Re: sysbus_create_simple Vs qdev_create

[Peter Maydell]

On Wed, 15 Jul 2020 at 14:59, Pratik Parvati <pratikp@vayavyalabs.com> wrote:
> Can you please explain to me in brief on buses and device hierarchies (i.e. 
> BusState and DeviceState) and how they are related to each other? As I can 
> see, the DeviceState class inherits the BusState
>
> struct DeviceState {
>     /*< private >*/
>     Object parent_obj;
>     /*< public >*/
>
>     const char *id;
>     char *canonical_path;
>     bool realized;
>     bool pending_deleted_event;
>     QemuOpts *opts;
>     int hotplugged;
>     bool allow_unplug_during_migration;
>     BusState *parent_bus; \\ BusState is inherited here

This is not inheritance. The DeviceState has-a BusState parent_bus.
Inheritance is the parent_obj at the top: a DeviceState is-a
Object.

>     QLIST_HEAD(, NamedGPIOList) gpios;
>     QLIST_HEAD(, BusState) child_bus;
>     int num_child_bus;
>     int instance_id_alias;
>     int alias_required_for_version;
>     ResettableState reset;
> };
>
> and BusState, in turn, inherits the DeviceState as
>
> /**
>  * BusState:
>  * @hotplug_handler: link to a hotplug handler associated with bus.
>  * @reset: ResettableState for the bus; handled by Resettable interface.
>  */
> struct BusState {
>     Object obj;
>     DeviceState *parent; \\ DeviceState is inherited here

This isn't inheritance either. A BusState is-a
Object (which is the inheritance for this class),
and it has-a DeviceState parent.

Anyway, the two form a tree: every Device may
be on exactly one Bus (that's the parent_bus link),
and may have one or more child Buses (that's the
child_bus list). Every Bus is owned by exactly
one Device (its parent in the tree), may have
multiple siblings (if its parent has more than
one child bus), and has children (any Devices
which are plugged into the bus). These parent-and-child
links form the qdev or qbus tree. Note that this is
an entirely separate thing from the QOM hierarchy
of parent-and-child object relationships. It is
also entirely separate from the class hierarchy
of classes and subclasses.

thanks
-- PMM