[FS#3990] lantiq: gswip: packets larger than 1496 bytes dropped when VLAN is used

[OpenWrt Bugs]

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A new Flyspray task has been opened.  Details are below. 

User who did this - Jan Hoffmann (janhoffmann) 

Attached to Project - OpenWrt/LEDE Project
Summary - lantiq: gswip: packets larger than 1496 bytes dropped when VLAN is used
Task Type - Bug Report
Category - Kernel
Status - Unconfirmed
Assigned To - 
Operating System - All
Severity - Low
Priority - Very Low
Reported Version - Trunk
Due in Version - Undecided
Due Date - Undecided
Details - This issue occurs with current master on a Fritzbox 7412 and 7362 SL.

When a switch port is using tagged VLAN, any packets sent from the device which are larger than 1496 bytes are dropped. Incoming packets are received correctly.

Comparison of the old swconfig driver with the new DSA driver shows that a lower value is now written to the GSWIP_MAC_FLEN register. Now it is 1526 (VLAN_ETH_FRAME_LEN + 8), while it was 1534 before (1518 + 8 + 4 * 2, which corresponds to "MAC frame + 8-byte special tag + 4-byte VLAN tag * 2" according to comments in the vendor driver). Increasing this to the old value seems to fix the issue.

More information can be found at the following URL:
https://bugs.openwrt.org/index.php?do=details&task_id=3990

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