src/core/isdnhdlc.c

/*
 * isdnhdlc.c  --  General purpose ISDN HDLC decoder.
 *
 * Copyright (C)
 *	2009	Karsten Keil		<keil@b1-systems.de>
 *	2002	Wolfgang Mües		<wolfgang@iksw-muees.de>
 *	2001	Frode Isaksen		<fisaksen@bewan.com>
 *      2001	Kai Germaschewski	<kai.germaschewski@gmx.de>
 *
 * slightly adapted for use in userspace / osmocom envrionment by Harald Welte
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that 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.
 */

#include <string.h>

#include <osmocom/core/crc16.h>
#include <osmocom/core/bits.h>
#include <osmocom/core/isdnhdlc.h>

enum {
	HDLC_FAST_IDLE, HDLC_GET_FLAG_B0, HDLC_GETFLAG_B1A6, HDLC_GETFLAG_B7,
	HDLC_GET_DATA, HDLC_FAST_FLAG
};

enum {
	HDLC_SEND_DATA, HDLC_SEND_CRC1, HDLC_SEND_FAST_FLAG,
	HDLC_SEND_FIRST_FLAG, HDLC_SEND_CRC2, HDLC_SEND_CLOSING_FLAG,
	HDLC_SEND_IDLE1, HDLC_SEND_FAST_IDLE, HDLC_SENDFLAG_B0,
	HDLC_SENDFLAG_B1A6, HDLC_SENDFLAG_B7, STOPPED, HDLC_SENDFLAG_ONE
};

#define crc_ccitt_byte osmo_crc16_ccitt_byte

void osmo_isdnhdlc_rcv_init(struct osmo_isdnhdlc_vars *hdlc, uint32_t features)
{
	memset(hdlc, 0, sizeof(*hdlc));
	hdlc->state = HDLC_GET_DATA;
	if (features & OSMO_HDLC_F_56KBIT)
		hdlc->do_adapt56 = 1;
	if (features & OSMO_HDLC_F_BITREVERSE)
		hdlc->do_bitreverse = 1;
}

void osmo_isdnhdlc_out_init(struct osmo_isdnhdlc_vars *hdlc, uint32_t features)
{
	memset(hdlc, 0, sizeof(*hdlc));
	if (features & OSMO_HDLC_F_DCHANNEL) {
		hdlc->dchannel = 1;
		hdlc->state = HDLC_SEND_FIRST_FLAG;
	} else {
		hdlc->dchannel = 0;
		hdlc->state = HDLC_SEND_FAST_FLAG;
		hdlc->ffvalue = 0x7e;
	}
	hdlc->cbin = 0x7e;
	if (features & OSMO_HDLC_F_56KBIT) {
		hdlc->do_adapt56 = 1;
		hdlc->state = HDLC_SENDFLAG_B0;
	} else
		hdlc->data_bits = 8;
	if (features & OSMO_HDLC_F_BITREVERSE)
		hdlc->do_bitreverse = 1;
}

static int
check_frame(struct osmo_isdnhdlc_vars *hdlc)
{
	int status;

	if (hdlc->dstpos < 2)	/* too small - framing error */
		status = -OSMO_HDLC_FRAMING_ERROR;
	else if (hdlc->crc != 0xf0b8)	/* crc error */
		status = -OSMO_HDLC_CRC_ERROR;
	else {
		/* remove CRC */
		hdlc->dstpos -= 2;
		/* good frame */
		status = hdlc->dstpos;
	}
	return status;
}

/*! decodes HDLC frames from a transparent bit stream.

  The source buffer is scanned for valid HDLC frames looking for
  flags (01111110) to indicate the start of a frame. If the start of
  the frame is found, the bit stuffing is removed (0 after 5 1's).
  When a new flag is found, the complete frame has been received
  and the CRC is checked.
  If a valid frame is found, the function returns the frame length
  excluding the CRC.
  If the beginning of a valid frame is found, the function returns
  the length.
  If a framing error is found (too many 1s and not a flag) the function
  returns -OSMO_HDLC_FRAMING_ERROR.
  If a CRC error is found the function returns -OSMO_HDLC_CRC_ERROR.
  If the frame length exceeds the destination buffer size, the function
  returns the length with the bit OSMO_HDLC_LENGTH_ERROR set.

