netdev - Re: [RFC 3/9] net: dsa: mv88e6xxx: add support for VTU ops

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Date:	Wed, 3 Jun 2015 10:56:18 -0400 (EDT)
From:	Vivien Didelot <vivien.didelot@...oirfairelinux.com>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	netdev <netdev@...r.kernel.org>, David <davem@...emloft.net>,
	Florian Fainelli <f.fainelli@...il.com>,
	Andrew Lunn <andrew@...n.ch>,
	Scott Feldman <sfeldma@...il.com>,
	Jiri Pirko <jiri@...nulli.us>,
	Jérome Oufella 
	<jerome.oufella@...oirfairelinux.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	kernel <kernel@...oirfairelinux.com>,
	Chris Healy <cphealy@...il.com>
Subject: Re: [RFC 3/9] net: dsa: mv88e6xxx: add support for VTU ops

Hi Guenter,

On Jun 2, 2015, at 10:17 PM, Guenter Roeck linux@...ck-us.net wrote:
> On Tue, Jun 02, 2015 at 09:39:50PM -0400, Vivien Didelot wrote:
>> Guenter,
>> 
>> On Jun 2, 2015, at 2:50 AM, Guenter Roeck linux@...ck-us.net wrote:
>> > On 06/01/2015 06:27 PM, Vivien Didelot wrote:
>> >> +    /* Bringing an interface up adds it to the VLAN 0. Ignore this. */
>> >> +    if (!vid)
>> >> +        return 0;
>> >> +
>> > 
>> > Me puzzled ;-). I brought this and the fid question up before.
>> > No idea if my e-mail got lost or what happened.
>> > 
>> > Can you explain why we don't need a configuration for vlan 0 ?
>> 
>> Sorry for late reply. Initially, when issuing "ip link set up dev swp0",
>> ndo_vlan_rx_add_vid was called to add the interface in the VLAN 0.
>> 
> Loading the 802.1q module has the same effect.
> 
> I think this may be on purpose; it is telling the switch to accept
> packets with vid==0 (and untagged packets).
> 
>> 2 things happen here:
>> 
>>   * this is inconsistent with the "bridge vlan" output which doesn't seem to
>>     know about a VID 0;
>>   * VID 0 seems special for this switch: if an ingressing frame has VID 0, the
>>     tagged port will override the VID bits with the port default VID at egress.
>> 
> As far as I can see, the switch treats packets with vid==0 and untaged packets
> as unknown if VLAN support is enabled.

I am not sure about the untagged frames. But for tagged frames, the
documentation says that frames with vid 0 will be overridden with the port's
default VID.

> Anyway, sounds odd. Sure this isn't a configuration problem somethere ?

If I'm not mistaken, other drivers do that. e.g. Rocker deals with VID >= 1:

    for (vid = 1; vid < VLAN_N_VID; vid++)

Maybe this VID overriding feature is what we want? But it doesn't look right to
me, even more since it is not exposed to the user.

Thanks,
-v
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