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

[Peter Maydell]

On Mon, 27 Mar 2023 at 23:10, Alex Bennée <alex.bennee@linaro.org> wrote:
>
>
> 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;
> +    }
>  }

I think this leaves the door open to bugs where you create
the property, somebody looks at it, and then you update
the default value afterwards...

-- PMM