linux-imx

// SPDX-License-Identifier: GPL-2.0+

/*

 * Safe Encapsulated USB Serial Driver

 *

 *      Copyright (C) 2010 Johan Hovold <jhovold@gmail.com>

 *      Copyright (C) 2001 Lineo

 *      Copyright (C) 2001 Hewlett-Packard

 *

 * By:

 *      Stuart Lynne <sl@lineo.com>, Tom Rushworth <tbr@lineo.com>

 */

​

/*

 * The encapsultaion is designed to overcome difficulties with some USB

 * hardware.

 *

 * While the USB protocol has a CRC over the data while in transit, i.e. while

 * being carried over the bus, there is no end to end protection. If the

 * hardware has any problems getting the data into or out of the USB transmit

 * and receive FIFO's then data can be lost.

 *

 * This protocol adds a two byte trailer to each USB packet to specify the

 * number of bytes of valid data and a 10 bit CRC that will allow the receiver

 * to verify that the entire USB packet was received without error.

 *

 * Because in this case the sender and receiver are the class and function

 * drivers there is now end to end protection.

 *

 * There is an additional option that can be used to force all transmitted

 * packets to be padded to the maximum packet size. This provides a work

 * around for some devices which have problems with small USB packets.

 *

 * Assuming a packetsize of N:

 *

 *      0..N-2  data and optional padding

 *

 *      N-2     bits 7-2 - number of bytes of valid data

 *              bits 1-0 top two bits of 10 bit CRC

 *      N-1     bottom 8 bits of 10 bit CRC

 *

 *

 *      | Data Length       | 10 bit CRC                                |

 *      + 7 . 6 . 5 . 4 . 3 . 2 . 1 . 0 | 7 . 6 . 5 . 4 . 3 . 2 . 1 . 0 +

 *

 * The 10 bit CRC is computed across the sent data, followed by the trailer

 * with the length set and the CRC set to zero. The CRC is then OR'd into

 * the trailer.

 *

 * When received a 10 bit CRC is computed over the entire frame including

 * the trailer and should be equal to zero.

 *

 * Two module parameters are used to control the encapsulation, if both are

 * turned of the module works as a simple serial device with NO

 * encapsulation.

 *

 * See linux/drivers/usbd/serial_fd for a device function driver

 * implementation of this.

 *

 */

​

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

​

#include <linux/kernel.h>