netdev - Re: [PATCH net-next v3 1/4] net: dsa: add support for DSA rx offloading via metadata dst

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Date:   Mon, 14 Nov 2022 13:55:59 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     Felix Fietkau <nbd@....name>
Cc:     Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>, Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v3 1/4] net: dsa: add support for DSA rx
 offloading via metadata dst

On Sat, Nov 12, 2022 at 12:13:15PM +0100, Felix Fietkau wrote:
> On 12.11.22 05:40, Jakub Kicinski wrote:
> I don't really see a valid use case in running generic XDP, TC and NFT on a
> DSA master dealing with packets before the tag receive function has been
> run. And after the tag has been processed, the metadata DST is cleared from
> the skb.

Oh, there are potentially many use cases, the problem is that maybe
there aren't as many actual implementations as ideas? At least XDP is,
I think, expected to be able to deal with DSA tags if run on a DSA
master (not sure how that applies when RX DSA tag is offloaded, but
whatever). Marek Behun had a prototype with Marvell tags, not sure how
far that went in the end:
https://www.mail-archive.com/netdev@vger.kernel.org/msg381018.html
In general, forwarding a packet to another switch port belonging to the
same master via XDP_TX should be relatively efficient.

> How about this: I send a v4 which uses skb_dst_drop instead of skb_dst_set,
> so that other drivers can use refcounting if it makes sense for them. For
> mtk_eth_soc, I prefer to leave out refcounting for performance reasons.
> Is that acceptable to you guys?

I don't think you can mix refcounting at consumer side with no-refcounting
at producer side, no?

I suppose that we could leave refcounting out for now, and bug you if
someone comes with a real need later and complains. Right now it's a bit
hard for me to imagine all the possibilities. How does that sound?