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Fri, 29 May 2026 23:18:45 +0000 Message-ID: <625ecceb-3b0a-45a8-817d-337b37df7335@intel.com> Date: Fri, 29 May 2026 16:18:43 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH net-next] net: ibm: emac: Reserve VLAN header in MJS limit To: Rosen Penev , CC: Andrew Lunn , "David S. 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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 If this results in dropped packets, that seems like a user visible bug.. Should this have a Fixes tag and target the net-next tree? I guess its not a "regression" since the driver has always behaved this way, but it still seems like a bug to me. > --- > 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 > #include > #include > +#include > #include > #include > #include > @@ -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;