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When I run Arch on my laptop without AC power I get the error "hda: irq probe failed" and the system is bloody slow. For example, it takes five seconds for 'password' to appear when I type my login. A good five minutes to start X/KDE. When I reconnect power it is still slow, unless I boot with the power on.
This is very annoying because I can only use Windows when I'm running on the battery.
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For me this happens when my battery is almost empty and the APM system starts "yelling" IRQ's and flooding the kernel with it.
Since I can't read any more information from your post, maybe it's something similar. Maybe the IRQ setup is wrong, the APM setup, the ACPI setup .... try turning APM and/or ACPI off or pass different parameters for whatever you have. There are APM and/or ACPI HOWTOs out there - also a Battery-powered-HOWTO
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Turning APM on fixed it :-)
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