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net: ibm: emac: Reserve VLAN header in MJS limit

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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>
Link: https://patch.msgid.link/20260526202247.13823-1-rosenp@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
This commit is contained in:
Rosen Penev 2026-05-26 13:22:47 -07:00 committed by Paolo Abeni
parent c2c0486c56
commit 0906c117f8
1 changed files with 2 additions and 1 deletions
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3
drivers/net/ethernet/ibm/emac/core.c
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@ -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;