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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , linux-kernel@vger.kernel.org (open list) Subject: [PATCH net-next] net: ibm: emac: Reserve VLAN header in MJS limit Date: Tue, 26 May 2026 13:22:47 -0700 Message-ID: <20260526202247.13823-1-rosenp@gmail.com> X-Mailer: git-send-email 2.54.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" 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 Reviewed-by: Jacob Keller --- 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/ib= m/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 #include #include +#include #include #include #include @@ -457,7 +458,7 @@ static inline u32 emac_iff2rmr(struct net_device *ndev) =20 if (emac_has_feature(dev, EMAC_APM821XX_REQ_JUMBO_FRAME_SIZE)) { r &=3D ~EMAC4_RMR_MJS_MASK; - r |=3D EMAC4_RMR_MJS(ndev->mtu); + r |=3D EMAC4_RMR_MJS(ndev->mtu + VLAN_HLEN); } =20 return r; --=20 2.54.0