[mesh-dev] Physical link state detection with an integrated switch

[Marc Juul]

SO!

Max had some trouble last night with notdhcp not detecting link state.
There turned out to be two problems. The first one was that I'd used an int
where it should have been a uin8_t which caused link state detection not to
work on any ar71xx hardware. That's fixed now and I tested it on a Bullet
M2.

The second issue was not so easily defeated. It turns out that there are no
netlink notifications for physical link state on switches. Instead, the
kernel always reports the linkstate as connected since the internal
ethernet interface is always connected to the internal switch. The
N600/N750 boards have integrated switches, and so do the nanostations. I'm
not sure about the nanobridges or picostations (I'd think not).

It wouldn't be completely insane to extend the switch driver with the
ability to report link state changes using netlink (the switch driver
already communicates to userland with netlink), and I've outlined my
findings on how this could be done here if anyone is really curious:

  https://github.com/sudomesh/notdhcp/issues/2

However, we have a deadline and it turns out that the link state info can
be gotten with the swlib library which exists only as part of the swconfig
utility. It's a bit ugly that we can only detect link state changes by
polling but real ethernet drivers actually just poll in-kernel for these
changes anyway. I've copied the swlib source into the notdhcp repository.
Unfortunately that utility uses libnl and libnl-gen so now notdhcp has
library dependencies (which needs to be added to the sudowrt-packages
makefile and to the packages list).

Writing notdhcp has been much more of a rabbit hole than expected. But! We
want to be able to query switch port link status for purposes of the GUI
anyway and now we know that this is currently impossible (short of allowing
the running of arbitrary commands from the gui). I think once we've gotten
all this working I'll go back and move the switch status polling code over
to a fork of netifd and then make notdhcp use ubus to listen for the link
state changes.

Now the skylights are no longer black and that's my queue to call it a
night. I'll try to get this working before Tuesday.

-- 
marc/juul
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