xref: /linux/drivers/isdn/hardware/mISDN/isdnhdlc.c (revision df561f66)

// SPDX-License-Identifier: GPL-2.0-or-later
/*
 * 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>
 */

#include <linux/module.h>
#include <linux/init.h>
#include <linux/crc-ccitt.h>
#include <linux/bitrev.h>
#include "isdnhdlc.h"

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

MODULE_AUTHOR("Wolfgang Mües <wolfgang@iksw-muees.de>, "
	      "Frode Isaksen <fisaksen@bewan.com>, "
	      "Kai Germaschewski <kai.germaschewski@gmx.de>");
MODULE_DESCRIPTION("General purpose ISDN HDLC decoder");
MODULE_LICENSE("GPL");

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

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
};

void isdnhdlc_rcv_init(struct isdnhdlc_vars *hdlc, u32 features)
{
	memset(hdlc, 0, sizeof(struct isdnhdlc_vars));
	hdlc->state = HDLC_GET_DATA;
	if (features & HDLC_56KBIT)
		hdlc->do_adapt56 = 1;
	if (features & HDLC_BITREVERSE)
		hdlc->do_bitreverse = 1;
}
EXPORT_SYMBOL(isdnhdlc_out_init);

void isdnhdlc_out_init(struct isdnhdlc_vars *hdlc, u32 features)
{
	memset(hdlc, 0, sizeof(struct isdnhdlc_vars));
	if (features & HDLC_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 & HDLC_56KBIT) {
		hdlc->do_adapt56 = 1;
		hdlc->state = HDLC_SENDFLAG_B0;
	} else
		hdlc->data_bits = 8;
	if (features & HDLC_BITREVERSE)
		hdlc->do_bitreverse = 1;
}
EXPORT_SYMBOL(isdnhdlc_rcv_init);

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

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

/*
  isdnhdlc_decode - 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 with the bit HDLC_END_OF_FRAME set.
  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 the length with the bit HDLC_FRAMING_ERROR set.
  If a CRC error is found the function returns the length with the
  bit HDLC_CRC_ERROR set.
  If the frame length exceeds the destination buffer size, the function
  returns the length with the bit HDLC_LENGTH_ERROR set.

  src - source buffer
  slen - source buffer length
  count - number of bytes removed (decoded) from the source buffer
  dst _ destination buffer
  dsize - destination buffer size
  returns - number of decoded bytes in the destination buffer and status
  flag.
*/
int isdnhdlc_decode(struct isdnhdlc_vars *hdlc, const u8 *src, int slen,
		    int *count, u8 *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 = bitrev8(*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 = -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 = -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;
}
EXPORT_SYMBOL(isdnhdlc_decode);
/*
  isdnhdlc_encode - 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.

  src - source buffer
  slen - source buffer length
  count - number of bytes removed (encoded) from source buffer
  dst _ destination buffer
  dsize - destination buffer size
  returns - number of encoded bytes in the destination buffer
*/
int isdnhdlc_encode(struct isdnhdlc_vars *hdlc, const u8 *src, u16 slen,
		    int *count, u8 *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++ = bitrev8(hdlc->ffvalue);
				else
					*dst++ = hdlc->ffvalue;
				len++;
				dsize--;
				break;
			}
			fallthrough;
		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++ = bitrev8(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++ = bitrev8(hdlc->cbin);
			else
				*dst++ = hdlc->cbin;
			hdlc->data_bits = 0;
			len++;
			dsize--;
		}
	}
	*count -= slen;

	return len;
}
EXPORT_SYMBOL(isdnhdlc_encode);