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Date:   Mon, 16 May 2022 12:20:48 -0700
From:   Jakub Kicinski <>
To:     Maxime Chevallier <>
Cc:, Rob Herring <>,,,,,
        Andrew Lunn <>,
        Florian Fainelli <>,
        Heiner Kallweit <>,
        Russell King <>,,
        Vladimir Oltean <>,
        Luka Perkov <>,
        Robert Marko <>
Subject: Re: [PATCH net-next v2 2/5] net: dsa: add out-of-band tagging

On Sat, 14 May 2022 17:06:53 +0200 Maxime Chevallier wrote:
> This tagging protocol is designed for the situation where the link
> between the MAC and the Switch is designed such that the Destination
> Port, which is usually embedded in some part of the Ethernet Header, is
> sent out-of-band, and isn't present at all in the Ethernet frame.
> This can happen when the MAC and Switch are tightly integrated on an
> SoC, as is the case with the Qualcomm IPQ4019 for example, where the DSA
> tag is inserted directly into the DMA descriptors. In that case,
> the MAC driver is responsible for sending the tag to the switch using
> the out-of-band medium. To do so, the MAC driver needs to have the
> information of the destination port for that skb.
> This out-of-band tagging protocol is using the very beggining of the skb
> headroom to store the tag. The drawback of this approch is that the
> headroom isn't initialized upon allocating it, therefore we have a
> chance that the garbage data that lies there at allocation time actually
> ressembles a valid oob tag. This is only problematic if we are
> sending/receiving traffic on the master port, which isn't a valid DSA
> use-case from the beggining. When dealing from traffic to/from a slave
> port, then the oob tag will be initialized properly by the tagger or the
> mac driver through the use of the dsa_oob_tag_push() call.
> Signed-off-by: Maxime Chevallier <>

This must had been asked on v1 but there's no trace of it in the
current submission afaict...

If the tag is passed in the descriptor how is this not a pure switchdev
driver? The explanation must be preserved somehow.

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