# rawhdlc.c

    /* $Id$

     * rawhdlc.c     support routines for cards that don't support HDLC
     *
     * Author     Karsten Keil (keil@temic-ech.spacenet.de)
     *            Brent Baccala <baccala@FreeSoft.org>
     *
     *
     * Some passive ISDN cards, such as the Traverse NETJet and the AMD 7930,
     * don't perform HDLC encapsulation over the B channel.  Drivers for
     * such cards use support routines in this file to perform B channel HDLC.
     *
     * Bit-synchronous HDLC encapsulation is a means of encapsulating packets
     * over a continuously transmitting serial communications link.
     * It looks like this:
     *
     *      11111111101111110...........0111111011111111111
     *      iiiiiiiiiffffffffdddddddddddffffffffiiiiiiiiiii
     *
     *      i = idle     f = flag     d = data
     *
     * When idle, the channel sends a continuous string of ones (mark
     * idle; illustrated), or a continuous string of flag characters (flag
     * idle).  The beginning of a data frame is marked by a flag character
     * (01111110), then comes the actual data, followed by another flag
     * character, after which another frame may be sent immediately (a
     * single flag may serve as both the end of one frame and the start of
     * the next), or the link may return to idle.  Obviously, the flag
     * character can not appear anywhere in the data (or a false
     * end-of-frame would occur), so the transmitter performs
     * "bit-stuffing" - inserting a zero bit after every five one bits,
     * irregardless of the original bit after the five ones.  Byte
     * ordering is irrelevent at this point - the data is treated as a
     * string of bits, not bytes.  Since no more than 5 ones may now occur
     * in a row, the flag sequence, with its 6 ones, is unique.
     *
     * Upon reception, a zero bit that occur after 5 one bits is simply
     * discarded.  A series of 6 one bits is end-of-frame, and a series of
     * 7 one bits is an abort.  Once bit-stuffing has been corrected for,
     * an integer number of bytes should now be present.  The last two
     * of these bytes form the Frame Check Sequence, a CRC that is verified
     * and then discarded.  Note that bit-stuffing is performed on the FCS
     * just as if it were regular data.
     *
     *
     *
     * int make_raw_hdlc_data(u_char *src, u_int slen,
     *                        u_char *dst, u_int dsize)
     *
     *   Used for transmission.  Copies slen bytes from src to dst, performing
     *   HDLC encapsulation (flag bytes, bit-stuffing, CRC) in the process.
     *   dsize is size of destination buffer, and should be at least
     *   ((6*slen)/5)+5 bytes to ensure adequate space will be available.
     *   Function returns length (in bytes) of valid destination buffer, or
     *   0 upon destination overflow.
     *
     * void init_hdlc_state(struct hdlc_state *stateptr, int mode)
     *
     *   Initializes hdlc_state structure before first call to read_raw_hdlc_data
     *
     *   mode = 0: Sane mode
     *   mode = 1/2:
     *             Insane mode; NETJet use a shared unsigned int memory block (
     * 	       with busmaster DMA), the bit pattern of every word is
     *  	       <8 B1> <8 B2> <8 Mon> <2 D> <4 C/I> <MX> <MR>
     *	       according to Siemens IOM-2 interface, so we have to handle
     *             the src buffer as unsigned int and have to shift/mask the
     *             B-channel bytes.
     *             mode 1 -> B1  mode 2  -> B2 data is used
     *
     * int read_raw_hdlc_data(struct hdlc_state *saved_state,
     *                        u_char *src, u_int slen,
     *                        u_char *dst, u_int dsize)
     *
     *   Used for reception.  Scans source buffer bit-by-bit looking for
     *   valid HDLC frames, which are copied to destination buffer.  HDLC
     *   state information is stored in a structure, which allows this
     *   function to process frames spread across several blocks of raw
     *   HDLC data.  Part of the state information is bit offsets into
     *   the source and destination buffers.
     *
     *   A return value >0 indicates the length of a valid frame, now
     *   stored in the destination buffer.  In this case, the source
     *   buffer might not be completely processed, so this function should
     *   be called again with the same source buffer, possibly with a
     *   different destination buffer.
     *
     *   A return value of zero indicates that the source buffer was
     *   completely processed without finding a valid end-of-packet;
     *   however, we might be in the middle of packet reception, so
     *   the function should be called again with the next block of
     *   raw HDLC data and the same destination buffer.  It is NOT
     *   permitted to change the destination buffer in this case,
     *   since data may already have begun to be stored there.
     *
     *   A return value of -1 indicates some kind of error - destination
     *   buffer overflow, CRC check failed, frame not a multiple of 8
     *   bits.  Destination buffer probably contains invalid data, which
     *   should be discarded.  Call function again with same source buffer
     *   and a new (or same) destination buffer.
     *
     *   Suggested calling sequence:
     *
     *      init_hdlc_state(...);
     *      for (EACH_RAW_DATA_BLOCK) {
     *         while (len = read_raw_hdlc_data(...)) {
     *             if (len == -1) DISCARD_FRAME;
     *             else PROCESS_FRAME;
     *         }
     *      }
     *
     *
     * Test the code in this file as follows:
     *    gcc -DDEBUGME -o rawhdlctest rawhdlc.c
     *    ./rawhdlctest < rawdata
     *
     * The file "rawdata" can be easily generated from a HISAX B-channel
     * hex dump (CF CF CF 02 ...) using the following perl script:
     *
     * while(<>) {
     *     @hexlist = split ' ';
     *     while ($hexstr = shift(@hexlist)) {
     *         printf "%c", hex($hexstr);
     *     }
     * }
     *
     */

