netdev - Re: [patch 5/7] CAN: Add virtual CAN netdevice driver

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Date:	Wed, 30 May 2007 21:16:32 +0200
From:	Patrick McHardy <kaber@...sh.net>
To:	Oliver Hartkopp <oliver@...tkopp.net>
CC:	Urs Thuermann <urs@...ogud.escape.de>,
	David Miller <davem@...emloft.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Oliver Hartkopp <oliver.hartkopp@...kswagen.de>,
	Urs Thuermann <urs.thuermann@...kswagen.de>,
	netdev@...r.kernel.org
Subject: Re: [patch 5/7] CAN: Add virtual CAN netdevice driver

Oliver Hartkopp wrote:
> Patrick McHardy wrote:
> 
>>I have a set of patches coming up that introduce a rtnetlink API
>>for adding/modifying/deleting software network devices. I would
>>prefer if you could switch this driver over instead of doing the
>>"create N devices during loading" that many current drivers do,
>>leaving you with 20 unused devices after boot.
>>  
> 
> 
> Hi Patrick,
> 
> next Friday? Too late ;-)


This friday :)

> Ok, your approach is indeed an interesting idea and we would look on it,
> as we also not interested in creating unused devices and do any double
> work.


Thanks.

>>(..)
>>Qdiscs might change skb->cb. Maybe use skb->sk?
>>
> 
> The loopback functionality in CAN is a bit tricky (maybe you can take a
> look into the Documentation patch [7/7] at chapter 3.2 and 4.1.4).
> 
> The problem is, that we need a per socket(!) option that enables the
> loopback for the sent CAN-Frames or not, so the information about
> loopback a skb content or not is contained inside the skb sent down to
> the CAN netdevice. If the CAN networkdevice is not capable to perform a
> loopback itself (see dev->flags, chapter 6) the CAN core has to do the
> loopback as a fallback.


skb->sk should work fine for a per socket option. In fact the
CAN core patch simply sets skb->cb to skb->sk.

> The use of skb->cb is also needed in the receive path to recognize and
> trash loopback'ed CAN-frames inside the *originating* socket (see
> chapter 4.1.4).


skb->sk should work here as well since you detect these frames
before queueing to the receiving socket.
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