airoha: account for L2 overhead in PPE MTU configuration

The PPE egress MTU register and WAN MTU register compare against L2
frame length without FCS, as confirmed by the hardware reset value of
0x05EA (1514 = ETH_HLEN + 1500).

Account for VLAN_ETH_HLEN when programming these registers to prevent
valid VLAN-tagged frames from being incorrectly dropped by hardware.

Signed-off-by: Sayantan Nandy <sayantann11@gmail.com>
[ add commit description ]
Link: https://github.com/openwrt/openwrt/pull/23875
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>

diff --git a/target/linux/airoha/patches-6.18/920-16-net-airoha-Account-for-L2-overhead-in-PPE-MTU-configuration.patch b/target/linux/airoha/patches-6.18/920-16-net-airoha-Account-for-L2-overhead-in-PPE-MTU-configuration.patch
new file mode 100644 (file)
index 0000000..5aac6bd
--- /dev/null
+++ b/target/linux/airoha/patches-6.18/920-16-net-airoha-Account-for-L2-overhead-in-PPE-MTU-configuration.patch
@@ -0,0 +1,79 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Sayantan Nandy <sayantann11@gmail.com>
+Date: Thu, 19 Jun 2026 12:00:00 +0530
+Subject: [PATCH] net: airoha: Account for L2 overhead in PPE MTU configuration
+
+The PPE egress MTU register and WAN MTU register compare against L2
+frame length without FCS, as confirmed by the hardware reset value of
+0x05EA (1514 = ETH_HLEN + 1500).
+
+Account for VLAN_ETH_HLEN when programming these registers to prevent
+valid VLAN-tagged frames from being incorrectly dropped by hardware.
+
+Rename the local variable from 'mtu' to 'len' in airoha_ppe_set_mtu()
+since it now represents a frame length rather than an L3 MTU value.
+
+Fixes: 99e204d255c9 ("net: airoha: Rework MTU configuration")
+
+Signed-off-by: Sayantan Nandy <sayantann11@gmail.com>
+---
+ drivers/net/ethernet/airoha/airoha_eth.c | 2 +-
+ drivers/net/ethernet/airoha/airoha_ppe.c | 7 ++++---
+ 2 files changed, 5 insertions(+), 4 deletions(-)
+
+--- a/drivers/net/ethernet/airoha/airoha_eth.c
++++ b/drivers/net/ethernet/airoha/airoha_eth.c
+@@ -8,6 +8,7 @@
+ #include <linux/of_reserved_mem.h>
+ #include <linux/platform_device.h>
+ #include <linux/tcp.h>
++#include <linux/if_vlan.h>
+ #include <linux/pcs/pcs.h>
+ #include <linux/u64_stats_sync.h>
+ #include <net/dst_metadata.h>
+@@ -2115,7 +2116,7 @@ static void airoha_dev_set_mtu(struct ne
+       if (!airoha_is_lan_gdm_dev(dev))
+               airoha_fe_rmw(dev->eth, REG_WAN_MTU0,
+                             WAN_MTU0_MASK,
+-                            FIELD_PREP(WAN_MTU0_MASK, netdev->mtu));
++                            FIELD_PREP(WAN_MTU0_MASK, netdev->mtu + VLAN_ETH_HLEN));
+ }
+ 
+ static int airoha_dev_open(struct net_device *netdev)
+--- a/drivers/net/ethernet/airoha/airoha_ppe.c
++++ b/drivers/net/ethernet/airoha/airoha_ppe.c
+@@ -10,6 +10,7 @@
+ #include <linux/platform_device.h>
+ #include <linux/rhashtable.h>
+ #include <net/ipv6.h>
++#include <linux/if_vlan.h>
+ #include <net/pkt_cls.h>
+ #include <net/route.h>
+ 
+@@ -107,7 +108,7 @@ void airoha_ppe_set_mtu(struct airoha_gd
+       struct airoha_eth *eth = dev->eth;
+       struct airoha_qdma *qdma;
+       int i, ppe_id, index;
+-      u32 mtu = 0;
++      u32 len = 0;
+ 
+       qdma = airoha_qdma_deref(dev);
+       for (i = 0; i < ARRAY_SIZE(port->devs); i++) {
+@@ -124,14 +125,15 @@ void airoha_ppe_set_mtu(struct airoha_gd
+ 
+               netdev = netdev_from_priv(d);
+               if (netif_running(netdev))
+-                      mtu = max_t(u32, mtu, netdev->mtu);
++                      len = max_t(u32, len, netdev->mtu);
+       }
+ 
++      len += VLAN_ETH_HLEN;
+       ppe_id = !airoha_is_lan_gdm_dev(dev) && airoha_ppe_is_enabled(eth, 1);
+       index = port->id == AIROHA_GDM4_IDX ? 7 : port->id;
+       airoha_fe_rmw(eth, REG_PPE_MTU(ppe_id, index),
+                     FP_EGRESS_MTU_MASK(index),
+-                    __field_prep(FP_EGRESS_MTU_MASK(index), mtu));
++                    __field_prep(FP_EGRESS_MTU_MASK(index), len));
+ }
+ 
+ static void airoha_ppe_hw_init(struct airoha_ppe *ppe)