Re: sysbus_create_simple Vs qdev_create

[Markus Armbruster]

Eduardo Habkost <ehabkost@redhat.com> writes:

> On Fri, Jul 17, 2020 at 07:10:57AM +0200, Markus Armbruster wrote:
>> Eduardo Habkost <ehabkost@redhat.com> writes:
>> 
>> > I'd also note that the use of "parent" in the code is also
>> > ambiguous.  It can mean:
>> >
>> > * QOM parent type, i.e. TypeInfo.parent.  Related fields:
>> >   * parent_class members of class structs
>> >   * parent_obj members of object structs
>> 
>> I hate the use of "parent" and "child" for a super- / subtype relation.
>> 
>> Correcting the terminology there would be short term pain for long term
>> gain.  Worthwhile?
>
> I don't know.  It looks like the terminology came from GObject.
>
>> 
>> > * QOM composition tree parent object, i.e. Object::parent
>> > * qdev device parent bus, i.e. DeviceState::parent_bus
>> > * parent device of of qdev bus, i.e. BusState::parent
>> 
>> These are tree relations.  Use of "parent" and "child" is perfectly
>> fine.
>
> The terms are fine but still ambiguous, as devices belong to two
> separate trees at the same time (the QOM composition tree, and
> the qdev tree).
>
> I never understood why we have two separate independent object
> trees.

When we rebased qdev onto QOM, we left the qdev tree alone, we did not
embed it into the QOM composition tree.

The qdev tree edge from bus to device providing the bus is commonly
mirrored in the QOM composition tree: both are QOM objects, and the bus
is commonly a QOM composition child of the device providing it.  I hedge
with "commonly", because nothing enforces this as far as I know.

We do not mirror the edge from device to the bus it's plugged into.  I
believe we could have.  I guess we could mirror it as a link even now
(but note links are not children).

I don't know why the rebase of qdev onto QOM was done that way.  Perhaps
Paolo remembers.