[FS#384] IPQ806X: CMD: swconfig on R7800 generate Kernel Panic!

[LEDE Bugs]

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FS#384 - IPQ806X: CMD: swconfig on R7800 generate Kernel Panic!
User who did this - Hannu Nyman (hnyman)

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Confirmed with R7800 using two firmware versions:
Reboot (SNAPSHOT, r2919-d1daf3f38d)
Reboot (SNAPSHOT, r2939-f44663c673)

During testing I noticed an aditional symptom:
The router does not react to disconnecting/connecting wan cable at all. There is no trace of loss of connectivity or acquisition of new connectivity in dmesg or logread output. The WAN LED stays lit, although the WAN cable has been disconnected from the device. Also WAN interface does no go down when the WAN cable is removed from the device.


> Only disconnect the `WAN-port` or run ‘swconfig’ later than 3 min after kernel boot, The router will run OK. 

The three minutes mentioned above is not exact: my router hanged after 4 minutes too.

(The error can be triggered by visiting the switch page in LuCI, too.)

Self-built firmware, so no issues from shared/nonshared components should materialise.

I first tested with the router having uptime of over 12 hours, and there were no problems. The swconfig command ran normally.

Then I tested with a recently rebooted router:
root at lede:~# uptime
 14:36:43 up 0 min,  load average: 1.14, 0.31, 0.10
root at lede:~# swconfig dev switch0 show
Global attributes:
        enable_vlan: 1
        enable_mirror_rx: 0
        enable_mirror_tx: 0
        mirror_monitor_port: 0
        mirror_source_port: 0
        arl_age_time: 300
---HANG---
ROUTER NEEDS TO BE MANUALLY REBOOTED

 14:42:39 up 4 min,  load average: 0.12, 0.20, 0.09
root at lede:~# swconfig dev switch0 show
Global attributes:
        enable_vlan: 1
        enable_mirror_rx: 0
        enable_mirror_tx: 0
        mirror_monitor_port: 0
        mirror_source_port: 0
        arl_age_time: 300
---HANG---
ROUTER NEEDS TO BE MANUALLY REBOOTED


root at lede:~# uptime
 14:48:56 up 5 min,  load average: 0.11, 0.16, 0.09
root at lede:~# swconfig dev switch0 show | head -n 30
Global attributes:
        enable_vlan: 1
        enable_mirror_rx: 0
        enable_mirror_tx: 0
        mirror_monitor_port: 0
        mirror_source_port: 0
        arl_age_time: 300
        arl_table: address resolution table
Port 0: MAC dc:ef:aa:bb:cc:e5
Port 2: MAC 78:24:dd:ee:ff:1c
... OK

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More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=384#comment1233