The reason I haven't (yet) written any particular directions up on
this is because I'm still trying to come up with relevant workarounds
for spectral scan on the AR9280, when it's active with traffic.

Since some of you will likely enable it with traffic and then discover
that things hang (and frequently), I'd like to make sure the general
case recoverable.

What I've discovered thus far:

* In STA mode, the beacon timer programming doesn't happen after a
watchdog reset.
* In STA mode, a beacon miss event can be indicative of RX deafness,
so the right thing to do there is a no-loss reset. A reset does fix
the RX deafness hang issue, but it doesn't reprogram the beacon
timers, so subsequent beacon miss interrupts don't occur.
* If I overflow the RX queue, the recovery doesn't .. recover. This
may be a failure mode that's very specific to "RX queue overflow with
PHY errors" rather than "RX queue overflow with data." At this stage,
the RX just totally hangs until a reset occurs.

I've fixed #1 and #2, and #3 is an interesting side-case. Once #2 was
fixed, #3 immediately results in missed beacons, which results in a
reset. But if too many missed beacons occur due to deafness, the
association session is reset.

So what I may do is:

* Add the code to -HEAD to correctly reprogram the beacon timer after
a missed beacon -> hardware reset;
* Always reset the hardware after a beacon miss, in case it's a hang
that we haven't yet detected;
* Add the spectral scan related signatures to the HAL, so the
ath_bmiss_proc() code doesn't call ieee80211_beacon_miss() and thus
disassociate things.

I'll also see if a full reset rather than a warm reset fixes whatever
the bad PHY hardware state is that's causing things to continuously
get stuck.

Once that's done, I'll release some updated instructions so people can
tinker with it, along with "this can hang all kinds of things and
decrease performance, be careful" warnings. I may even wrap the code
up in a github project or something and make a port.

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