netdev - Re: DSA and underlying 802.1Q encapsulation

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Date:	Tue, 26 May 2015 15:51:42 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Vivien Didelot <vivien.didelot@...oirfairelinux.com>
Cc:	netdev <netdev@...r.kernel.org>,
	Florian Fainelli <f.fainelli@...il.com>,
	Andrew Lunn <andrew@...n.ch>, Chris Healy <cphealy@...il.com>,
	Jérome Oufella 
	<jerome.oufella@...oirfairelinux.com>
Subject: Re: DSA and underlying 802.1Q encapsulation

On Tue, May 26, 2015 at 06:29:57PM -0400, Vivien Didelot wrote:
> Hi,
> 
> I'm doing tests with VLAN support in DSA and I noticed that the EDSA 
> frame is prepended with a 802.1q header once queued to the underlying 
> network device, in net/dsa/tag_edsa.c:
> 
>     skb->dev = p->parent->dst->master_netdev;
>     dev_queue_xmit(skb);
> 
> This issue can be observed with the following dump:
> 
>     curl -s http://ix.io/iIv | tcpdump -en -r -
> 
> I suspect that the DSA code must clear some VLAN flags in the skb
> structure, in order to prevent the additional encapsulation by the lower
> level. Does this make sense?
> 

Hi Vivien,

Interesting question. Does the underlying network device support VLAN HW
acceleration (NETIF_F_HW_VLAN_CTAG_RX, NETIF_F_HW_VLAN_CTAG_TX) ?

If yes, the dsa code may need to move the tag into the header.
If we are lucky, a call to vlan_hwaccel_push_inside() might do it.

Do you have some vlan dsa code to share, by any chance ? That might
save me some time, as I am looking into it as well.

Thanks,
Guenter
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