Re: [dpdk-dev] [PATCH v3] net/af_packet: reinsert the stripped vlan tag

[Tudor Cornea <tudor.cornea@gmail.com>]

Thanks Stephen, for the suggestion

I've sent v4 of the patch, which adds the succinct description in the
af_packet documentation..


On Fri, 24 Sept 2021 at 18:11, Stephen Hemminger <stephen@networkplumber.org>
wrote:

> On Fri, 24 Sep 2021 14:44:45 +0300
> Tudor Cornea <tudor.cornea@gmail.com> wrote:
>
> > +Features and Limitations of the af_packet PMD
> > +---------------------------------------------
> > +
> > +Since the following commit, the Linux kernel strips the vlan tag
> > +
> > +.. code-block:: console
> > +
> > +    commit bcc6d47903612c3861201cc3a866fb604f26b8b2
> > +    Author: Jiri Pirko <jpirko@xxxxxxxxxx>
> > +    Date:   Thu Apr 7 19:48:33 2011 +0000
> > +
> > +     net: vlan: make non-hw-accel rx path similar to hw-accel
> > +
> > +Running on such a kernel results in receiving untagged frames while
> using
> > +the af_packet PMD. Fortunately, the stripped information is still
> available
> > +for use in ``mbuf->vlan_tci``, and applications could check
> ``PKT_RX_VLAN_STRIPPED``.
> > +
> > +However, we currently don't have a way to describe offloads which can't
> be
> > +disabled by PMDs, and this creates an inconsistency with the way
> applications
> > +expect the PMD offloads to work, and requires them to be aware of which
> > +underlying driver they use.
> > +
> > +Since release 21.11 the af_packet PMD will implement support for the
> > +``DEV_RX_OFFLOAD_VLAN_STRIP`` offload, and users can control the
> desired vlan
> > +stripping behavior.
> > +
> > +It's important to note that the default case will change. If previously,
> > +the vlan tag was stripped, if the application now requires the same
> behavior,
> > +it will need to configure ``rxmode.offloads`` with
> ``DEV_RX_OFFLOAD_VLAN_STRIP``.
> > +
> > +The PMD driver will re-insert the vlan tag transparently to the
> application
> > +if the kernel strips it, as long as the ``DEV_RX_OFFLOAD_VLAN_STRIP``
> is not
> > +enabled.
>
> This seems like the wrong place for this text.
> It is not a new feature, it is a bug fix.
>
> If you want to describe it as an enhancement, the text should be succinct
> and describe
> the situation from user point of view. Something like:
>
>     Af_packet PMD now works with VLAN's on Linux
>
>