kern/intr: implement specialized SWI class

The event compatibility implementation is targeted for compatibility
for source not converted to devices.  Notably architectures being
maintained outside the main tree.  SWIs can have lower overhead by
removing the full compatibility.

Author: ehem

sys/kern/kern_intr.c

@@ -1138,18 +1138,39 @@ intr_event_schedule_thread(struct intr_event *ie, struct trapframe *frame)
 	return (0);
 }
 
+/*
+ * Pseudo-PIC implementation for SWIs swi_add().  Lower overhead than
+ * intr_event_create()'s full implementation.
+ */
+
+static void
+swi_event_func(device_t pic, interrupt_t *intr)
+{
+}
+
 /*
  * Allow interrupt event binding for software interrupt handlers -- a no-op,
  * since interrupts are generated in software rather than being directed by
  * a PIC.
  */
 static int
-swi_assign_cpu(void *arg, int cpu)
+swi_event_assign(device_t pic, interrupt_t *intr, u_int cpu)
 {
 
 	return (0);
 }
 
+static device_method_t swi_event_methods[] = {
+	DEVMETHOD(intr_event_pre_ithread,	swi_event_func),
+	DEVMETHOD(intr_event_post_ithread,	swi_event_func),
+	DEVMETHOD(intr_event_post_filter,	swi_event_func),
+	DEVMETHOD(intr_event_assign_cpu,	swi_event_assign),
+
+	DEVMETHOD_END
+};
+
+PRIVATE_DEFINE_CLASSN(swi_event, swi_event_class, swi_event_methods, 0);
+
 /*
  * Add a software interrupt handler to a specified event.  If a given event
  * is not specified, then a new event is created.
@@ -1170,10 +1191,22 @@ swi_add(struct intr_event **eventp, const char *name, driver_intr_t handler,
 		if (!(ie->ie_flags & IE_SOFT))
 			return (EINVAL);
 	} else {
-		error = intr_event_create(&ie, NULL, IE_SOFT, 0,
-		    NULL, NULL, NULL, swi_assign_cpu, "swi%d:", pri);
-		if (error)
+		static device_t handler = NULL;
+
+		if (__predict_false(handler == NULL)) {
+			handler = bus_generic_add_child(root_bus,
+			    BUS_PASS_ORDER_FIRST, "swi-event", 0);
+			device_set_driver(handler, &swi_event_class);
+		}
+
+		ie = malloc(sizeof(struct intr_event), M_ITHREAD,
+		    M_WAITOK | M_ZERO);
+		error = intr_event_init_(ie, handler, 0, IE_SOFT, "swi%d:",
+		    pri);
+		if (error) {
+			free(ie, M_ITHREAD);
 			return (error);
+		}
 		if (eventp != NULL)
 			*eventp = ie;
 	}