[RFC PATCH] doc: clarify VLAN and QinQ stripping behaviour

[Dean Marx]

On Mon, Jul 14, 2025 at 9:30 AM Bruce Richardson
<bruce.richardson at intel.com> wrote:
>
> The behaviour of VLAN tag stripping Rx offloads is unclear in DPDK, and
> not very well documented. Even the documentation that does exist appears
> contradictory.
>
> For example, the doxygen docs for the mbuf flag
> RTE_MBUF_F_RX_QINQ_STRIPPED says:
>
>   "If RTE_MBUF_F_RX_QINQ_STRIPPED is set and RTE_MBUF_F_RX_VLAN_STRIPPED
>   is unset, only the outer VLAN is removed from packet data,..."
>
> but the docs for RTE_MBUF_F_RX_QINQ says:
>
>   "If the flag RTE_MBUF_F_RX_QINQ_STRIPPED is also present, both VLANs
>   headers have been stripped from mbuf data, ..."
>
> Without a good definition of what the correct behaviour is, it's not
> possible to assess and ensure conformance across drivers. Update the
> documentation for NIC features, ethdev and mbuf library to all report
> the same information: that VLAN strip feature is stripping one flag, and
> QinQ strip feature is removing two.

I'm working on testing QinQ/VLAN stripping features across PMDs, and
so far I've found that our Intel devices are capable of QinQ
stripping, while our Mellanox/Broadcom devices are not. When QinQ
stripping is enabled on an Intel PMD, the test packet is received with
its outer VLAN layer stripped, but the inner VLAN layer remains
intact. Thus, the doxygen example is more accurate for what is
currently supported. I'm also running some tests on VLAN stripping
behavior, I'll update this thread with the results once these are
finished.