Re: [Socratic RFC PATCH] include: attempt to document device_class_set_props

[Alex Bennée]

Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk> writes:

> On 27/03/2023 14:15, Alex Bennée wrote:
>
>> I'm still not sure how I achieve by use case of the parent class
>> defining the following properties:
>>    static Property vud_properties[] = {
>>        DEFINE_PROP_CHR("chardev", VHostUserDevice, chardev),
>>        DEFINE_PROP_UINT16("id", VHostUserDevice, id, 0),
>>        DEFINE_PROP_UINT32("num_vqs", VHostUserDevice, num_vqs, 1),
>>        DEFINE_PROP_END_OF_LIST(),
>>    };
>> But for the specialisation of the class I want the id to default to
>> the actual device id, e.g.:
>>    static Property vu_rng_properties[] = {
>>        DEFINE_PROP_UINT16("id", VHostUserDevice, id, VIRTIO_ID_RNG),
>>        DEFINE_PROP_UINT32("num_vqs", VHostUserDevice, num_vqs, 1),
>>        DEFINE_PROP_END_OF_LIST(),
>>    };
>> And so far the API for doing that isn't super clear.
>> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
>> ---
>>   include/hw/qdev-core.h | 9 +++++++++
>>   1 file changed, 9 insertions(+)
>> diff --git a/include/hw/qdev-core.h b/include/hw/qdev-core.h
>> index bd50ad5ee1..d4bbc30c92 100644
>> --- a/include/hw/qdev-core.h
>> +++ b/include/hw/qdev-core.h
>> @@ -776,6 +776,15 @@ BusState *sysbus_get_default(void);
>>   char *qdev_get_fw_dev_path(DeviceState *dev);
>>   char *qdev_get_own_fw_dev_path_from_handler(BusState *bus, DeviceState 
>> *dev);
>>   +/**
>> + * device_class_set_props(): add a set of properties to an device
>> + * @dc: the parent DeviceClass all devices inherit
>> + * @props: an array of properties, terminate by DEFINE_PROP_END_OF_LIST()
>> + *
>> + * This will add a set of properties to the object. It will fault if
>> + * you attempt to add an existing property defined by a parent class.
>> + * To modify an inherited property you need to use????
>> + */
>>   void device_class_set_props(DeviceClass *dc, Property *props);
>>     /**
>
> Hmmm that's an interesting one. Looking at the source in
> hw/core/qdev-properties.c you could possibly get away with something
> like this in vu_rng_class_init():
>
>     ObjectProperty *op = object_class_property_find(klass, "id");
>     object_property_set_default_uint(op, VIRTIO_ID_RNG);
>
> Of course this is all completely untested :)

Sadly we assert on the existing prop->defval:

  static void object_property_set_default(ObjectProperty *prop, QObject *defval)
  {
      assert(!prop->defval);
      assert(!prop->init);

      prop->defval = defval;
      prop->init = object_property_init_defval;
  }

Maybe the assert is too aggressive or we need a different helper, maybe
a:

  void object_property_update_default_uint(ObjectProperty *prop, uint64_t value)

?

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro