SPL (9) | Kernel routines | Unix Manual Pages

SPL (9) | Kernel routines | Unix Manual Pages | :man▋

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 >= spl xxx ().

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.