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

[Mark Cave-Ayland]

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.


ATB,

Mark.