qdev: Support marking individual buses as 'full'

By default, QEMU will allow devices to be plugged into a bus up to
the bus class's device count limit.  If the user creates a device on
the command line or via the monitor and doesn't explicitly specify
the bus to plug it in, QEMU will plug it into the first non-full bus
that it finds.

This is fine in most cases, but some machines have multiple buses of
a given type, some of which are dedicated to on-board devices and
some of which have an externally exposed connector for user-pluggable
devices. One example is I2C buses.

Provide a new function qbus_mark_full() so that a machine model can
mark this kind of "internal only" bus as 'full' after it has created
all the devices that should be plugged into that bus. The "find a
non-full bus" algorithm will then skip the internal-only bus when
looking for a place to plug in user-created devices.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210903151435.22379-2-peter.maydell@linaro.org

include/hw/qdev-core.h

@@ -264,6 +264,7 @@ struct BusState {
     HotplugHandler *hotplug_handler;
     int max_index;
     bool realized;
+    bool full;
     int num_children;
 
     /*
@@ -798,6 +799,29 @@ static inline bool qbus_is_hotpluggable(BusState *bus)
    return bus->hotplug_handler;
 }
 
+/**
+ * qbus_mark_full: Mark this bus as full, so no more devices can be attached
+ * @bus: Bus to mark as full
+ *
+ * By default, QEMU will allow devices to be plugged into a bus up
+ * to the bus class's device count limit. Calling this function
+ * marks a particular bus as full, so that no more devices can be
+ * plugged into it. In particular this means that the bus will not
+ * be considered as a candidate for plugging in devices created by
+ * the user on the commandline or via the monitor.
+ * If a machine has multiple buses of a given type, such as I2C,
+ * where some of those buses in the real hardware are used only for
+ * internal devices and some are exposed via expansion ports, you
+ * can use this function to mark the internal-only buses as full
+ * after you have created all their internal devices. Then user
+ * created devices will appear on the expansion-port bus where
+ * guest software expects them.
+ */
+static inline void qbus_mark_full(BusState *bus)
+{
+    bus->full = true;
+}
+
 void device_listener_register(DeviceListener *listener);
 void device_listener_unregister(DeviceListener *listener);
 

softmmu/qdev-monitor.c

@@ -435,7 +435,12 @@ static DeviceState *qbus_find_dev(BusState *bus, char *elem)
 
 static inline bool qbus_is_full(BusState *bus)
 {
-    BusClass *bus_class = BUS_GET_CLASS(bus);
+    BusClass *bus_class;
+
+    if (bus->full) {
+        return true;
+    }
+    bus_class = BUS_GET_CLASS(bus);
     return bus_class->max_dev && bus->num_children >= bus_class->max_dev;
 }