[Virtio-fs] [PATCH v2 01/13] include: attempt to document device_class_set_props

[Stefan Hajnoczi]

On Tue, Apr 18, 2023 at 05:21:28PM +0100, 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.

Does this mean this patch is an RFC and this patch is not intended to be
merged?

> 
> Signed-off-by: Alex Bennée <alex.bennee at 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);

I don't know the answer. There doesn't seem to be a way for child
classes to override parent DeviceClass properties. The assumption is the
sets of properties are disjoint (no property name collisions).

Here is a workaround in the vhost-user-rng code:

  /* Set our default if the user didn't specify the id */
  if (vud->id == 0) {
      vud->id = VIRTIO_ID_RNG;
  }

This could be a problem because the value 0 may be valid and there is no
way to distinguish between a user setting 0 and the default 0 value.

Stefan
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