    #ifdef DEBUGME
    #include <stdio.h>
    #endif

    #include <linux/types.h>
    #include <linux/ppp_defs.h>
    #include "rawhdlc.h"

    /* There's actually an identical copy of this table in the PPP code
     * (ppp_crc16_table), but I don't want this code dependant on PPP
     */

    // static
    __u16 fcstab[256] =
    {
    	0x0000, 0x1189, 0x2312, 0x329b, 0x4624, 0x57ad, 0x6536, 0x74bf,
    	0x8c48, 0x9dc1, 0xaf5a, 0xbed3, 0xca6c, 0xdbe5, 0xe97e, 0xf8f7,
    	0x1081, 0x0108, 0x3393, 0x221a, 0x56a5, 0x472c, 0x75b7, 0x643e,
    	0x9cc9, 0x8d40, 0xbfdb, 0xae52, 0xdaed, 0xcb64, 0xf9ff, 0xe876,
    	0x2102, 0x308b, 0x0210, 0x1399, 0x6726, 0x76af, 0x4434, 0x55bd,
    	0xad4a, 0xbcc3, 0x8e58, 0x9fd1, 0xeb6e, 0xfae7, 0xc87c, 0xd9f5,
    	0x3183, 0x200a, 0x1291, 0x0318, 0x77a7, 0x662e, 0x54b5, 0x453c,
    	0xbdcb, 0xac42, 0x9ed9, 0x8f50, 0xfbef, 0xea66, 0xd8fd, 0xc974,
    	0x4204, 0x538d, 0x6116, 0x709f, 0x0420, 0x15a9, 0x2732, 0x36bb,
    	0xce4c, 0xdfc5, 0xed5e, 0xfcd7, 0x8868, 0x99e1, 0xab7a, 0xbaf3,
    	0x5285, 0x430c, 0x7197, 0x601e, 0x14a1, 0x0528, 0x37b3, 0x263a,
    	0xdecd, 0xcf44, 0xfddf, 0xec56, 0x98e9, 0x8960, 0xbbfb, 0xaa72,
    	0x6306, 0x728f, 0x4014, 0x519d, 0x2522, 0x34ab, 0x0630, 0x17b9,
    	0xef4e, 0xfec7, 0xcc5c, 0xddd5, 0xa96a, 0xb8e3, 0x8a78, 0x9bf1,
    	0x7387, 0x620e, 0x5095, 0x411c, 0x35a3, 0x242a, 0x16b1, 0x0738,
    	0xffcf, 0xee46, 0xdcdd, 0xcd54, 0xb9eb, 0xa862, 0x9af9, 0x8b70,
    	0x8408, 0x9581, 0xa71a, 0xb693, 0xc22c, 0xd3a5, 0xe13e, 0xf0b7,
    	0x0840, 0x19c9, 0x2b52, 0x3adb, 0x4e64, 0x5fed, 0x6d76, 0x7cff,
    	0x9489, 0x8500, 0xb79b, 0xa612, 0xd2ad, 0xc324, 0xf1bf, 0xe036,
    	0x18c1, 0x0948, 0x3bd3, 0x2a5a, 0x5ee5, 0x4f6c, 0x7df7, 0x6c7e,
    	0xa50a, 0xb483, 0x8618, 0x9791, 0xe32e, 0xf2a7, 0xc03c, 0xd1b5,
    	0x2942, 0x38cb, 0x0a50, 0x1bd9, 0x6f66, 0x7eef, 0x4c74, 0x5dfd,
    	0xb58b, 0xa402, 0x9699, 0x8710, 0xf3af, 0xe226, 0xd0bd, 0xc134,
    	0x39c3, 0x284a, 0x1ad1, 0x0b58, 0x7fe7, 0x6e6e, 0x5cf5, 0x4d7c,
    	0xc60c, 0xd785, 0xe51e, 0xf497, 0x8028, 0x91a1, 0xa33a, 0xb2b3,
    	0x4a44, 0x5bcd, 0x6956, 0x78df, 0x0c60, 0x1de9, 0x2f72, 0x3efb,
    	0xd68d, 0xc704, 0xf59f, 0xe416, 0x90a9, 0x8120, 0xb3bb, 0xa232,
    	0x5ac5, 0x4b4c, 0x79d7, 0x685e, 0x1ce1, 0x0d68, 0x3ff3, 0x2e7a,
    	0xe70e, 0xf687, 0xc41c, 0xd595, 0xa12a, 0xb0a3, 0x8238, 0x93b1,
    	0x6b46, 0x7acf, 0x4854, 0x59dd, 0x2d62, 0x3ceb, 0x0e70, 0x1ff9,
    	0xf78f, 0xe606, 0xd49d, 0xc514, 0xb1ab, 0xa022, 0x92b9, 0x8330,
    	0x7bc7, 0x6a4e, 0x58d5, 0x495c, 0x3de3, 0x2c6a, 0x1ef1, 0x0f78
    };

