[PATCH v7] doc: add PMD known issue

[Stephen Hemminger]

On Tue, 27 Dec 2022 17:00:40 +0800
Mingjin Ye <mingjinx.ye at intel.com> wrote:

> +
> +ice: Rx path is not supported after PF or DCF add vlan offload
> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> +
> +If pmd does not enable Vlan offload during initialization, it will
> +automatically select Rx paths that do not support offload. Even if
> +Vlan offload is subsequently enabled through the API, Vlan offload
> +will not work because the selected Rx path does not support Vlan
> +offload.
> +
> +cmd_vlan_offload_parsed() goes down to the follow ethdev API functions:
> +    - rte_eth_dev_set_vlan_strip_on_queue()
> +    - rte_eth_dev_set_vlan_offload()
> +
> +These functions add offload settings when the port is started, running
> +and processing traffic. At this time, ``rte_eth_rx_queue_setup`` api is
> +needed to reroute rxq to the RX path with offload function. But at this
> +time, it is possible that the original Rx path is handling packages, so
> +this is not thread-safe.
> +
> +When applying offload on the PF or DCF, starting the ``testpmd``
> +application, use the ``--rx-offloads`` startup parameter to force the
> +dpdk lib to choose the Rx path with the offload function by default.
> +

This seems like just making excuses in the documentation for something
that should be fixed instead.

This situation is probably common to many PMD's.
Ideally, the drivers should reject changes to settings after device
is started if they can not support it.