hw/core/bus.c: Only the main system bus can have no parent

In commit 80376c3fc2 in 2010 we added a workaround for
some qbus buses not being connected to qdev devices -- if the
bus has no parent object then we register a reset function which
resets the bus on system reset (and unregister it when the
bus is unparented).

Nearly a decade later, we have now no buses in the tree which
are created with non-NULL parents, so we can remove the
workaround and instead just assert that if the bus has a NULL
parent then it is the main system bus.

(The absence of other parentless buses was confirmed by
code inspection of all the callsites of qbus_create() and
qbus_create_inplace() and cross-checked by 'make check'.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Damien Hedde <damien.hedde@greensocs.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20190523150543.22676-1-peter.maydell@linaro.org

hw/core/bus.c

@@ -97,10 +97,9 @@ static void qbus_realize(BusState *bus, DeviceState *parent, const char *name)
         bus->parent->num_child_bus++;
         object_property_add_child(OBJECT(bus->parent), bus->name, OBJECT(bus), NULL);
         object_unref(OBJECT(bus));
-    } else if (bus != sysbus_get_default()) {
-        /* TODO: once all bus devices are qdevified,
-           only reset handler for main_system_bus should be registered here. */
-        qemu_register_reset(qbus_reset_all_fn, bus);
+    } else {
+        /* The only bus without a parent is the main system bus */
+        assert(bus == sysbus_get_default());
     }
 }
 
@@ -109,18 +108,16 @@ static void bus_unparent(Object *obj)
     BusState *bus = BUS(obj);
     BusChild *kid;
 
+    /* Only the main system bus has no parent, and that bus is never freed */
+    assert(bus->parent);
+
     while ((kid = QTAILQ_FIRST(&bus->children)) != NULL) {
         DeviceState *dev = kid->child;
         object_unparent(OBJECT(dev));
     }
-    if (bus->parent) {
-        QLIST_REMOVE(bus, sibling);
-        bus->parent->num_child_bus--;
-        bus->parent = NULL;
-    } else {
-        assert(bus != sysbus_get_default()); /* main_system_bus is never freed */
-        qemu_unregister_reset(qbus_reset_all_fn, bus);
-    }
+    QLIST_REMOVE(bus, sibling);
+    bus->parent->num_child_bus--;
+    bus->parent = NULL;
 }
 
 void qbus_create_inplace(void *bus, size_t size, const char *typename,