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     swi_add, swi_sched — register and schedule software interrupt handlers


     #include <sys/param.h>
     #include <sys/bus.h>
     #include <sys/interrupt.h>

     extern struct ithd *tty_ithd;
     extern struct ithd *clk_ithd;
     extern void *net_ih;
     extern void *softclock_ih;
     extern void *vm_ih;

     swi_add(struct ithd **ithdp, const char *name, driver_intr_t handler,
         void *arg, int pri, enum intr_type flags, void **cookiep);

     swi_sched(void *cookie, int flags);


     These functions are used to register and schedule software interrupt
     handlers.  Software interrupt handlers are attached to a software
     interrupt thread, just as hardware interrupt handlers are attached to a
     hardware interrupt thread.  Multiple handlers can be attached to the same
     thread.  Software interrupt handlers can be used to queue up less
     critical processing inside of hardware interrupt handlers so that the
     work can be done at a later time.  Software interrupt threads are
     different from other kernel threads in that they are treated as an
     interrupt thread.  This means that time spent executing these threads is
     counted as interrupt time, and that they can be run via a lightweight
     context switch.

     The swi_add() function is used to register a new software interrupt
     handler.  The ithdp argument is an optional pointer to a struct ithd
     pointer.  If this argument points to an existing software interrupt
     thread, then this handler will be attached to that thread.  Otherwise a
     new thread will be created, and if ithdp is not NULL, then the pointer at
     that address to will be modified to point to the newly created thread.
     The name argument is used to associate a name with a specific handler.
     This name is appended to the name of the software interrupt thread that
     this handler is attached to.  The handler argument is the function that
     will be executed when the handler is scheduled to run.  The arg parameter
     will be passed in as the only parameter to handler when the function is
     executed.  The pri value specifies the priority of this interrupt handler
     relative to other software interrupt handlers.  If an interrupt thread is
     created, then this value is used as the vector, and the flags argument is
     used to specify the attributes of a handler such as INTR_MPSAFE.  The
     cookiep argument points to a void * cookie.  This cookie will be set to a
     value that uniquely identifies this handler, and is used to schedule the
     handler for execution later on.

     The swi_sched() function is used to schedule an interrupt handler and its
     associated thread to run.  The cookie argument specifies which software
     interrupt handler should be scheduled to run.  The flags argument
     specifies how and when the handler should be run and is a mask of one or
     more of the following flags:

     SWI_DELAY  Specifies that the kernel should mark the specified handler as
                needing to run, but the kernel should not schedule the
                software interrupt thread to run.  Instead, handler will be
                executed the next time that the software interrupt thread runs
                after being scheduled by another event.  Attaching a handler
                to the clock software interrupt thread and using this flag
                when scheduling a software interrupt handler can be used to
                implement the functionality performed by setdelayed() in
                earlier versions of FreeBSD.

     The tty_ithd and clk_ithd variables contain pointers to the software
     interrupt threads for the tty and clock software interrupts,
     respectively.  tty_ithd is used to hang tty software interrupt handlers
     off of the same thread.  clk_ithd is used to hang delayed handlers off of
     the clock software interrupt thread so that the functionality of
     setdelayed() can be obtained in conjunction with SWI_DELAY.  The net_ih,
     softclock_ih, and vm_ih handler cookies are used to schedule software
     interrupt threads to run for the networking stack, clock interrupt, and
     VM subsystem respectively.


     The swi_add() function returns zero on success and non-zero on failure.


     The swi_add() function will fail if:

     [EAGAIN]           The system-imposed limit on the total number of
                        processes under execution would be exceeded.  The
                        limit is given by the sysctl(3) MIB variable

     [EINVAL]           The flags argument specifies either INTR_ENTROPY or

     [EINVAL]           The ithdp argument points to a hardware interrupt

     [EINVAL]           Either of the name or handler arguments are NULL.

     [EINVAL]           The INTR_EXCL flag is specified and the interrupt
                        thread pointed to by ithdp already has at least one
                        handler, or the interrupt thread already has an
                        exclusive handler.


     ithread(9), taskqueue(9)


     The swi_add() and swi_sched() functions first appeared in FreeBSD 5.0.
     They replaced the register_swi() function which appeared in FreeBSD 3.0
     and the setsoft*(), and schedsoft*() functions which date back to at
     least 4.4BSD.


     Most of the global variables described in this manual page should not be
     global, or at the very least should not be declared in <sys/interrupt.h>.