  \param[in] src source buffer
  \param[in] slen source buffer length
  \param[out] count number of bytes removed (decoded) from the source buffer
  \param[out] dst destination buffer
  \param[in] dsize destination buffer size
  \returns number of decoded bytes in the destination buffer and status flag.
*/
int osmo_isdnhdlc_decode(struct osmo_isdnhdlc_vars *hdlc, const uint8_t *src, int slen,
			 int *count, uint8_t *dst, int dsize)
{
	int status = 0;

	static const unsigned char fast_flag[] = {
		0x00, 0x00, 0x00, 0x20, 0x30, 0x38, 0x3c, 0x3e, 0x3f
	};

	static const unsigned char fast_flag_value[] = {
		0x00, 0x7e, 0xfc, 0xf9, 0xf3, 0xe7, 0xcf, 0x9f, 0x3f
	};

	static const unsigned char fast_abort[] = {
		0x00, 0x00, 0x80, 0xc0, 0xe0, 0xf0, 0xf8, 0xfc, 0xfe, 0xff
	};

#define handle_fast_flag(h)						\
	do {								\
		if (h->cbin == fast_flag[h->bit_shift]) {		\
			h->ffvalue = fast_flag_value[h->bit_shift];	\
			h->state = HDLC_FAST_FLAG;			\
			h->ffbit_shift = h->bit_shift;			\
			h->bit_shift = 1;				\
		} else {						\
			h->state = HDLC_GET_DATA;			\
			h->data_received = 0;				\
		}							\
	} while (0)

#define handle_abort(h)						\
	do {							\
		h->shift_reg = fast_abort[h->ffbit_shift - 1];	\
		h->hdlc_bits1 = h->ffbit_shift - 2;		\
		if (h->hdlc_bits1 < 0)				\
			h->hdlc_bits1 = 0;			\
		h->data_bits = h->ffbit_shift - 1;		\
		h->state = HDLC_GET_DATA;			\
		h->data_received = 0;				\
	} while (0)

	*count = slen;

	while (slen > 0) {
		if (hdlc->bit_shift == 0) {
			/* the code is for bitreverse streams */
			if (hdlc->do_bitreverse == 0)
				hdlc->cbin = osmo_revbytebits_8(*src++);
			else
				hdlc->cbin = *src++;
			slen--;
			hdlc->bit_shift = 8;
			if (hdlc->do_adapt56)
				hdlc->bit_shift--;
		}

		switch (hdlc->state) {
		case STOPPED:
			return 0;
		case HDLC_FAST_IDLE:
			if (hdlc->cbin == 0xff) {
				hdlc->bit_shift = 0;
				break;
			}
			hdlc->state = HDLC_GET_FLAG_B0;
			hdlc->hdlc_bits1 = 0;
			hdlc->bit_shift = 8;
			break;
		case HDLC_GET_FLAG_B0:
			if (!(hdlc->cbin & 0x80)) {
				hdlc->state = HDLC_GETFLAG_B1A6;
				hdlc->hdlc_bits1 = 0;
			} else {
				if ((!hdlc->do_adapt56) &&
				    (++hdlc->hdlc_bits1 >= 8) &&
				    (hdlc->bit_shift == 1))
					hdlc->state = HDLC_FAST_IDLE;
			}
			hdlc->cbin <<= 1;
			hdlc->bit_shift--;
			break;
		case HDLC_GETFLAG_B1A6:
			if (hdlc->cbin & 0x80) {
				hdlc->hdlc_bits1++;
				if (hdlc->hdlc_bits1 == 6)
					hdlc->state = HDLC_GETFLAG_B7;
			} else
				hdlc->hdlc_bits1 = 0;
			hdlc->cbin <<= 1;
			hdlc->bit_shift--;
			break;
		case HDLC_GETFLAG_B7:
			if (hdlc->cbin & 0x80) {
				hdlc->state = HDLC_GET_FLAG_B0;
			} else {
				hdlc->state = HDLC_GET_DATA;
				hdlc->crc = 0xffff;
				hdlc->shift_reg = 0;
				hdlc->hdlc_bits1 = 0;
				hdlc->data_bits = 0;
				hdlc->data_received = 0;
			}
			hdlc->cbin <<= 1;
			hdlc->bit_shift--;
			break;
		case HDLC_GET_DATA:
			if (hdlc->cbin & 0x80) {
				hdlc->hdlc_bits1++;
				switch (hdlc->hdlc_bits1) {
				case 6:
					break;
				case 7:
					if (hdlc->data_received)
						/* bad frame */
						status = -OSMO_HDLC_FRAMING_ERROR;
					if (!hdlc->do_adapt56) {
						if (hdlc->cbin == fast_abort
						    [hdlc->bit_shift + 1]) {
							hdlc->state =
								HDLC_FAST_IDLE;
							hdlc->bit_shift = 1;
							break;
						}
					} else
						hdlc->state = HDLC_GET_FLAG_B0;
					break;
				default:
					hdlc->shift_reg >>= 1;
					hdlc->shift_reg |= 0x80;
					hdlc->data_bits++;
					break;
				}
			} else {
				switch (hdlc->hdlc_bits1) {
				case 5:
					break;
				case 6:
					if (hdlc->data_received)
						status = check_frame(hdlc);
					hdlc->crc = 0xffff;
					hdlc->shift_reg = 0;
					hdlc->data_bits = 0;
					if (!hdlc->do_adapt56)
						handle_fast_flag(hdlc);
					else {
						hdlc->state = HDLC_GET_DATA;
						hdlc->data_received = 0;
					}
					break;
				default:
					hdlc->shift_reg >>= 1;
					hdlc->data_bits++;
					break;
				}
				hdlc->hdlc_bits1 = 0;
			}
			if (status) {
				hdlc->dstpos = 0;
				*count -= slen;
				hdlc->cbin <<= 1;
				hdlc->bit_shift--;
				return status;
			}
			if (hdlc->data_bits == 8) {
				hdlc->data_bits = 0;
				hdlc->data_received = 1;
				hdlc->crc = crc_ccitt_byte(hdlc->crc,
							   hdlc->shift_reg);

