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Date:	Mon, 25 Jun 2007 12:37:33 +0200
From:	Patrick McHardy <>
To:	Oliver Hartkopp <>
CC:, David Miller <>,
	Urs Thuermann <>,
	Thomas Gleixner <>,
Subject: Re: [CAN] [RFC] skb->iif usage and vcan driver background

Oliver Hartkopp wrote:
> Hello Patrick and Jamal,
> as i felt a bit misunderstood in the discussion about the usage of
> skb->iif and the idea behind the virtual CAN driver i created four
> PDF-slides to clarify some issues. The slides may give you the
> appropriate background why the incoming (receiving) interface is
> relevant at user level (which is unusual e.g. for PF_INET). Additionally
> i collected some points what the VCAN driver does - and especially what
> it is not for. So you will see, that approaches like VLAN (regarding
> IEEE 802.1Q) is nothing that can be done with the CAN bus by design. The
> PDF can be found at the BerliOS OSS server:

I normally wouldn't have gone reading some PDF to explain a patch,
but this one was really worth it .. a couple of pictures of cars
with four applications using can0-can3 :)

> After reading the PDF ...
> @Patrick: The (optional) loading of the vcan module and the
> specification of the needed number of vcan devices (for the wanted
> use-case) was a very easy thing up to now that did not require any
> additional configuration nor additional userspace tools (except saying
> 'ifconfig vcan0 up'). As only the use-case required number of interfaces
> are allocated at module load time, i do not see a need for an extra
> netlink interface to implement an IMHO obsolete vcan add/remove
> mechanism. What could the implementation of the netlink API bring for
> the vcan driver use-case?

You keep talkign about "the use-case". This is *your* usage case
any just because *you* need four interfaces doesn't mean everyone
else on the world does too.

Your last slide brings it to the point: "... configured at module
load time for the needed use-case:
- number of created vcan devices (current default=4)
- perform the loopback on driver level (current default=off)"

So you *do* have parameters for configuration and you're using the
wrong interface. Either drop them or use the correct interface.

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