.\"	$NetBSD: spl.9,v 1.6 2000/03/27 09:36:29 soda Exp $
.\"
.\" Copyright (c) 1997 Michael Long.
.\" Copyright (c) 1997 Jonathan Stone.
.\" All rights reserved.
.\"
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.\" modification, are permitted provided that the following conditions
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.Dd March 11, 1997
.Dt SPL 9
.Os
.Sh NAME
.Nm spl ,
.Nm spl0 ,
.Nm splbio ,
.Nm splclock ,
.Nm splhigh ,
.Nm splimp ,
.Nm spllowersoftclock ,
.Nm splnet ,
.Nm splsched ,
.Nm splserial ,
.Nm splsoftclock ,
.Nm splsoftnet ,
.Nm splsoftserial ,
.Nm splstatclock ,
.Nm spltty ,
.Nm splx
.Nd modify system interrupt priority level
.Sh SYNOPSIS
.Fd #include <machine/intr.h>
.Ft int
.Fn splhigh void
.Ft int
.Fn splsched void
.Ft int
.Fn splserial void
.Ft int
.Fn splclock void
.Ft int
.Fn splstatclock void
.Ft int
.Fn splimp void
.Ft int
.Fn spltty void
.Ft int
.Fn splsoftserial void
.Ft int
.Fn splnet void
.Ft int
.Fn splbio void
.Ft int
.Fn splsoftnet void
.Ft int
.Fn splsoftclock void
.Ft int
.Fn spllowersoftclock void
.Ft int
.Fn spl0 void
.Ft void
.Fn splx "int s"
.Sh DESCRIPTION
These functions raise and lower the system priority level.
They are used by kernel code to block interrupts with priority less
than or equal to the named level (e.g.
.Fn spltty
blocks interrupts of priority less than or equal to
.Dv IPL_TTY Ns ).
The code may then safely access variables or data structures which are
read or modified by interrupt service routines that run at the named
level.
.Pp
A
.Nm
function exists for each distinct priority level which can exist in
the system.  These functions and the corresponding priority levels are
used for various defined purposes, and may be divided into two main
types: hard and soft.  Hard interrupts are generated by hardware
devices.  Soft interrupts are generated by callouts, and are called
from the kernel's periodic timer interrupt service routine.
.Pp
In order of highest to lowest priority, the priority-raising functions
are:
.Bl -tag -width splsoftserialXX
.It Fn splhigh
blocks all hard and soft interrupts.  It is used for code that cannot
tolerate any interrupts, like hardware context switching code and 
the
.Xr ddb 4
in-kernel debugger.
.It Fn splserial
blocks hard interrupts from serial interfaces.  Code running at this
level may not access the tty subsystem.
.It Fn splsched
blocks interrupts that may access scheduler data structures.  Code
running at or above this level may not call
.Fn sleep ,
.Fn tsleep ,
or
.Fn wakeup ,
nor may it post signals.
.It Fn splclock
blocks the hardware clock interrupt.  It is used by
.Fn hardclock
to update kernel and process times, and must be used by any other code
that accesses time-related data.
.It Fn splstatclock
blocks the hardware statistics clock interrupt.  It is used by
.Fn statclock
to update kernel profiling and other statistics, and must be used by
any code that accesses that data.
This level is identical to
.Fn splclock
if there is no separate statistics clock.
.It Fn splimp
blocks hard interrupts from all devices that are allowed to use the
kernel
.Xr malloc 9 .
That includes all disk, network, and tty device interrupts.
.It Fn spltty
blocks hard interrupts from TTY devices.
.It Fn splsoftserial
blocks soft interrupts generated by serial devices.
.It Fn splnet
blocks hard interrupts from network interfaces.
.It Fn splbio
blocks hard interrupts from disks and other mass-storage devices.
.It Fn splsoftnet
blocks soft network interrupts.
.It Fn splsoftclock
blocks soft clock interrupts.
.El
.Pp
Two functions lower the system priority level.  They are:
.Bl -tag -width spllowersoftclockXX
.It Fn spllowersoftclock
unblocks all interrupts but the soft clock interrupt.
.It Fn spl0
unblocks all interrupts.
.El
.Pp
The
.Fn splx
function restores the system priority level to the one encoded in
.Fa s ,
which must be a value previously returned by one of the other
.Nm
functions.
.Pp
.Sh HISTORY
.Pp
Originally,
.Fn splsoftclock
lowered the system priority level.
During the
.Nx 1.5
development cycle,
.Fn spllowersoftclock
was introduced and the semantics of
.Fn splsoftclock
were changed.
.Pp