*
 612  * Send a packet to the peer over an sync tty line.
 613  * Returns 1 iff the packet was accepted.
 614  * If the packet was not accepted, we will call ppp_output_wakeup
 615  * at some later time.
 616  */
 617 static int
 618 ppp_sync_send(struct ppp_channel *chan, struct sk_buff *skb)
 619 {
 620         struct syncppp *ap = chan->private;
 621
 622         ppp_sync_push(ap);
 623
 624         if (test_and_set_bit(XMIT_FULL, &ap->xmit_flags))
 625                 return 0;       /* already full */
 626         skb = ppp_sync_txmunge(ap, skb);
 627         if (skb != NULL)
 628                 ap->tpkt = skb;
 629         else
 630                 clear_bit(XMIT_FULL, &ap->xmit_flags);
 631
 632         ppp_sync_push(ap);
 633         return 1;
 634 }
 635
 636 /*
 637  * Push as much data as possible out to the tty.
 638  */
 639 static int
 640 ppp_sync_push(struct syncppp *ap)
 641 {
 642         int sent, done = 0;
 643         struct tty_struct *tty = ap->tty;
 644         int tty_stuffed = 0;
 645
 646         if (!spin_trylock_bh(&ap->xmit_lock))
 647                 return 0;
 648         for (;;) {
 649                 if (test_and_clear_bit(XMIT_WAKEUP, &ap->xmit_flags))
 650                         tty_stuffed = 0;
 651                 if (!tty_stuffed && ap->tpkt) {
 652                         set_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
 653                         sent = tty->ops->write(tty, ap->tpkt->data, ap->tpkt->len);
 654                         if (sent < 0)
 655                                 goto flush;     /* error, e.g. loss of CD */
 656                         if (sent < ap->tpkt->len) {
 657                                 tty_stuffed = 1;
 658                         } else {
 659                                 kfree_skb(ap->tpkt);
 660                                 ap->tpkt = NULL;
 661                                 clear_bit(XMIT_FULL, &ap->xmit_flags);
 662                                 done = 1;
 663                         }
 664                         continue;
 665                 }
 666                 /* haven't made any progress */
 667                 spin_unlock_bh(&ap->xmit_lock);
 668                 if (!(test_bit(XMIT_WAKEUP, &ap->xmit_flags)
 669                       || (!tty_stuffed && ap->tpkt)))
 670                         break;
 671                 if (!spin_trylock_bh(&ap->xmit_lock))
 672                         break;
 673         }
 674         return done;
 675
 676 flush:
 677         if (ap->tpkt) {
 678                 kfree_skb(ap->tpkt);
 679                 ap->tpkt = NULL;
 680                 clear_bit(XMIT_FULL, &ap->xmit_flags);
 681                 done = 1;
 682         }
 683         spin_unlock_bh(&ap->xmit_lock);
 684         return done;
 685 }
 686
 687 /*
 688  * Flush output from our internal buffers.
 689  * Called for the TCFLSH ioctl.
 690  */
 691 static void
 692 ppp_sync_flush_output(struct syncppp *ap)
 693 {
 694         int done = 0;
 695
 696         spin_lock_bh(&ap->xmit_lock);
 697         if (ap->tpkt != NULL) {
 698                 kfree_skb(ap->tpkt);
 699                 ap->tpkt = NULL;
 700                 clear_bit(XMIT_FULL, &ap->xmit_flags);
 701                 done = 1;
 702         }
 703         spin_unlock_bh(&ap->xmit_lock);
 704         if (done)
 705                 ppp_output_wakeup(&ap->chan);
 706 }
 707
 708 /*
 709  * Receive-side routines.
 710  */
 711
 712 /* called when the tty driver has data for us.
 713  *
 714  * Data is frame oriented: each call to ppp_sync_input is considered
 715  * a whole frame. If the 1st flag byte is non-zero then the whole
 716  * frame is considered to be in error and is tossed.
 717  */
 718 static void
 719 ppp_sync_input(struct syncppp *ap, const unsigned char *buf,
 720                 char *flags, int count)
 721 {
 722         struct sk_buff *skb;
 723         unsigned char *p;
 724
 725         if (count == 0)
 726                 return;
 727
 728         if (ap->flags & SC_LOG_INPKT)
 729                 ppp_print_buffer ("receive buffer", buf, count);
 730
 731         /* stuff the chars in the skb */
 732         skb = dev_alloc_skb(ap->mru + PPP_HDRLEN + 2);
 733         if (!skb) {
 734                 printk(KERN_ERR "PPPsync: no memory (input pkt)\n");
 735                 goto err;
 736         }
 737         /* Try to get the payload 4-byte aligned */
 738         if (buf[0] != PPP_ALLSTATIONS)
 739                 skb_reserve(skb, 2 + (buf[0] & 1));
 740
 741         if (flags && *flags) {
 742                 /* error flag set, ignore frame */
 743                 goto err;
 744         } else if (count > skb_tailroom(skb)) {
 745                 /* packet overflowed MRU */
 746                 goto err;
 747         }
 748
 749         p = skb_put(skb, count);
 750         memcpy(p, buf, count);
 751
 752         /* strip address/control field if present */
 753         p = skb->data;
 754         if (p[0] == PPP_ALLSTATIONS && p[1] == PPP_UI) {
 755                 /* chop off address/control */
 756                 if (skb->len < 3)
 757                         goto err;
 758                 p = skb_pull(skb, 2);
 759         }
 760
 761         /* decompress protocol field if compressed */
 762         if (p[0] & 1) {
 763                 /* protocol is compressed */
 764                 skb_push(skb, 1)[0] = 0;
 765         } else if (skb->len < 2)
 766                 goto err;
 767
 768         /* queue the frame to be processed */
 769         skb_queue_tail(&ap->rqueue, skb);
 770         return;
 771
 772 err:
 773         /* queue zero length packet as error indication */
 774         if (skb || (skb = dev_alloc_skb(0))) {
 775                 skb_trim(skb, 0);
 776                 skb_queue_tail(&ap->rqueue, skb);
 777         }
 778 }
 779
 780 static void __exit
 781 ppp_sync_cleanup(void)
 782 {
 783         if (tty_unregister_ldisc(N_SYNC_PPP) != 0)
 784                 printk(KERN_ERR "failed to unregister Sync PPP line discipline\n");
 785 }
 786
 787 module_init(ppp_sync_init);
 788 module_exit(ppp_sync_cleanup);
 789 MODULE_LICENSE("GPL");
 790 MODULE_ALIAS_LDISC(N_SYNC_PPP);