[FS#4116] qca9560-eth supports jumbo frames but is limited to 1516

[OpenWrt Bugs]

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User who did this - Geoffrey McRae (gnif) 

Attached to Project - OpenWrt/LEDE Project
Summary - qca9560-eth supports jumbo frames but is limited to 1516
Task Type - Bug Report
Category - Kernel
Status - Unconfirmed
Assigned To - 
Operating System - All
Severity - High
Priority - Very Low
Reported Version - Trunk
Due in Version - Undecided
Due Date - Undecided
Details - - Device problem occurs on
GL.iNet GL-AR750S (NOR)

- Software versions of OpenWrt/LEDE release, packages, etc.
Trunk

- Steps to reproduce

Attempt to increase the mtu beyond 1516

ifconifg eth0 mtu 1517

----

According to all resources the QCA9560 supports jumbo frames, as does the switch (AR8337) in this device however for an undocumented unknown reason it's hard coded in `linux/ath79/files/drivers/net/ethernet/atheros/ag71xx/ag71xx_main.c:1640` to `1540`. Also for reasons I do not understand I can not increase the mtu beyond 1516 up to this limit of `1540` which I need for 802.11q (1522 frame size).

This code seems to need to be reviewed by someone with access to the datasheets for the QCA9560 to properly resolve this issue.

More information can be found at the following URL:
https://bugs.openwrt.org/index.php?do=details&task_id=4116

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