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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

     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 INTR_FAST.

     [EINVAL]           The ithdp argument points to a hardware interrupt thread.

     [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>.