Understanding RX_OFFLOAD_VLAN_EXTEND

[Ivan Malov]

Thank you, Ferruh. You have been most helpful.

On Mon, 31 Oct 2022, Ferruh Yigit wrote:

> On 10/31/2022 6:46 PM, Ivan Malov wrote:
>> Hi!
>> 
>> We have a hard time figuring out what the API contract of
>> RX_OFFLOAD_VLAN_EXTEND might be. The best educated guess
>> we can make is that the feature might have something to
>> do with identifying VLAN packets and extracting TCI
>> without stripping the tags from incoming packets.
>> 
>> Is this understanding correct?
>> 
>> You see, things aren't helped by the offload bit having
>> almost no commentary. Such could've shed light on its
>> meaning. Perhaps this gap in documentation should
>> be addressed somehow. Any opinions?
>> 
>> Thank you.
>
>
> Hi Ivan,
>
> It is legacy from ixgbe driver, you can find more details on the ixgbe 
> (82599) datasheet [1].
>
> RX_OFFLOAD_VLAN_EXTEND is *like*, QinQ but not, that is why we have 
> 'QINQ_STRIP' offload.
>
> And RX_OFFLOAD_VLAN_EXTEND is more a configuration option, briefly you can 
> ignore it.
> But in detail that is to configure device in a mode that it knows that 
> received packets always has at least one VLAN tag, I assume it is for a case 
> that some in the middle networking device inserts/requires VLAN tags. But 
> optionally packet can have two VLAN tags. But as far as I can see this is not 
> for to strip the VLAN tag or to filter packet based on it, this is just to 
> configure device for this environment.
>
>
>
> [1] copy/paste from a public datasheet 
> (http://iommu.com/datasheets/ixgbe-datasheets/82599-datasheet-v3-4.pdf), not 
> sure if this is up to date version, but I think it is OK for this context:
>
> Double VLAN and Single VLAN Support
> The 82599 supports a mode where all received and sent packets have at least 
> one VLAN tag in addition to the regular tagging that might optionally be 
> added. In this document, when a packet carries two VLAN headers, the first 
> header is referred to as an outer VLAN and the second header as an inner VLAN 
> header (as listed in the table that follows). This mode is used for systems 
> where the near end switch adds the outer VLAN header containing switching 
> information. This mode is enabled by the following configuration:
> • This mode is activated by setting the DMATXCTL.GDV and the Extended VLAN 
> bit in the CTRL_EXT register.
> • The EtherType of the VLAN tag used for the additional VLAN is defined in 
> the VET EXT field in the EXVET register.
>