[LEDE-DEV] Broken hotplug of USB 3G modems (packages wwan and uqmi vs. netifd) — advice needed

[Ivan Shapovalov]

Hello,

I've noticed a bug in LEDE's handling of USB 3G modems using QMI
protocol (at least), which leads to broken hotplug. It's actually been
present for quite long (since BB or CC). I've tracked down the root
cause but I'm unsure where (as in which project's bugtracker) would be
the best place to report it or how would be the best way to fix it (see
below for the actual question).

The problem is that QMI proto handler clashes with netifd proper over
the interface's "device" property. QMI proto handler uses "device" to
specify the cdc-wdmN device node path[1], e. g.:

root at router:~# cat /etc/config/network

<...>
config interface 'wan'
        option proto 'qmi'
        option device '/dev/cdc-wdm0'
        option apn 'internet.megafon.ru'
        option auth 'none'

[1]: https://wiki.openwrt.org/doc/uci/network#protocol_qmi_usb_modems_using_qmi_protocol

At the same time, netifd clobbers "device" in the UCI state files in
runtime, setting it at least from the following places:

/etc/hotplug/iface/00_netstate.sh[2]

[2]: https://github.com/lede-project/source/blob/master/package/network/config/netifd/files/etc/hotplug.d/iface/00-netstate#L4
[3]: https://github.com/lede-project/source/blob/master/package/network/config/netifd/files/lib/network/config.sh#L46

The QMI proto handler reads UCI configs directly, so it isn't affected
by this clobbering. However, the wwan's hotplug handler reads UCI via
config_load/config_get and thus reads the clobbered value and is thus
broken.

The most easy way to fix this would be to read UCI configs directly in
the hotplug handler, thus ignoring state. However, this property name
clash feels dirty and I'd like to fix this properly.

Felix, what is the "device" property used for in netifd? Why is it
needed? Could you please explain what you meant by "further remapping
issues" in 477c3c4[4] and especially which "legacy scripts" you were
talking about in bffa830[5]?

[4]: https://github.com/lede-project/source/commit/477c3c40156cca124bec0520a733f455d0085914
[5]: https://github.com/lede-project/source/commit/bffa830f13d610bce0d9357c35759dec099a9ea5

Cheers,
-- 
Ivan Shapovalov / intelfx /
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