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

[Rosen Penev]

On Fri, May 29, 2026 at 4:18 PM Jacob Keller <jacob.e.keller@xxxxxxxxx> wrote:
>
> On 5/26/2026 1:22 PM, Rosen Penev wrote:
> > 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@xxxxxxxxx>
>
> 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.
Good question. This bug is because of DSA. When this driver was
written, DSA didn't exist.

net instead of net-next makes sense but it's not my call to make.

Trivia: downstream OpenWrt used its own downstream switching mechanism
called swconfig. Before I transitioned this device to DSA, it worked
fine. After DSA it started to fail on tagged VLAN.
>
> > ---
> >  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;
>