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

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Message-ID: <80774235-c0d7-ad94-b644-490069f8e087@gmail.com>
Date:   Tue, 26 Mar 2019 15:13:49 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Vladimir Oltean <olteanv@...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 3:03 PM, Vladimir Oltean wrote:
> 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?

So that queue could be used to steer management traffic, but it would
still attempt to perform a dev_queue_xmit() using the master DSA network
device unless you somehow change that and/or parent that queue to a
different network device that the sja1105 switch driver creates (which
is doable).
-- 
Florian