ath(4) interrupt time

[Kostik Belousov]

I have Acer laptop with the Atheros card, identified as

ath0 at pci6:2:0:  class=0x020000 card=0x04181468 chip=0x001a168c rev=0x01 hdr=0x00
	vendor     = 'Atheros Communications Inc.'
	class      = network
	subclass   = ethernet

Corresponding dmesg lines are

ath0: <Atheros 5212> mem 0xd0000000-0xd000ffff irq 22 at device 2.0 on pci6
ath0: Reserved 0x10000 bytes for rid 0x10 type 3 at 0xd0000000
ath0: [GIANT-LOCKED]

Driver successfully attaches to the card and wifi connection works nice.

There is some problem. At home, where I have no access point, I did
ifconfig ath0 up
ifconfig ath0 list scan
Shortly after then, interrupt time goes to ~95%, top -S accounted the time
to the ath0 interrupt thread. After
ifconfig ath0 down
machine returned to working condition.

Some time later I did the same (in different location), while laptop was
powered from battery and powerd ran. After short time, machine locked
(presumably, due to interrupts being steadily delivered and lowered frequency
of the CPU, that cannot cope with the interrupts stream).

Other than that, machine is stable, buildworld/port build for several hours
did not cause the problems.

Any advise how can I help to debug the issue ?
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