				/* good byte received */
				if (hdlc->dstpos < dsize)
					dst[hdlc->dstpos++] = hdlc->shift_reg;
				else {
					/* frame too long */
					status = -OSMO_HDLC_LENGTH_ERROR;
					hdlc->dstpos = 0;
				}
			}
			hdlc->cbin <<= 1;
			hdlc->bit_shift--;
			break;
		case HDLC_FAST_FLAG:
			if (hdlc->cbin == hdlc->ffvalue) {
				hdlc->bit_shift = 0;
				break;
			} else {
				if (hdlc->cbin == 0xff) {
					hdlc->state = HDLC_FAST_IDLE;
					hdlc->bit_shift = 0;
				} else if (hdlc->ffbit_shift == 8) {
					hdlc->state = HDLC_GETFLAG_B7;
					break;
				} else
					handle_abort(hdlc);
			}
			break;
		default:
			break;
		}
	}
	*count -= slen;
	return 0;
}
/*! encodes HDLC frames to a transparent bit stream.

  The bit stream starts with a beginning flag (01111110). After
  that each byte is added to the bit stream with bit stuffing added
  (0 after 5 1's).
  When the last byte has been removed from the source buffer, the
  CRC (2 bytes is added) and the frame terminates with the ending flag.
  For the dchannel, the idle character (all 1's) is also added at the end.
  If this function is called with empty source buffer (slen=0), flags or
  idle character will be generated.

  \param[in] src source buffer
  \param[in] slen source buffer length
  \param[out] count number of bytes removed (encoded) from source buffer
  \param[out] dst destination buffer
  \param[in] dsize destination buffer size
  \returns - number of encoded bytes in the destination buffer
*/
int osmo_isdnhdlc_encode(struct osmo_isdnhdlc_vars *hdlc, const uint8_t *src, uint16_t slen,
			 int *count, uint8_t *dst, int dsize)
{
	static const unsigned char xfast_flag_value[] = {
		0x7e, 0x3f, 0x9f, 0xcf, 0xe7, 0xf3, 0xf9, 0xfc, 0x7e
	};

	int len = 0;

	*count = slen;

