can: Add documentation for virtual CAN driver usage

This patch adds a usage documentation for the virtual CAN driver (vcan).
Signed-off-by: Oliver Hartkopp <oliver@hartkopp.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/can.txt

@@ -35,8 +35,9 @@ This file contains
     6.1 general settings
     6.2 local loopback of sent frames
     6.3 CAN controller hardware filters
-    6.4 currently supported CAN hardware
-    6.5 todo
+    6.4 The virtual CAN driver (vcan)
+    6.5 currently supported CAN hardware
+    6.6 todo
 
   7 Credits
 
@@ -584,7 +585,42 @@ solution for a couple of reasons:
   @133MHz with four SJA1000 CAN controllers from 2002 under heavy bus
   load without any problems ...
 
-  6.4 currently supported CAN hardware (September 2007)
+  6.4 The virtual CAN driver (vcan)
+
+  Similar to the network loopback devices, vcan offers a virtual local
+  CAN interface. A full qualified address on CAN consists of
+
+  - a unique CAN Identifier (CAN ID)
+  - the CAN bus this CAN ID is transmitted on (e.g. can0)
+
+  so in common use cases more than one virtual CAN interface is needed.
+
+  The virtual CAN interfaces allow the transmission and reception of CAN
+  frames without real CAN controller hardware. Virtual CAN network
+  devices are usually named 'vcanX', like vcan0 vcan1 vcan2 ...
+  When compiled as a module the virtual CAN driver module is called vcan.ko
+
+  Since Linux Kernel version 2.6.24 the vcan driver supports the Kernel
+  netlink interface to create vcan network devices. The creation and
+  removal of vcan network devices can be managed with the ip(8) tool:
+
+  - Create a virtual CAN network interface:
+       ip link add type vcan
+
+  - Create a virtual CAN network interface with a specific name 'vcan42':
+       ip link add dev vcan42 type vcan
+
+  - Remove a (virtual CAN) network interface 'vcan42':
+       ip link del vcan42
+
+  The tool 'vcan' from the SocketCAN SVN repository on BerliOS is obsolete.
+
+  Virtual CAN network device creation in older Kernels:
+  In Linux Kernel versions < 2.6.24 the vcan driver creates 4 vcan
+  netdevices at module load time by default. This value can be changed
+  with the module parameter 'numdev'. E.g. 'modprobe vcan numdev=8'
+
+  6.5 currently supported CAN hardware
 
   On the project website http://developer.berlios.de/projects/socketcan
   there are different drivers available:
@@ -603,7 +639,7 @@ solution for a couple of reasons:
 
   Please check the Mailing Lists on the berlios OSS project website.
 
-  6.5 todo (September 2007)
+  6.6 todo
 
   The configuration interface for CAN network drivers is still an open
   issue that has not been finalized in the socketcan project. Also the