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

["Alex Bennée" <alex.bennee@linaro.org>]

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

> On 27/03/2023 17:12, Alex Bennée wrote:
>
>> 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)
>> ?
>
> It seems in that case once the default has been set, it is impossible
> to change. The only other immediate option I can think of is to define
> a specific DEFINE_VHOST_PROPERTIES macro in a similar way to
> DEFINE_AUDIO_PROPERTIES which you can use to set the common properties
> for all VHostUserDevice devices, including providing the default ID.

I tried this: allow the default to change

modified   qom/object.c
@@ -1557,11 +1557,16 @@ static void object_property_init_defval(Object *obj, ObjectProperty *prop)
 
 static void object_property_set_default(ObjectProperty *prop, QObject *defval)
 {
-    assert(!prop->defval);
-    assert(!prop->init);
+    if (prop->init == object_property_init_defval) {
+        fprintf(stderr, "%s: updating existing defval\n", __func__);
+        prop->defval = defval;
+    } else {
+        assert(!prop->defval);
+        assert(!prop->init);
 
-    prop->defval = defval;
-    prop->init = object_property_init_defval;
+        prop->defval = defval;
+        prop->init = object_property_init_defval;
+    }
 }
 


>
>
> ATB,
>
> Mark.


-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro