[Bug] ag71xx: MTU limited to 1516

[Dominik Kleiser]

Hello,

I run a batman-adv mesh consisting of several Ubiquiti UniFi AC Mesh Pro. Some of the devices are connected over gigabit ethernet and I want to use the wired connection as a mesh backhaul. This requires setting the ethernet MTU to at least 1528 bytes, since batman-adv prepends its own header to packets entering the mesh. This shouldn't be a problem since both the Qualcomm Atheros QCA956X SoC and the Atheros AR8337 switch support jumbo frames. However, after doing some testing I figured out that I can only set the MTU to a value up to 1516. If I go higher, ifconfig fails with the error message 'SIOCSIFMTU: Invalid argument':

# ifconfig eth0 mtu 9000
ifconfig: SIOCSIFMTU: Invalid argument
# ifconfig eth0 mtu 4072
ifconfig: SIOCSIFMTU: Invalid argument
# ifconfig eth0 mtu 1500
# ifconfig eth0 mtu 1490
# ifconfig eth0 mtu 1520
ifconfig: SIOCSIFMTU: Invalid argument
# ifconfig eth0 mtu 1516
# ifconfig eth0 mtu 1517
ifconfig: SIOCSIFMTU: Invalid argument

This issue exists both in OpenWrt 19.07.7 and 21.02.0-rc1. I did some research and found older reports suggesting that other devices are also affected by this issue. 
(https://forum.openwrt.org/t/ag71xx-gigabit-e-g-archer-c7-v5-eth-raising-mtu-problem-regression/44138, https://dev.archive.openwrt.org/ticket/18296) 

Is anybody around who can help fixing the problem? I think https://forum.openwrt.org/t/ag71xx-gigabit-e-g-archer-c7-v5-eth-raising-mtu-problem-regression/44138 already describes a potential patch.

Thanks!

Regards,
Dominik