rawhdlc.c
/* rawhdlc.c.shtml,v 1.1.1.1 2002/08/19 05:50:13 fritz Exp
* 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
