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NAME

     splbio, splclock, splhigh, splimp, splnet, splsoftclock, splsofttty,
     splstatclock, spltty, splvm, spl0, splx -- manipulate interrupt
     priorities

SYNOPSIS

     #include <sys/types.h>
     #include <sys/systm.h>

     intrmask_t
     splbio(void);

     intrmask_t
     splclock(void);

     intrmask_t
     splhigh(void);

     intrmask_t
     splimp(void);

     intrmask_t
     splnet(void);

     intrmask_t
     splsoftclock(void);

     intrmask_t
     splsofttty(void);

     intrmask_t
     splstatclock(void);

     intrmask_t
     spltty(void);

     void
     spl0(void);

     void
     splx(intrmask_t ipl);

DESCRIPTION

     This API is deprecated.  Use mutexes to protect data structures instead.
     See mutex(9) for more information.  The API is now a complete NOP.  This
     man page documents historical behavior so you can understand the code
     locking that the spl did when converting code from versions of the kernel
     prior to FreeBSD 5.0.  The examples in this man page are also obsolete
     and should not be viewed as documenting FreeBSD 5.0 and newer.

     The spl() function family sets the interrupt priority ``level'' of the
     CPU.  This prevents interrupt handlers of the blocked priority level from
     being run.  This is used in the ``synchronous'' part of a driver (the
     part that runs on behalf of the user process) to examine or modify data
     areas that might be examined or modified by interrupt handlers.

     Each driver that uses interrupts is normally assigned to an interrupt
     priority group by a keyword in its config line.  For example:

           device foo0 at isa? port 0x0815 irq 12 tty

     assigns interrupt 12 to the ``tty'' priority group.  The system
     automatically arranges for interrupts in the xxx group to be called at a
     priority >= splxxx ().

     The function splx() sets the interrupt priority to an absolute value.
     The intent is that the value returned by the other functions should be
     saved in a local variable, and later passed to splx() in order to restore
     the previous priority.

     The function spl0() lowers the priority to a value where all interrupt
     handlers are unblocked, but ASTs (asynchronous system traps) remain
     blocked until the system is about to return to user mode.

     The traditional assignment of the various device drivers to the interrupt
     priority groups can be roughly classified as:

     splnet()          Software part of the network interface drivers.

     splimp()          All network interface drivers.

     splbio()          All buffered IO (i.e., disk and the like) drivers.

     spltty()          Basically, all non-network communications devices, but
                       effectively used for all drivers that are neither
                       network nor disks.

RETURN VALUES

     All functions except splx() and spl0() return the previous priority
     value.

EXAMPLES

     This is a typical example demonstrating the usage:

     struct foo_softc {
             ...
             int flags;
     #define FOO_ASLEEP      1
     #define FOO_READY       2

     } foo_softc[NFOO];

     int
     foowrite(...)
     {
             struct foo_softc *sc;
             int s, error;

             ...
             s = spltty();
             if (!(sc->flags & FOO_READY)) {
                     /* Not ready, must sleep on resource. */
                     sc->flags |= FOO_ASLEEP;
                     error = tsleep(sc, PZERO, "foordy", 0);
                     sc->flags &= ~FOO_ASLEEP;
             }
             sc->flags &= ~FOO_READY;
             splx(s);

             ...
     }

     void
     foointr(...)
     {
             struct foo_softc *sc;

             ...
             sc->flags |= FOO_READY;
             if (sc->flags & FOO_ASLEEP)
                     /* Somebody was waiting for us, awake him. */
                     wakeup(sc);
             ...
     }

     Note that the interrupt handler should never reduce the priority level.
     It is automatically called as it had raised the interrupt priority to its
     own level, i.e., further interrupts of the same group are being blocked.

HISTORY

     The interrupt priority levels appeared in a very early version of UNIX.
     They have been traditionally known by number instead of by names, and
     were inclusive up to higher priority levels (i.e., priority 5 has been
     blocking everything up to level 5).  This is no longer the case in
     FreeBSD.  The traditional name 'level' for them is still reflected in the
     letter 'l' of the respective functions and variables, although they are
     not really levels anymore, but rather different (partially inclusive)
     sets of functions to be blocked during some periods of the life of the
     system.  The historical number scheme can be considered as a simple
     linearly ordered set of interrupt priority groups.

     FreeBSD 5.0 eliminated spl entirely in favor of locking primitives which
     scale to more than one processor.

AUTHORS

     This manual page was written by Jorg Wunsch.