    #define HDLC_ZERO_SEARCH 0
    #define HDLC_FLAG_SEARCH 1
    #define HDLC_FLAG_FOUND  2
    #define HDLC_FRAME_FOUND 3
    #define HDLC_NULL 4
    #define HDLC_PART 5
    #define HDLC_FULL 6

    #define HDLC_FLAG_VALUE	0x7e

    #define MAKE_RAW_BYTE for (j=0; j<8; j++) { \
    			bitcnt++;\
    			out_val >>= 1;\
    			if (val & 1) {\
    				s_one++;\
    				out_val |= 0x80;\
    			} else {\
    				s_one = 0;\
    				out_val &= 0x7f;\
    			}\
    			if (bitcnt==8) {\
    				if (d_cnt == dsize) return 0;\
    				dst[d_cnt++] = out_val;\
    				bitcnt = 0;\
    			}\
    			if (s_one == 5) {\
    				out_val >>= 1;\
    				out_val &= 0x7f;\
    				bitcnt++;\
    				s_one = 0;\
    			}\
    			if (bitcnt==8) {\
    				if (d_cnt == dsize) return 0;\
    				dst[d_cnt++] = out_val;\
    				bitcnt = 0;\
    			}\
    			val >>= 1;\
    		}

    /* Optimization suggestion: If needed, this function could be
     * dramatically sped up using a state machine.  Each state would
     * correspond to having seen N one bits, and being offset M bits into
     * the current output byte.  N ranges from 0 to 4, M from 0 to 7, so
     * we need 5*8 = 35 states.  Each state would have a table with 256
     * entries, one for each input character.  Each entry would contain
     * three output characters, an output state, an a byte increment
     * that's either 1 or 2.  All this could fit in four bytes; so we need
     * 4 bytes * 256 characters = 1 KB for each state (35 KB total).  Zero
     * the output buffer before you start.  For each character in your
     * input, you look it up in the current state's table and get three
     * bytes to be or'ed into the output at the current byte offset, and
     * an byte increment to move your pointer forward.  A simple Perl
     * script could generate the tables.  Given HDLC semantics, probably
     * would be better to set output to all 1s, then use ands instead of ors.
     * A smaller state machine could operate on nibbles instead of bytes.
     * A state machine for 32-bit architectures could use word offsets
     * instead of byte offsets, requiring 5*32 = 160 states; probably
     * best to work on nibbles in such a case.
     */

    int make_raw_hdlc_data(u_char *src, u_int slen, u_char *dst, u_int dsize)
    {
    	register u_int i,d_cnt=0;
    	register u_char j;
    	register u_char val;
    	register u_char s_one = 0;
    	register u_char out_val = 0;
    	register u_char bitcnt = 0;
    	u_int fcs;

