FS#3285 - Can't change MTU on IPQ806x devices

[openwrt-bot]

jacekowski:

• I've personally tested it on ZyXEL NBG6817
• Any version with https://patchwork.kernel.org/patch/11283121/ patch applied
• To reproduce:

ifdown wan

ifconfig (wan_interface) MTU 1499 (or 1501 or anything other than 1500)

This bug is caused by txfifosz being 0 on IPQ806x devices (and possibly more) unless tx-fifo-depth is specified in device tree, potential fix would be to specify tx-fifo-depth in device tree for all affected devices, or, check if txfifosz == 0 and then allow MTU change.

[openwrt-bot]

kebe:

Same here. Archer C2600 (ipq8064)

[openwrt-bot]

Olipro:

Ditto on Netgear R7800, this is a new regression since I updated from an early 2019 build. It blocks me from using baby jumbo frames (1508 MTU) over PPPoE.