Warning on mac80211/tx.c:4292

Openwrt 19.07.4
Router: ZBT-WE1326
SoC Type: MediaTek MT7621

The router seemed to have recovered.

[62003.671208] ------------[ cut here ]------------
[62003.675906] WARNING: CPU: 2 PID: 19 at backports-4.19.137-1/net/mac80211/tx.c:4292 0x8f1acbc4 [mac80211@8f180000+0x6cca0]
[62003.686885] Modules linked in: pppoe ppp_async pppox ppp_generic nf_conntrack_ipv6 mt76x2e mt76x2_common mt76x02_lib mt7603e mt76 mac80211 iptable_nat ipt_REJECT ipt_MASQUERADE cfg80211 xt_time xt_tcpudp xt_state xt_nat xt_multiport xt_mark xt_mac xt_limit xt_conntrack xt_comment xt_TCPMSS xt_REDIRECT xt_LOG xt_FLOWOFFLOAD xt_CT slhc nf_reject_ipv4 nf_nat_redirect nf_nat_masquerade_ipv4 nf_conntrack_ipv4 nf_nat_ipv4 nf_nat nf_log_ipv4 nf_flow_table_hw nf_flow_table nf_defrag_ipv6 nf_defrag_ipv4 nf_conntrack_rtcache nf_conntrack iptable_mangle iptable_filter ip_tables crc_ccitt compat nf_log_ipv6 nf_log_common ip6table_mangle ip6table_filter ip6_tables ip6t_REJECT x_tables nf_reject_ipv6 tun mmc_block mtk_sd mmc_core leds_gpio xhci_plat_hcd xhci_pci xhci_mtk xhci_hcd gpio_button_hotplug usbcore nls_base
[62003.758052]  usb_common
[62003.760558] CPU: 2 PID: 19 Comm: ksoftirqd/2 Not tainted 4.14.195 #0
[62003.766881] Stack : 00000000 00000000 00000000 8e899030 00000000 00000000 00000000 00000000
[62003.775214]         00000000 00000000 00000000 00000000 00000000 00000001 8fc83b70 ac07f5da
[62003.783551]         8fc83c08 00000000 00000000 00005d70 00000038 8049c858 00000007 00000000
[62003.791888]         00000000 80550000 000b9aee 00000000 8fc83b50 00000000 00000000 8f1e7fd0
[62003.800224]         8f1acbc4 000010c4 8fc13480 8e899030 00000003 802ad210 00000008 806b0008
[62003.808558]         ...
[62003.810999] Call Trace:
[62003.811059] [<8049c858>] 0x8049c858
[62003.816967] [<8f1acbc4>] 0x8f1acbc4 [mac80211@8f180000+0x6cca0]
[62003.822865] [<802ad210>] 0x802ad210
[62003.826344] [<8000c1a0>] 0x8000c1a0
[62003.829819] [<8000c1a8>] 0x8000c1a8
[62003.833298] [<804856c4>] 0x804856c4
[62003.836773] [<8f1acbc4>] 0x8f1acbc4 [mac80211@8f180000+0x6cca0]
[62003.842663] [<80072a84>] 0x80072a84
[62003.846134] [<8002e608>] 0x8002e608
[62003.849613] [<8f1acbc4>] 0x8f1acbc4 [mac80211@8f180000+0x6cca0]
[62003.855519] [<8002e6f0>] 0x8002e6f0
[62003.858992] [<80053b7c>] 0x80053b7c
[62003.862467] [<8f1acbc4>] 0x8f1acbc4 [mac80211@8f180000+0x6cca0]
[62003.868374] [<8f1af408>] 0x8f1af408 [mac80211@8f180000+0x6cca0]
[62003.874298] [<8e907b90>] 0x8e907b90 [mt76x02_lib@8e900000+0x96c0]
[62003.880374] [<8f1af86c>] 0x8f1af86c [mac80211@8f180000+0x6cca0]
[62003.886283] [<8e907bd0>] 0x8e907bd0 [mt76x02_lib@8e900000+0x96c0]
[62003.892362] [<8f1b65e0>] 0x8f1b65e0 [mac80211@8f180000+0x6cca0]
[62003.898272] [<8f1b66bc>] 0x8f1b66bc [mac80211@8f180000+0x6cca0]
[62003.904173] [<8e904a94>] 0x8e904a94 [mt76x02_lib@8e900000+0x96c0]
[62003.910247] [<8f1f5000>] 0x8f1f5000 [mt76@8f1f0000+0x79c0]
[62003.915719] [<800329f4>] 0x800329f4
[62003.919194] [<804a3658>] 0x804a3658
[62003.922664] [<8004fd10>] 0x8004fd10
[62003.926139] [<8004fd10>] 0x8004fd10
[62003.929615] [<80032cc8>] 0x80032cc8
[62003.933092] [<8004feb8>] 0x8004feb8
[62003.936560] [<8049ebd8>] 0x8049ebd8
[62003.940044] [<8004bf38>] 0x8004bf38
[62003.943520] [<8004be08>] 0x8004be08
[62003.947006] [<8004be08>] 0x8004be08
[62003.950493] [<8004be08>] 0x8004be08
[62003.953969] [<80006f78>] 0x80006f78
[62003.957448] 
[62003.959125] ---[ end trace 76264dc1c9d8024b ]---
root@OpenWrt:~# 

Start by examining net/mac80211/tx.c:4292 and the mac80211 call trace in the report. Reproduce the warning on OpenWrt 19.07.4 with the ZBT-WE1326 and its MediaTek MT7621 hardware, then trace the interaction with the mt76x2e and related modules. Done means identifying the cause and preventing the warning without regressing wireless operation.