    	dst[d_cnt++] = HDLC_FLAG_VALUE;
    	fcs = PPP_INITFCS;
    	for (i=0; i<slen; i++) {
    		val = src[i];
    		fcs = PPP_FCS (fcs, val);
    		MAKE_RAW_BYTE;
    	}
    	fcs ^= 0xffff;
    	val = fcs & 0xff;
    	MAKE_RAW_BYTE;
    	val = (fcs>>8) & 0xff;
    	MAKE_RAW_BYTE;
    	val = HDLC_FLAG_VALUE;
    	for (j=0; j<8; j++) {
    		bitcnt++;
    		out_val >>= 1;
    		if (val & 1)
    			out_val |= 0x80;
    		else
    			out_val &= 0x7f;
    		if (bitcnt==8) {
    			if (d_cnt == dsize) return 0;
    			dst[d_cnt++] = out_val;
    			bitcnt = 0;
    		}
    		val >>= 1;
    	}
    	if (bitcnt) {
    		while (8>bitcnt++) {
    			out_val >>= 1;
    			out_val |= 0x80;
    		}
    		if (d_cnt == dsize) return 0;
    		dst[d_cnt++] = out_val;
    	}

    	return d_cnt;
    }

    void init_hdlc_state(struct hdlc_state *stateptr, int mode)
    {
    	stateptr->state = HDLC_ZERO_SEARCH;
    	stateptr->r_one = 0;
    	stateptr->r_val = 0;
    	stateptr->o_bitcnt = 0;
    	stateptr->i_bitcnt = 0;
    	stateptr->insane_mode = mode;
    }

    /* Optimization suggestion: A similar state machine could surely
     * be developed for this function as well.
     */

    int read_raw_hdlc_data(struct hdlc_state *saved_state,
                           u_char *src, u_int slen, u_char *dst, u_int dsize)
    {
    	int retval=0;
    	register u_char val;
    	register u_char state = saved_state->state;
    	register u_char r_one = saved_state->r_one;
    	register u_char r_val = saved_state->r_val;
    	register u_int o_bitcnt = saved_state->o_bitcnt;
    	register u_int i_bitcnt = saved_state->i_bitcnt;
    	register u_int fcs    = saved_state->fcs;
    	register u_int *isrc = (u_int *) src;

    	/* Use i_bitcnt (bit offset into source buffer) to reload "val"
    	 * in case we're starting up again partway through a source buffer
    	 */

    	if ((i_bitcnt >> 3) < slen) {
    		if (saved_state->insane_mode==1) {
    			val = isrc[(i_bitcnt >> 3)] & 0xff;
    		} else if (saved_state->insane_mode==2) {
    			val = (isrc[i_bitcnt >> 3] >>8) & 0xff;
    		} else {
    			val = src[i_bitcnt >> 3];
    		}
    		val >>= i_bitcnt & 7;
    	}

    	/* One bit per loop.  Keep going until we've got something to
    	 * report (retval != 0), or we exhaust the source buffer
    	 */

    	while ((retval == 0) && ((i_bitcnt >> 3) < slen)) {
    		if ((i_bitcnt & 7) == 0) {
    			if (saved_state->insane_mode==1) {
    				val = isrc[(i_bitcnt >> 3)] & 0xff;
    			} else if (saved_state->insane_mode==2) {
    				val = (isrc[i_bitcnt >> 3] >>8) & 0xff;
    			} else {
    				val = src[i_bitcnt >> 3];
    			}
    #ifdef DEBUGME
    			printf("Input byte %d: 0x%2x\n", i_bitcnt>>3, val);
    #endif
    			if (val == 0xff) {
    				state = HDLC_ZERO_SEARCH;
    				o_bitcnt = 0;
    				r_one = 0;
    				i_bitcnt += 8;
    				continue;
    			}
    		}

    #ifdef DEBUGME
    		/* printf("Data bit=%d (%d/%d)\n", val&1, i_bitcnt>>3, i_bitcnt&7);*/
    #endif

