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

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Message-ID: <465F2F20.6090700@hartkopp.net>
Date:	Thu, 31 May 2007 22:25:04 +0200
From:	Oliver Hartkopp <oliver@...tkopp.net>
To:	Patrick McHardy <kaber@...sh.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

Patrick McHardy wrote:
> Urs Thuermann wrote:
>   
>> +	/* tx socket reference pointer: Loopback required if not NULL */
>> +	loop = *(struct sock **)skb->cb != NULL;
>>     
>
>
> Qdiscs might change skb->cb. Maybe use skb->sk?
>
>   

Hi Patrick,

due to current projects Urs and me had only a short time today to
discuss the use of other skbuff-elements to implement the needed
loopback functionality. Urs remembered that the value of skb->sk was not
'stable' in the whole loopback and therefore the use of skb->cb was a
reasonable approach - and it was indeed working. Btw. triggered by your
remark, we're looking for alternatives.

Another thing is, that i would like to know, if Qdiscs would ever touch
skb's containing CAN-frames?!?
The MTU is 16 bytes (sizeof(struct can_frame)) and each skb contains
exact one CAN-frame.
What would a packet scheduler do with these skb's? IMO there is nothing
that could be optimized and linked together like in IPv4 / IPv6 data.

Would therefore skb->cb left unchanged in my skb's? Or is there any flag
that can be set in the skb to keep the packet scheduler's hands off?

Best regards,
Oliver

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