power4
Power button and power fail support
Usage
power0 at mainbus?
Overview
HP PA-RISC machines support power failure detection in different forms:
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“soft” power button
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power failure interrupt
The power driver tries its best to determine the power failure condition that is a result of a power button being depressed or a power supply system failure or a similar condition. Upon detection of any of the aforementioned conditions, it tries to gracefully (if possible) shutdown the system through a call to the boot(9) function.
On systems equipped with the lasi(4) bus adapter, we find a “soft” power button which upon depression gives us a signal in the “power register” and thus polling periodically we detect the event. Since that bit comes directly from the power button without any dampening, we perform a second long delay; thus quick transitions off and on will not result in a shutdown.
On modern systems, an interruption is signalled upon the power button depression and thus this avoids polling and does not require (as does being implemented in hardware) dampening.