[RFC PATCH 17/27] net/i40e: document requirement for QinQ support

[Bruce Richardson <bruce.richardson@intel.com>]

In order to get multiple VLANs inserted in an outgoing packet with QinQ
offload the i40e driver needs to be set to double vlan mode. This is
done by using the VLAN_EXTEND Rx config flag. Add a code check for this
dependency and update the docs about it.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
---
 doc/guides/nics/i40e.rst           | 18 ++++++++++++++++++
 drivers/net/intel/i40e/i40e_rxtx.c |  9 +++++++++
 2 files changed, 27 insertions(+)

diff --git a/doc/guides/nics/i40e.rst b/doc/guides/nics/i40e.rst
index 45dc083c94..cbfaddbdd8 100644
--- a/doc/guides/nics/i40e.rst
+++ b/doc/guides/nics/i40e.rst
@@ -245,6 +245,24 @@ Runtime Configuration
   * ``segment``: Check number of mbuf segments not exceed hw limitation.
   * ``offload``: Check any unsupported offload flag.
 
+QinQ Configuration
+~~~~~~~~~~~~~~~~~~
+
+When using QinQ TX offload (``RTE_ETH_TX_OFFLOAD_QINQ_INSERT``), you must also
+enable ``RTE_ETH_RX_OFFLOAD_VLAN_EXTEND`` to configure the hardware for double
+VLAN mode. Without this, only the inner VLAN tag will be inserted.
+
+Example::
+
+  struct rte_eth_conf port_conf = {
+      .rxmode = {
+          .offloads = RTE_ETH_RX_OFFLOAD_VLAN_EXTEND,
+      },
+      .txmode = {
+          .offloads = RTE_ETH_TX_OFFLOAD_QINQ_INSERT,
+      },
+  };
+
 Vector RX Pre-conditions
 ~~~~~~~~~~~~~~~~~~~~~~~~
 For Vector RX it is assumed that the number of descriptor rings will be a power
diff --git a/drivers/net/intel/i40e/i40e_rxtx.c b/drivers/net/intel/i40e/i40e_rxtx.c
index 2d12e6dd1a..aef78c5358 100644
--- a/drivers/net/intel/i40e/i40e_rxtx.c
+++ b/drivers/net/intel/i40e/i40e_rxtx.c
@@ -2171,6 +2171,15 @@ i40e_dev_tx_queue_setup(struct rte_eth_dev *dev,
 	vsi = i40e_pf_get_vsi_by_qindex(pf, queue_idx);
 	if (!vsi)
 		return -EINVAL;
+
+	/* Check if QinQ TX offload requires VLAN extend mode */
+	if ((offloads & RTE_ETH_TX_OFFLOAD_QINQ_INSERT) &&
+			!(dev->data->dev_conf.rxmode.offloads & RTE_ETH_RX_OFFLOAD_VLAN_EXTEND)) {
+		PMD_DRV_LOG(WARNING, "Port %u: QinQ TX offload is enabled but VLAN extend mode is not set. ",
+				dev->data->port_id);
+		PMD_DRV_LOG(WARNING, "Double VLAN insertion may not work correctly without RTE_ETH_RX_OFFLOAD_VLAN_EXTEND set in Rx configuration.");
+	}
+
 	q_offset = i40e_get_queue_offset_by_qindex(pf, queue_idx);
 	if (q_offset < 0)
 		return -EINVAL;
-- 
2.51.0