.\"	$OpenBSD: power.4,v 1.5 2007/05/31 19:19:54 jmc Exp $
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.\" Copyright (c) 2003 Michael Shalayeff
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.Dd $Mdocdate: May 31 2007 $
.Dt POWER 4 hppa
.Os
.Sh NAME
.Nm power
.Nd power button and power fail support
.Sh SYNOPSIS
.Cd "power0 at mainbus?"
.Sh DESCRIPTION
.Tn HP PA-RISC
machines support power failure detection in different forms:
.Pp
.Bl -bullet -offset indent -compact
.It
.Dq soft
power button
.It
power failure interrupt
.El
.Pp
The
.Nm
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
.Pq if possible
shutdown the system through a call to the
.Xr boot 9
function.
.Pp
On systems equipped with the
.Xr lasi 4
bus adapter, we find a
.Dq soft
power button which upon depression gives us a signal in the
.Dq 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
.Em not
result in a shutdown.
.Pp
On modern systems, an interruption is signalled upon the power
button depression and thus this avoids polling and does not require
.Pq as does being implemented in hardware
dampening.
.Sh SEE ALSO
.Xr intro 4 ,
.Xr lasi 4 ,
.Xr pdc 4 ,
.Xr boot 9
.Sh HISTORY
The
.Nm
driver
appeared in
.Ox 3.4 .