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+.. SPDX-License-Identifier: GPL-2.0
+
+X.25 Device Driver Interface
+============================
+
+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.