n_hdlc.c

// SPDX-License-Identifier: GPL-1.0+
/* generic HDLC line discipline for Linux
 *
 * Written by Paul Fulghum paulkf@microgate.com
 * for Microgate Corporation
 *
 * Microgate and SyncLink are registered trademarks of Microgate Corporation
 *
 * Adapted from ppp.c, written by Michael Callahan <callahan@maths.ox.ac.uk>,
 *	Al Longyear <longyear@netcom.com>,
 *	Paul Mackerras <Paul.Mackerras@cs.anu.edu.au>
 *
 * Original release 01/11/99
 *
 * This module implements the tty line discipline N_HDLC for use with
 * tty device drivers that support bit-synchronous HDLC communications.
 *
 * All HDLC data is frame oriented which means:
 *
 * 1. tty write calls represent one complete transmit frame of data
 *    The device driver should accept the complete frame or none of 
 *    the frame (busy) in the write method. Each write call should have
 *    a byte count in the range of 2-65535 bytes (2 is min HDLC frame
 *    with 1 addr byte and 1 ctrl byte). The max byte count of 65535
 *    should include any crc bytes required. For example, when using
 *    CCITT CRC32, 4 crc bytes are required, so the maximum size frame
 *    the application may transmit is limited to 65531 bytes. For CCITT
 *    CRC16, the maximum application frame size would be 65533.
 *
 *
 * 2. receive callbacks from the device driver represents
 *    one received frame. The device driver should bypass
 *    the tty flip buffer and call the line discipline receive
 *    callback directly to avoid fragmenting or concatenating
 *    multiple frames into a single receive callback.
 *
 *    The HDLC line discipline queues the receive frames in separate
 *    buffers so complete receive frames can be returned by the
 *    tty read calls.
 *
 * 3. tty read calls returns an entire frame of data or nothing.
 *    
 * 4. all send and receive data is considered raw. No processing
 *    or translation is performed by the line discipline, regardless
 *    of the tty flags
 *
 * 5. When line discipline is queried for the amount of receive
 *    data available (FIOC), 0 is returned if no data available,
 *    otherwise the count of the next available frame is returned.
 *    (instead of the sum of all received frame counts).
 *
 * These conventions allow the standard tty programming interface
 * to be used for synchronous HDLC applications when used with
 * this line discipline (or another line discipline that is frame
 * oriented such as N_PPP).
 *
 * The SyncLink driver (synclink.c) implements both asynchronous
 * (using standard line discipline N_TTY) and synchronous HDLC
 * (using N_HDLC) communications, with the latter using the above
 * conventions.
 *
 * This implementation is very basic and does not maintain
 * any statistics. The main point is to enforce the raw data
 * and frame orientation of HDLC communications.
 *
 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED
 * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT,
 * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES