OpenWrt/LEDE Project

  • Status Unconfirmed
  • Percent Complete
    0%
  • Task Type Bug Report
  • Category Kernel
  • Assigned To No-one
  • Operating System All
  • Severity High
  • Priority Very Low
  • Reported Version Trunk
  • Due in Version Undecided
  • Due Date Undecided
  • Private
Attached to Project: OpenWrt/LEDE Project
Opened by Geoffrey McRae - 30.10.2021

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

- 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.

Geoffrey McRae commented on 30.10.2021 12:18

Correction, the limit of 1516 is caused by the following code:

dev->max_mtu = max_frame_len - ag71xx_max_frame_len(0);
static inline unsigned int ag71xx_max_frame_len(unsigned int mtu)
{
	return ETH_SWITCH_HEADER_LEN + ETH_HLEN + VLAN_HLEN + mtu + ETH_FCS_LEN;
}

Where

ETH_SWITCH_HEADER_LEN = 2
ETH_HLEN              = 14
VLAN_HLEN             = 4
ETH_FCS_LEN           = 4

1540 - 2 - 14 - 4 - 4 = 1516

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