The jumbo frame uses the 'RTE_ETHER_MAX_LEN' as boundary condition.
If the Ether overhead is larger than 18 when it supports VLAN tag,
that will cause the jumbo flag rx offload is wrong when MTU size is
'RTE_ETHER_MTU'.

This fix will normalize the boundary condition with 'RTE_ETHER_MTU'
and overhead even though current overhead is 18.

Fixes: 5d5589b0c858 ("net/enetc: support MTU update and jumbo frames")

Signed-off-by: Steve Yang <redacted>
---
 drivers/net/enetc/enetc.h        | 4 ++++
 drivers/net/enetc/enetc_ethdev.c | 2 +-
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/net/enetc/enetc.h b/drivers/net/enetc/enetc.h
index 14ef3bc18b..7163633bce 100644
--- a/drivers/net/enetc/enetc.h
+++ b/drivers/net/enetc/enetc.h

@@ -29,6 +29,10 @@
 /* maximum frame size supported */
 #define ENETC_MAC_MAXFRM_SIZE	9600
 
+/* The max frame size with default MTU */
+#define ENETC_ETH_MAX_LEN (RTE_ETHER_MTU + \
+		RTE_ETHER_HDR_LEN + RTE_ETHER_CRC_LEN)
+
 /*
  * upper_32_bits - return bits 32-63 of a number
  * @n: the number we're accessing

diff --git a/drivers/net/enetc/enetc_ethdev.c b/drivers/net/enetc/enetc_ethdev.c
index 6ff3022874..4d2c9c0474 100644
--- a/drivers/net/enetc/enetc_ethdev.c
+++ b/drivers/net/enetc/enetc_ethdev.c

@@ -677,7 +677,7 @@ enetc_mtu_set(struct rte_eth_dev *dev, uint16_t mtu)
 		return -EINVAL;
 	}
 
-	if (frame_size > RTE_ETHER_MAX_LEN)
+	if (frame_size > ENETC_ETH_MAX_LEN)
 		dev->data->dev_conf.rxmode.offloads &=
 						DEV_RX_OFFLOAD_JUMBO_FRAME;
 	else

-- 
2.17.1