Re: sysbus_create_simple Vs qdev_create

[Eduardo Habkost <ehabkost@redhat.com>]

On Mon, Jul 20, 2020 at 09:38:24AM +0200, Markus Armbruster wrote:
> 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).

They are already mirrored as links, and guess what's the link
name: "child[...]".

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

I'm guessing this is because QOM parent/child relationships
represent ownership, while there's no ownership relationship
between buses and devices.

-- 
Eduardo