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FS#4116 - qca9560-eth supports jumbo frames but is limited to 1516 #9097

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openwrt-bot opened this issue Oct 30, 2021 · 1 comment
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openwrt-bot commented Oct 30, 2021

gnif:

  • 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.

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openwrt-bot commented Oct 30, 2021

gnif:

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

@aparcar aparcar added the kernel label Feb 22, 2022
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