qdev: support properties which don't set a default value

In some situations it's useful to have a qdev property which doesn't
automatically set its default value when qdev_property_add_static is
called (for instance when the default value is not constant).

Support this by adding a flag to the Property struct indicating
whether to set the default value.  This replaces the existing test
for whether the PropertyInfo set_default_value function pointer is
NULL, and we set the .set_default field to true for all those cases
of struct Property which use a PropertyInfo with a non-NULL
set_default_value, so behaviour remains the same as before.

This gives us the semantics of:
 * if .set_default is true, then .info->set_default_value must
   be not NULL, and .defval is used as the the default value of
   the property
 * otherwise, the property system does not set any default, and
   the field will retain whatever initial value it was given by
   the device's .instance_init method

We define two new macros DEFINE_PROP_SIGNED_NODEFAULT and
DEFINE_PROP_UNSIGNED_NODEFAULT, to cover the most plausible use cases
of wanting to set an integer property with no default value.

Suggested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-id: 1499788408-10096-3-git-send-email-peter.maydell@linaro.org

hw/core/qdev.c

@@ -800,7 +800,7 @@ void qdev_property_add_static(DeviceState *dev, Property *prop,
                                     prop->info->description,
                                     &error_abort);
 
-    if (prop->info->set_default_value) {
+    if (prop->set_default) {
         prop->info->set_default_value(obj, prop);
     }
 }

include/hw/qdev-core.h

@@ -221,11 +221,21 @@ struct BusState {
     QLIST_ENTRY(BusState) sibling;
 };
 
+/**
+ * Property:
+ * @set_default: true if the default value should be set from @defval,
+ *    in which case @info->set_default_value must not be NULL
+ *    (if false then no default value is set by the property system
+ *     and the field retains whatever value it was given by instance_init).
+ * @defval: default value for the property. This is used only if @set_default
+ *     is true.
+ */
 struct Property {
     const char   *name;
     const PropertyInfo *info;
     ptrdiff_t    offset;
     uint8_t      bitnr;
+    bool         set_default;
     union {
         int64_t i;
         uint64_t u;

include/hw/qdev-properties.h

@@ -44,15 +44,24 @@ extern const PropertyInfo qdev_prop_link;
         .info      = &(_prop),                                          \
         .offset    = offsetof(_state, _field)                           \
             + type_check(_type,typeof_field(_state, _field)),           \
+        .set_default = true,                                            \
         .defval.i  = (_type)_defval,                                    \
         }
 
+#define DEFINE_PROP_SIGNED_NODEFAULT(_name, _state, _field, _prop, _type) { \
+        .name      = (_name),                                           \
+        .info      = &(_prop),                                          \
+        .offset    = offsetof(_state, _field)                           \
+            + type_check(_type, typeof_field(_state, _field)),          \
+        }
+
 #define DEFINE_PROP_BIT(_name, _state, _field, _bit, _defval) {  \
         .name      = (_name),                                    \
         .info      = &(qdev_prop_bit),                           \
         .bitnr    = (_bit),                                      \
         .offset    = offsetof(_state, _field)                    \
             + type_check(uint32_t,typeof_field(_state, _field)), \
+        .set_default = true,                                     \
         .defval.u  = (bool)_defval,                              \
         }
 
@@ -61,15 +70,24 @@ extern const PropertyInfo qdev_prop_link;
         .info      = &(_prop),                                          \
         .offset    = offsetof(_state, _field)                           \
             + type_check(_type, typeof_field(_state, _field)),          \
+        .set_default = true,                                            \
         .defval.u  = (_type)_defval,                                    \
         }
 
+#define DEFINE_PROP_UNSIGNED_NODEFAULT(_name, _state, _field, _prop, _type) { \
+        .name      = (_name),                                           \
+        .info      = &(_prop),                                          \
+        .offset    = offsetof(_state, _field)                           \
+            + type_check(_type, typeof_field(_state, _field)),          \
+        }
+
 #define DEFINE_PROP_BIT64(_name, _state, _field, _bit, _defval) {       \
         .name      = (_name),                                           \
         .info      = &(qdev_prop_bit64),                                \
         .bitnr    = (_bit),                                             \
         .offset    = offsetof(_state, _field)                           \
             + type_check(uint64_t, typeof_field(_state, _field)),       \
+        .set_default = true,                                            \
         .defval.u  = (bool)_defval,                                     \
         }
 
@@ -78,6 +96,7 @@ extern const PropertyInfo qdev_prop_link;
         .info      = &(qdev_prop_bool),                          \
         .offset    = offsetof(_state, _field)                    \
             + type_check(bool, typeof_field(_state, _field)),    \
+        .set_default = true,                                     \
         .defval.u    = (bool)_defval,                            \
         }
 
@@ -111,6 +130,7 @@ extern const PropertyInfo qdev_prop_link;
                           _arrayfield, _arrayprop, _arraytype) {        \
         .name = (PROP_ARRAY_LEN_PREFIX _name),                          \
         .info = &(qdev_prop_arraylen),                                  \
+        .set_default = true,                                            \
         .defval.u = 0,                                                  \
         .offset = offsetof(_state, _field)                              \
             + type_check(uint32_t, typeof_field(_state, _field)),       \