From: Enrico Pozzobon <redacted>

The hardware VLAN filter (RFE_CTL_VLAN_FILTER_) drops VLAN-tagged frames
whose VID has not been registered via lan78xx_vlan_rx_add_vid(). It is
left enabled in promiscuous mode, so packet capture (e.g. tcpdump or
Wireshark) does not see tagged frames for unregistered VIDs.

Clear the filter while the interface is promiscuous and restore it from
NETIF_F_HW_VLAN_CTAG_FILTER otherwise. Enforce the same condition in
lan78xx_set_features() so netdev_update_features() cannot re-enable the
filter while promiscuous.

Fixes: 55d7de9de6c3 ("Microchip's LAN7800 family USB 2/3 to 10/100/1000 Ethernet device driver")
Signed-off-by: Enrico Pozzobon <redacted>
---
Currently, on microchip lan7801, enabling promiscuous mode does not
result in VLAN tagged packets being captured. This patch fixes this,
forcing the RFE_CTL_VLAN_FILTER_ flag to be off when promiscuous mode is
enabled.
---
 drivers/net/usb/lan78xx.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/drivers/net/usb/lan78xx.c b/drivers/net/usb/lan78xx.c
index c4cebacabcb5..a1a53ef85cb9 100644
--- a/drivers/net/usb/lan78xx.c
+++ b/drivers/net/usb/lan78xx.c

@@ -1525,7 +1525,14 @@ static void lan78xx_set_multicast(struct net_device *netdev)
 	if (dev->net->flags & IFF_PROMISC) {
 		netif_dbg(dev, drv, dev->net, "promiscuous mode enabled");
 		pdata->rfe_ctl |= RFE_CTL_MCAST_EN_ | RFE_CTL_UCAST_EN_;
+		/* bypass VLAN filter so all tagged frames are captured */
+		pdata->rfe_ctl &= ~RFE_CTL_VLAN_FILTER_;
 	} else {
+		if (dev->net->features & NETIF_F_HW_VLAN_CTAG_FILTER)
+			pdata->rfe_ctl |= RFE_CTL_VLAN_FILTER_;
+		else
+			pdata->rfe_ctl &= ~RFE_CTL_VLAN_FILTER_;
+
 		if (dev->net->flags & IFF_ALLMULTI) {
 			netif_dbg(dev, drv, dev->net,
 				  "receive all multicast enabled");

@@ -3074,7 +3081,9 @@ static int lan78xx_set_features(struct net_device *netdev,
 	else
 		pdata->rfe_ctl &= ~RFE_CTL_VLAN_STRIP_;
 
-	if (features & NETIF_F_HW_VLAN_CTAG_FILTER)
+	/* keep VLAN filter off while promiscuous */
+	if ((features & NETIF_F_HW_VLAN_CTAG_FILTER) &&
+	    !(netdev->flags & IFF_PROMISC))
 		pdata->rfe_ctl |= RFE_CTL_VLAN_FILTER_;
 	else
 		pdata->rfe_ctl &= ~RFE_CTL_VLAN_FILTER_;

---
base-commit: dc59e4fea9d83f03bad6bddf3fa2e52491777482
change-id: 20260623-lan78xx-vlan-promisc-83af8a48a7ec

Best regards,
--  
Enrico Pozzobon [off-list ref]