    		if (state == HDLC_ZERO_SEARCH) {
    			if (val & 1) {
    				r_one++;
    			} else {
    				r_one=0;
    				state= HDLC_FLAG_SEARCH;
    			}
    		} else if (state == HDLC_FLAG_SEARCH) {
    			if (val & 1) {
    				r_one++;
    				if (r_one>6) {
    					state=HDLC_ZERO_SEARCH;
    				}
    			} else {
    				if (r_one==6) {
    					o_bitcnt=0;
    					r_val=0;
    					state=HDLC_FLAG_FOUND;
    				}
    				r_one=0;
    			}
    		} else if (state ==  HDLC_FLAG_FOUND) {
    			if (val & 1) {
    				r_one++;
    				if (r_one>6) {
    					state=HDLC_ZERO_SEARCH;
    				} else {
    					r_val >>= 1;
    					r_val |= 0x80;
    					o_bitcnt++;
    				}
    			} else {
    				if (r_one==6) {
    					o_bitcnt=0;
    					r_val=0;
    					r_one=0;
    					i_bitcnt++;
    					val >>= 1;
    					continue;
    				} else if (r_one!=5) {
    					r_val >>= 1;
    					r_val &= 0x7f;
    					o_bitcnt++;
    				}
    				r_one=0;
    			}
    			if ((state != HDLC_ZERO_SEARCH) &&
    				!(o_bitcnt & 7)) {
    #ifdef DEBUGME
    				printf("HDLC_FRAME_FOUND at i_bitcnt:%d\n",i_bitcnt);
    #endif
    				state=HDLC_FRAME_FOUND;
    				fcs = PPP_INITFCS;
    				dst[0] = r_val;
    				fcs = PPP_FCS (fcs, r_val);
    			}
    		} else if (state ==  HDLC_FRAME_FOUND) {
    			if (val & 1) {
    				r_one++;
    				if (r_one>6) {
    					state=HDLC_ZERO_SEARCH;
    					o_bitcnt=0;
    				} else {
    					r_val >>= 1;
    					r_val |= 0x80;
    					o_bitcnt++;
    				}
    			} else {
    				if (r_one==6) {
    					r_val=0;
    					r_one=0;
    					o_bitcnt++;
    					if (o_bitcnt & 7) {
    						/* Alignment error */
    #ifdef DEBUGME
    						printf("Alignment error\n");
    #endif
    						state=HDLC_FLAG_SEARCH;
    						retval = -1;
    					} else if (fcs==PPP_GOODFCS) {
    						/* Valid frame */
    						state=HDLC_FLAG_FOUND;
    						retval = (o_bitcnt>>3)-3;
    					} else {
    						/* CRC error */
    #ifdef DEBUGME
    						printf("CRC error; fcs was 0x%x, should have been 0x%x\n", fcs, PPP_GOODFCS);
    #endif
    						state=HDLC_FLAG_FOUND;
    						retval = -1;
    					}
    				} else if (r_one==5) {
    					r_one=0;
    					i_bitcnt++;
    					val >>= 1;
    					continue;
    				} else {
    					r_val >>= 1;
    					r_val &= 0x7f;
    					o_bitcnt++;
    				}
    				r_one=0;
    			}
    			if ((state == HDLC_FRAME_FOUND) &&
    				!(o_bitcnt & 7)) {
    				if ((o_bitcnt>>3)>=dsize) {
    					/* Buffer overflow error */
    #ifdef DEBUGME
    					printf("Buffer overflow error\n");
    #endif
    					r_val=0;
    					state=HDLC_FLAG_SEARCH;
    					retval = -1;
    				} else {
    					dst[(o_bitcnt>>3)-1] = r_val;
    					fcs = PPP_FCS (fcs, r_val);
    #ifdef DEBUGME
    					printf("Output byte %d: 0x%02x; FCS 0x%04x\n", (o_bitcnt>>3)-1, r_val, fcs);
    #endif
    				}
    			}
    		}
    		i_bitcnt ++;
    		val >>= 1;
    	}

    	/* We exhausted the source buffer before anything else happened
    	 * (retval==0).  Reset i_bitcnt in expectation of a new source
    	 * buffer.  Other, we either had an error or a valid frame, so
    	 * reset o_bitcnt in expectation of a new destination buffer.
    	 */

    	if (retval == 0) {
    		i_bitcnt = 0;
    	} else {
    		o_bitcnt = 0;
    	}

    	saved_state->state = state;
    	saved_state->r_one = r_one;
    	saved_state->r_val = r_val;
    	saved_state->fcs = fcs;
    	saved_state->o_bitcnt = o_bitcnt;
    	saved_state->i_bitcnt = i_bitcnt;

    	return (retval);
    }

    #ifdef DEBUGME

    char buffer[1024];
    char obuffer[1024];

    main()
    {
      int buflen=0;
      int len;
      struct hdlc_state hdlc_state;

      while((buffer[buflen] = getc(stdin)) != EOF && buflen<1024) buflen++;

      printf("buflen = %d\n", buflen);

      init_hdlc_state(&hdlc_state, 0);

      while (len = read_raw_hdlc_data(&hdlc_state,buffer,buflen,obuffer,1024)) {
        if (len == -1) printf("Error @ byte %d/bit %d\n",
    			  hdlc_state.i_bitcnt>>3, hdlc_state.i_bitcnt & 7);
        else {
          printf("Frame received: len %d\n", len);
        }
      }

      printf("Done\n");
    }

    #endif