	/* special handling for one byte frames */
	if ((slen == 1) && (hdlc->state == HDLC_SEND_FAST_FLAG))
		hdlc->state = HDLC_SENDFLAG_ONE;
	while (dsize > 0) {
		if (hdlc->bit_shift == 0) {
			if (slen && !hdlc->do_closing) {
				hdlc->shift_reg = *src++;
				slen--;
				if (slen == 0)
					/* closing sequence, CRC + flag(s) */
					hdlc->do_closing = 1;
				hdlc->bit_shift = 8;
			} else {
				if (hdlc->state == HDLC_SEND_DATA) {
					if (hdlc->data_received) {
						hdlc->state = HDLC_SEND_CRC1;
						hdlc->crc ^= 0xffff;
						hdlc->bit_shift = 8;
						hdlc->shift_reg =
							hdlc->crc & 0xff;
					} else if (!hdlc->do_adapt56)
						hdlc->state =
							HDLC_SEND_FAST_FLAG;
					else
						hdlc->state =
							HDLC_SENDFLAG_B0;
				}

			}
		}

		switch (hdlc->state) {
		case STOPPED:
			while (dsize--)
				*dst++ = 0xff;
			return dsize;
		case HDLC_SEND_FAST_FLAG:
			hdlc->do_closing = 0;
			if (slen == 0) {
				/* the code is for bitreverse streams */
				if (hdlc->do_bitreverse == 0)
					*dst++ = osmo_revbytebits_8(hdlc->ffvalue);
				else
					*dst++ = hdlc->ffvalue;
				len++;
				dsize--;
				break;
			}
			/* fall through */
		case HDLC_SENDFLAG_ONE:
			if (hdlc->bit_shift == 8) {
				hdlc->cbin = hdlc->ffvalue >>
					(8 - hdlc->data_bits);
				hdlc->state = HDLC_SEND_DATA;
				hdlc->crc = 0xffff;
				hdlc->hdlc_bits1 = 0;
				hdlc->data_received = 1;
			}
			break;
		case HDLC_SENDFLAG_B0:
			hdlc->do_closing = 0;
			hdlc->cbin <<= 1;
			hdlc->data_bits++;
			hdlc->hdlc_bits1 = 0;
			hdlc->state = HDLC_SENDFLAG_B1A6;
			break;
		case HDLC_SENDFLAG_B1A6:
			hdlc->cbin <<= 1;
			hdlc->data_bits++;
			hdlc->cbin++;
			if (++hdlc->hdlc_bits1 == 6)
				hdlc->state = HDLC_SENDFLAG_B7;
			break;
		case HDLC_SENDFLAG_B7:
			hdlc->cbin <<= 1;
			hdlc->data_bits++;
			if (slen == 0) {
				hdlc->state = HDLC_SENDFLAG_B0;
				break;
			}
			if (hdlc->bit_shift == 8) {
				hdlc->state = HDLC_SEND_DATA;
				hdlc->crc = 0xffff;
				hdlc->hdlc_bits1 = 0;
				hdlc->data_received = 1;
			}
			break;
		case HDLC_SEND_FIRST_FLAG:
			hdlc->data_received = 1;
			if (hdlc->data_bits == 8) {
				hdlc->state = HDLC_SEND_DATA;
				hdlc->crc = 0xffff;
				hdlc->hdlc_bits1 = 0;
				break;
			}
			hdlc->cbin <<= 1;
			hdlc->data_bits++;
			if (hdlc->shift_reg & 0x01)
				hdlc->cbin++;
			hdlc->shift_reg >>= 1;
			hdlc->bit_shift--;
			if (hdlc->bit_shift == 0) {
				hdlc->state = HDLC_SEND_DATA;
				hdlc->crc = 0xffff;
				hdlc->hdlc_bits1 = 0;
			}
			break;
		case HDLC_SEND_DATA:
			hdlc->cbin <<= 1;
			hdlc->data_bits++;
			if (hdlc->hdlc_bits1 == 5) {
				hdlc->hdlc_bits1 = 0;
				break;
			}
			if (hdlc->bit_shift == 8)
				hdlc->crc = crc_ccitt_byte(hdlc->crc,
							   hdlc->shift_reg);
			if (hdlc->shift_reg & 0x01) {
				hdlc->hdlc_bits1++;
				hdlc->cbin++;
				hdlc->shift_reg >>= 1;
				hdlc->bit_shift--;
			} else {
				hdlc->hdlc_bits1 = 0;
				hdlc->shift_reg >>= 1;
				hdlc->bit_shift--;
			}
			break;
		case HDLC_SEND_CRC1:
