linux/Documentation/networking/x25-iface.rst

.. SPDX-License-Identifier: GPL-2.0

============================-
X.25 Device Driver Interface
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Version 1.1

			   Jonathan Naylor 26.12.96

This is a description of the messages to be passed between the X.25 Packet
Layer and the X.25 device driver. They are designed to allow for the easy
setting of the LAPB mode from within the Packet Layer.

The X.25 device driver will be coded normally as per the Linux device driver
standards. Most X.25 device drivers will be moderately similar to the
already existing Ethernet device drivers. However unlike those drivers, the
X.25 device driver has a state associated with it, and this information
needs to be passed to and from the Packet Layer for proper operation.

All messages are held in sk_buff's just like real data to be transmitted
over the LAPB link. The first byte of the skbuff indicates the meaning of
the rest of the skbuff, if any more information does exist.


Packet Layer to Device Driver
-----------------------------

First Byte = 0x00 (X25_IFACE_DATA)

This indicates that the rest of the skbuff contains data to be transmitted
over the LAPB link. The LAPB link should already exist before any data is
passed down.

First Byte = 0x01 (X25_IFACE_CONNECT)

Establish the LAPB link. If the link is already established then the connect
confirmation message should be returned as soon as possible.

First Byte = 0x02 (X25_IFACE_DISCONNECT)

Terminate the LAPB link. If it is already disconnected then the disconnect
confirmation message should be returned as soon as possible.

First Byte = 0x03 (X25_IFACE_PARAMS)

LAPB parameters. To be defined.


Device Driver to Packet Layer
-----------------------------

First Byte = 0x00 (X25_IFACE_DATA)

This indicates that the rest of the skbuff contains data that has been
received over the LAPB link.

First Byte = 0x01 (X25_IFACE_CONNECT)

LAPB link has been established. The same message is used for both a LAPB
link connect_confirmation and a connect_indication.

First Byte = 0x02 (X25_IFACE_DISCONNECT)

LAPB link has been terminated. This same message is used for both a LAPB
link disconnect_confirmation and a disconnect_indication.

First Byte = 0x03 (X25_IFACE_PARAMS)

LAPB parameters. To be defined.


Requirements for the device driver
----------------------------------

Packets should not be reordered or dropped when delivering between the
Packet Layer and the device driver.

To avoid packets from being reordered or dropped when delivering from
the device driver to the Packet Layer, the device driver should not
call "netif_rx" to deliver the received packets. Instead, it should
call "netif_receive_skb_core" from softirq context to deliver them.