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Date:   Tue, 8 Nov 2022 12:33:30 +0200
From:   Vladimir Oltean <olteanv@gmail.com>
To:     Felix Fietkau <nbd@nbd.name>
Cc:     netdev@vger.kernel.org, "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 08/14] net: vlan: remove invalid VLAN protocol warning
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On Tue, Nov 08, 2022 at 11:24:51AM +0100, Felix Fietkau wrote:
> On 08.11.22 11:08, Vladimir Oltean wrote:
> > On Tue, Nov 08, 2022 at 10:46:54AM +0100, Felix Fietkau wrote:
> > > I think it depends on the hardware. If you can rely on the hardware always
> > > reporting the port out-of-band, a generic "oob" tagger would be fine.
> > > In my case, it depends on whether a second non-DSA ethernet MAC is active on
> > > the same device, so I do need to continue using the "mtk" tag driver.
> > 
> > It's not only about the time dimension (always OOB, vs sometimes OOB),
> > but also about what is conveyed through the OOB tag. I can see 2 vendors
> > agreeing today on a common "oob" tagger only to diverge in the future
> > when they'll need to convey more information than just port id. What do
> > you do with the tagging protocol string names then. Gotta make them
> > unique from the get go, can't export "oob" to user space IMO.
> > 
> > > The flow dissector part is already solved: I simply used the existing
> > > .flow_dissect() tag op.
> > 
> > Yes, well this seems like a generic enough case (DSA offload tag present
> > => don't shift network header because it's where it should be) to treat
> > it in the generic flow dissector and not have to invoke any tagger-specific
> > fixup method.
> 
> In that case I think we shouldn't use METADATA_HW_PORT_MUX, since it is
> already used for other purposes. I will add a new metadata type METADATA_DSA
> instead.

Which case, flow dissector figuring out that DSA offload tag is present?
Well, the skb can only carry one dst pointer ATM, so if it's METADATA_HW_PORT_MUX
but it belongs to SR-IOV on the DSA master, we have bigger problems anyway.
So, proto == ETH_P_XDSA && have METADATA_HW_PORT_MUX should be pretty
good indication that DSA offload tag is present.

Anyway I raised this concern as well to Jakub as well. Seems to be
theoretical at the moment. Using METADATA_HW_PORT_MUX seems to be fine
right now. Can be changed later if needed; it's not ABI.