qdev: Introduce DEFINE_PROP_LINK

This property can be used to replace the object_property_add_link in
device code, to add a link to other objects, which is a common pattern.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <20170714021509.23681-4-famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/core/qdev-properties.c

@@ -1214,3 +1214,21 @@ PropertyInfo qdev_prop_size = {
     .set = set_size,
     .set_default_value = set_default_value_uint,
 };
+
+/* --- object link property --- */
+
+static void create_link_property(Object *obj, Property *prop, Error **errp)
+{
+    Object **child = qdev_get_prop_ptr(DEVICE(obj), prop);
+
+    object_property_add_link(obj, prop->name, prop->link_type,
+                             child,
+                             qdev_prop_allow_set_link_before_realize,
+                             OBJ_PROP_LINK_UNREF_ON_RELEASE,
+                             errp);
+}
+
+PropertyInfo qdev_prop_link = {
+    .name = "link",
+    .create = create_link_property,
+};

include/hw/qdev-core.h

@@ -233,6 +233,7 @@ struct Property {
     int          arrayoffset;
     PropertyInfo *arrayinfo;
     int          arrayfieldsize;
+    const char   *link_type;
 };
 
 struct PropertyInfo {

include/hw/qdev-properties.h

@@ -30,6 +30,7 @@ extern PropertyInfo qdev_prop_pci_devfn;
 extern PropertyInfo qdev_prop_blocksize;
 extern PropertyInfo qdev_prop_pci_host_devaddr;
 extern PropertyInfo qdev_prop_arraylen;
+extern PropertyInfo qdev_prop_link;
 
 #define DEFINE_PROP(_name, _state, _field, _prop, _type) { \
         .name      = (_name),                                    \
@@ -117,6 +118,14 @@ extern PropertyInfo qdev_prop_arraylen;
         .arrayoffset = offsetof(_state, _arrayfield),                   \
         }
 
+#define DEFINE_PROP_LINK(_name, _state, _field, _type, _ptr_type) {     \
+        .name = (_name),                                                \
+        .info = &(qdev_prop_link),                                      \
+        .offset = offsetof(_state, _field)                              \
+            + type_check(_ptr_type, typeof_field(_state, _field)),      \
+        .link_type  = _type,                                            \
+        }
+
 #define DEFINE_PROP_UINT8(_n, _s, _f, _d)                       \
     DEFINE_PROP_UNSIGNED(_n, _s, _f, _d, qdev_prop_uint8, uint8_t)
 #define DEFINE_PROP_UINT16(_n, _s, _f, _d)                      \