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Re: [Socratic RFC PATCH] include: attempt to document device_class_set_p
From: |
Alex Bennée |
Subject: |
Re: [Socratic RFC PATCH] include: attempt to document device_class_set_props |
Date: |
Tue, 28 Mar 2023 12:05:28 +0100 |
User-agent: |
mu4e 1.10.0; emacs 29.0.60 |
Peter Maydell <peter.maydell@linaro.org> writes:
> 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...
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