Peter Maydell <peter.mayd...@linaro.org> writes:
> On Mon, 27 Mar 2023 at 23:10, Alex Bennée <alex.ben...@linaro.org> wrote: >> >> >> Mark Cave-Ayland <mark.cave-ayl...@ilande.co.uk> writes: >> >> > On 27/03/2023 17:12, Alex Bennée wrote: >> > >> >> Mark Cave-Ayland <mark.cave-ayl...@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.ben...@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... Really the pattern I have is: vhost-user-device has the property and is configurable vhost-user-rng-device specialises vhost-user-device and fixes the value I'm not sure how best to represent that. This should all be happening at class_init time. -- Alex Bennée Virtualisation Tech Lead @ Linaro