			hdlc->cbin <<= 1;
			hdlc->data_bits++;
			if (hdlc->hdlc_bits1 == 5) {
				hdlc->hdlc_bits1 = 0;
				break;
			}
			if (hdlc->shift_reg & 0x01) {
				hdlc->hdlc_bits1++;
				hdlc->cbin++;
				hdlc->shift_reg >>= 1;
				hdlc->bit_shift--;
			} else {
				hdlc->hdlc_bits1 = 0;
				hdlc->shift_reg >>= 1;
				hdlc->bit_shift--;
			}
			if (hdlc->bit_shift == 0) {
				hdlc->shift_reg = (hdlc->crc >> 8);
				hdlc->state = HDLC_SEND_CRC2;
				hdlc->bit_shift = 8;
			}
			break;
		case HDLC_SEND_CRC2:
			hdlc->cbin <<= 1;
			hdlc->data_bits++;
			if (hdlc->hdlc_bits1 == 5) {
				hdlc->hdlc_bits1 = 0;
				break;
			}
			if (hdlc->shift_reg & 0x01) {
				hdlc->hdlc_bits1++;
				hdlc->cbin++;
				hdlc->shift_reg >>= 1;
				hdlc->bit_shift--;
			} else {
				hdlc->hdlc_bits1 = 0;
				hdlc->shift_reg >>= 1;
				hdlc->bit_shift--;
			}
			if (hdlc->bit_shift == 0) {
				hdlc->shift_reg = 0x7e;
				hdlc->state = HDLC_SEND_CLOSING_FLAG;
				hdlc->bit_shift = 8;
			}
			break;
		case HDLC_SEND_CLOSING_FLAG:
			hdlc->cbin <<= 1;
			hdlc->data_bits++;
			if (hdlc->hdlc_bits1 == 5) {
				hdlc->hdlc_bits1 = 0;
				break;
			}
			if (hdlc->shift_reg & 0x01)
				hdlc->cbin++;
			hdlc->shift_reg >>= 1;
			hdlc->bit_shift--;
			if (hdlc->bit_shift == 0) {
				hdlc->ffvalue =
					xfast_flag_value[hdlc->data_bits];
				if (hdlc->dchannel) {
					hdlc->ffvalue = 0x7e;
					hdlc->state = HDLC_SEND_IDLE1;
					hdlc->bit_shift = 8-hdlc->data_bits;
					if (hdlc->bit_shift == 0)
						hdlc->state =
							HDLC_SEND_FAST_IDLE;
				} else {
					if (!hdlc->do_adapt56) {
						hdlc->state =
							HDLC_SEND_FAST_FLAG;
						hdlc->data_received = 0;
					} else {
						hdlc->state = HDLC_SENDFLAG_B0;
						hdlc->data_received = 0;
					}
					/* Finished this frame, send flags */
					if (dsize > 1)
						dsize = 1;
				}
			}
			break;
		case HDLC_SEND_IDLE1:
			hdlc->do_closing = 0;
			hdlc->cbin <<= 1;
			hdlc->cbin++;
			hdlc->data_bits++;
			hdlc->bit_shift--;
			if (hdlc->bit_shift == 0) {
				hdlc->state = HDLC_SEND_FAST_IDLE;
				hdlc->bit_shift = 0;
			}
			break;
		case HDLC_SEND_FAST_IDLE:
			hdlc->do_closing = 0;
			hdlc->cbin = 0xff;
			hdlc->data_bits = 8;
			if (hdlc->bit_shift == 8) {
				hdlc->cbin = 0x7e;
				hdlc->state = HDLC_SEND_FIRST_FLAG;
			} else {
				/* the code is for bitreverse streams */
				if (hdlc->do_bitreverse == 0)
					*dst++ = osmo_revbytebits_8(hdlc->cbin);
				else
					*dst++ = hdlc->cbin;
				hdlc->bit_shift = 0;
				hdlc->data_bits = 0;
				len++;
				dsize = 0;
			}
			break;
		default:
			break;
		}
		if (hdlc->do_adapt56) {
			if (hdlc->data_bits == 7) {
				hdlc->cbin <<= 1;
				hdlc->cbin++;
				hdlc->data_bits++;
			}
		}
		if (hdlc->data_bits == 8) {
			/* the code is for bitreverse streams */
			if (hdlc->do_bitreverse == 0)
				*dst++ = osmo_revbytebits_8(hdlc->cbin);
			else
				*dst++ = hdlc->cbin;
			hdlc->data_bits = 0;
			len++;
			dsize--;
		}
	}
	*count -= slen;

	return len;
}