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

[Paolo Abeni]

On 5/30/26 1:18 AM, Jacob Keller 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.

Given it never worked and we are trying to keep the amount of changes
into net under control, I tend to consider this suitable for net-next
i.e. enable DSA support.

/P