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

[Jacob Keller <jacob.e.keller@intel.com>]

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@gmail.com>

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 <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;