Re: [Qemu-devel] [RFC 1/5] qdev: add child alias properties

[Peter Crosthwaite <peter.crosthwaite@xilinx.com>]

On Wed, Mar 19, 2014 at 3:48 PM, Stefan Hajnoczi <stefanha@redhat.com> wrote:
> Child alias properties allow a parent object to expose child object
> properties.  This makes it possible for a parent object to forward all
> or a subset of a child's properties.
>

So this sounds useful to my MemoryRegion and GPIO QOMification plan.
One of the missing pieces is how do you set a child property when you
only have a ref to the parent which I guess is a big motivator here.
The idea is, memory region properties can be set by the parent device
that created them (Seems similar to what you are doing with blk stuffs
here really).

This implementation however seems to assume a 1:1 mapping between the
parent and child property namespaces limiting its use in cases where
multiple child objects of the same type want to expose on the parent
level. Is it somehow possible to change the name of the property as
exposed by the parent ...

> Currently we achieve similar behavior by duplicating property
> definitions and copying the fields around.  It turns out virtio has
> several double-frees since we didn't get the memory management right.
>
> Using child alias properties it will be much easier for a parent object
> to set a child's properties without fragile memory management issues
> since the actual field only exists once in the child object.
>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  hw/core/qdev-properties.c    | 28 ++++++++++++++++++++++++++++
>  include/hw/qdev-properties.h | 28 ++++++++++++++++++++++++++++
>  2 files changed, 56 insertions(+)
>
> diff --git a/hw/core/qdev-properties.c b/hw/core/qdev-properties.c
> index 77d0c66..e62a4fd 100644
> --- a/hw/core/qdev-properties.c
> +++ b/hw/core/qdev-properties.c
> @@ -1002,3 +1002,31 @@ PropertyInfo qdev_prop_size = {
>      .get = get_size,
>      .set = set_size,
>  };
> +
> +/* --- alias that forwards to a child object's property --- */
> +
> +static void get_child_alias(Object *obj, Visitor *v, void *opaque,
> +                            const char *name, Error **errp)
> +{
> +    DeviceState *dev = DEVICE(obj);
> +    Property *prop = opaque;
> +    Object *child = OBJECT(qdev_get_prop_ptr(dev, prop));
> +
> +    object_property_get(child, v, name, errp);
> +}
> +
> +static void set_child_alias(Object *obj, Visitor *v, void *opaque,
> +                            const char *name, Error **errp)
> +{
> +    DeviceState *dev = DEVICE(obj);
> +    Property *prop = opaque;
> +    Object *child = OBJECT(qdev_get_prop_ptr(dev, prop));
> +
> +    object_property_set(child, v, name, errp);
> +}
> +
> +PropertyInfo qdev_prop_child_alias = {
> +    .name  = "ChildAlias",
> +    .get = get_child_alias,
> +    .set = set_child_alias,
> +};
> diff --git a/include/hw/qdev-properties.h b/include/hw/qdev-properties.h
> index 3c000ee..5ab1cac 100644
> --- a/include/hw/qdev-properties.h
> +++ b/include/hw/qdev-properties.h
> @@ -13,6 +13,7 @@ extern PropertyInfo qdev_prop_uint32;
>  extern PropertyInfo qdev_prop_int32;
>  extern PropertyInfo qdev_prop_uint64;
>  extern PropertyInfo qdev_prop_size;
> +extern PropertyInfo qdev_prop_child_alias;
>  extern PropertyInfo qdev_prop_string;
>  extern PropertyInfo qdev_prop_chr;
>  extern PropertyInfo qdev_prop_ptr;
> @@ -61,6 +62,33 @@ extern PropertyInfo qdev_prop_arraylen;
>          .defval    = (bool)_defval,                              \
>          }
>
> +/**
> + * DEFINE_PROP_CHILD_ALIAS:
> + * @_name: property name
> + * @_state: name of the device state structure type
> + * @_field: name of field in @_state, must be Object subclass
> + *
> + * Define device properties that alias a child object's property.  For example,
> + * use the following to forward the 'baz' property where Foo embeds a Bar

... such that the parent renames the property to "bar-baz" to clarify
whose "baz" is being forwarded?

Regards,
Peter

> + * object:
> + *
> + *   typedef struct {
> + *       Object parent_obj;
> + *
> + *       Bar bar_obj;
> + *   } Foo;
> + *
> + *   DEFINE_PROP_CHILD_ALIAS("baz", Foo, bar_obj)
> + *
> + * Any access to Foo's 'baz' property actually accesses bar_obj's 'baz'
> + * property.
> + */
> +#define DEFINE_PROP_CHILD_ALIAS(_name, _state, _field) {         \
> +        .name      = (_name),                                    \
> +        .info      = &(qdev_prop_child_alias),                   \
> +        .offset    = offsetof(_state, _field),                   \
> +        }
> +
>  #define PROP_ARRAY_LEN_PREFIX "len-"
>
>  /**
> --
> 1.8.5.3
>
>