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 [off-list ref]
wrote:

On Fri, 24 Sep 2021 14:44:45 +0300
Tudor Cornea [off-list ref] wrote:

quoted

+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

quoted

+the af_packet PMD. Fortunately, the stripped information is still

available

quoted

+for use in ``mbuf->vlan_tci``, and applications could check

``PKT_RX_VLAN_STRIPPED``.

quoted

+
+However, we currently don't have a way to describe offloads which can't

be

quoted

+disabled by PMDs, and this creates an inconsistency with the way

applications

quoted

+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

quoted

+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,

quoted

+it will need to configure ``rxmode.offloads`` with

``DEV_RX_OFFLOAD_VLAN_STRIP``.

quoted

+
+The PMD driver will re-insert the vlan tag transparently to the

application

quoted

+if the kernel strips it, as long as the ``DEV_RX_OFFLOAD_VLAN_STRIP``

is not

quoted

+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