netdev - Re: [RFC PATCH net-next 10/13] net: dsa: sja1105: Add support for traffic through standalone ports

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Date:   Wed, 27 Mar 2019 00:03:59 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     Florian Fainelli <f.fainelli@...il.com>, davem@...emloft.net,
        netdev@...r.kernel.org
Cc:     andrew@...n.ch, vivien.didelot@...il.com, linus.walleij@...aro.org
Subject: Re: [RFC PATCH net-next 10/13] net: dsa: sja1105: Add support for
 traffic through standalone ports

On 3/26/19 4:31 AM, Florian Fainelli wrote:
> 
> 
> On 3/23/2019 8:23 PM, Vladimir Oltean wrote:
>> In order to support this, we are creating a make-shift switch tag out of
>> a VLAN trunk configured on the CPU port. Termination on switch ports
>> only works when not under a vlan_filtering bridge. We are making use of
>> the generic CONFIG_NET_DSA_TAG_8021Q code and leveraging it from our own
>> CONFIG_NET_DSA_TAG_SJA1105.
>>
>> There are two types of traffic: regular and link-local.
>> The link-local traffic received on the CPU port is trapped from the
>> switch's regular forwarding decisions because it matched one of the two
>> DMAC filters for management traffic.
>> On transmission, the switch requires special massaging for these
>> link-local frames. Due to a weird implementation of the switching IP, by
>> default it drops link-local frames that originate on the CPU port. It
>> needs to be told where to forward them to, through an SPI command
>> ("management route") that is valid for only a single frame.
>> So when we're sending link-local traffic, we need to clone skb's from
>> DSA and send them in our custom xmit worker that also performs SPI access.
>>
>> For that purpose, the DSA xmit handler and the xmit worker communicate
>> through a per-port "skb ring" software structure, with a producer and a
>> consumer index. At the moment this structure is rather fragile
>> (ping-flooding to a link-local DMAC would cause most of the frames to
>> get dropped). I would like to move the management traffic on a separate
>> netdev queue that I can stop when the skb ring got full and hardware is
>> busy processing, so that we are not forced to drop traffic.
>>
>> Signed-off-by: Vladimir Oltean <olteanv@...il.com>
> 
> Reviewed-by: Florian Fainelli <f.fainelli@...il.com>
> 
> I do like the idea of setting up specific management queue later on,
> although it is not clear to me how you would go about integrating it as
> a network device, given the DSA slave and master devices, do you know
> roughly how you would proceed?
> 

Actually I was thinking about leveraging the multiqueue support that you 
added in 55199df6d2af ("net: dsa: Allow switch drivers to indicate 
number of TX queues") and expose the slave netdev .ndo_select_queue 
callback towards DSA ports. There I would return queue #0 if 
sja1105_is_link_local(skb), and queue #1 otherwise.
Are there any complications that I'm missing?

Thanks,
-Vladimir