[PATCH net-next] net: ibm: emac: Reserve VLAN header in MJS limit

[Rosen Penev <rosenp@gmail.com>]

The IBM EMAC programs its Maximum Jumbo Size (MJS) drop
threshold from ndev->mtu directly. The hardware sizes the threshold
against the L2 frame minus the ethernet header, but does not
discount the 802.1Q tag, so a frame carrying a VLAN tag and a full
1500-byte payload exceeds MJS by exactly 4 bytes and is dropped.

This is normally hidden because JPSM (and therefore the MJS check)
only engages when the MTU is raised above ETH_DATA_LEN.  With the
qca8k DSA tagger the conduit MTU is bumped by QCA_HDR_LEN to 1502
during dsa_conduit_setup(), which is enough to enable JPSM and
expose the off-by-VLAN-tag in the limit.

Pad MJS by VLAN_HLEN so a VLAN-tagged full-MTU frame passes.

Reported on Meraki MX60 (qca8k switch): tagged VLAN
traffic drops at 1500-byte payload, while 1496 bytes works
and untagged 1500 bytes works.

Assisted-by: Claude:Opus-4.7
Signed-off-by: Rosen Penev <rosenp@gmail.com>
---
 drivers/net/ethernet/ibm/emac/core.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/ibm/emac/core.c b/drivers/net/ethernet/ibm/emac/core.c
index 11d673a9f119..90471cff37dd 100644
--- a/drivers/net/ethernet/ibm/emac/core.c
+++ b/drivers/net/ethernet/ibm/emac/core.c
@@ -30,6 +30,7 @@
 #include <linux/skbuff.h>
 #include <linux/crc32.h>
 #include <linux/ethtool.h>
+#include <linux/if_vlan.h>
 #include <linux/mii.h>
 #include <linux/bitops.h>
 #include <linux/of.h>
@@ -457,7 +458,7 @@ static inline u32 emac_iff2rmr(struct net_device *ndev)
 
 	if (emac_has_feature(dev, EMAC_APM821XX_REQ_JUMBO_FRAME_SIZE)) {
 		r &= ~EMAC4_RMR_MJS_MASK;
-		r |= EMAC4_RMR_MJS(ndev->mtu);
+		r |= EMAC4_RMR_MJS(ndev->mtu + VLAN_HLEN);
 	}
 
 	return r;
-- 
2.54.0