docs/devel: introduce some key concepts for QOM development

Using QOM correctly is increasingly important to maintaining a modern
code base. However the current documentation skips some important
concepts before launching into a simple example. Lets:

  - at least mention properties
  - mention TYPE_OBJECT and TYPE_DEVICE
  - talk about why we have realize/unrealize
  - mention the QOM tree
  - lightly re-arrange the order we mention things

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230630180423.558337-28-alex.bennee@linaro.org>

docs/devel/qom.rst

@@ -13,6 +13,24 @@ features:
 - System for dynamically registering types
 - Support for single-inheritance of types
 - Multiple inheritance of stateless interfaces
+- Mapping internal members to publicly exposed properties
+
+The root object class is TYPE_OBJECT which provides for the basic
+object methods.
+
+The QOM tree
+============
+
+The QOM tree is a composition tree which represents all of the objects
+that make up a QEMU "machine". You can view this tree by running
+``info qom-tree`` in the :ref:`QEMU monitor`. It will contain both
+objects created by the machine itself as well those created due to
+user configuration.
+
+Creating a QOM class
+====================
+
+A simple minimal device implementation may look something like bellow:
 
 .. code-block:: c
    :caption: Creating a minimal type
@@ -48,6 +66,12 @@ In the above example, we create a simple type that is described by #TypeInfo.
 #TypeInfo describes information about the type including what it inherits
 from, the instance and class size, and constructor/destructor hooks.
 
+The TYPE_DEVICE class is the parent class for all modern devices
+implemented in QEMU and adds some specific methods to handle QEMU
+device model. This includes managing the lifetime of devices from
+creation through to when they become visible to the guest and
+eventually unrealized.
+
 Alternatively several static types could be registered using helper macro
 DEFINE_TYPES()
 
@@ -98,7 +122,7 @@ when the object is needed.
    module_obj(TYPE_MY_DEVICE);
 
 Class Initialization
-====================
+--------------------
 
 Before an object is initialized, the class for the object must be
 initialized.  There is only one class object for all instance objects
@@ -168,7 +192,7 @@ will also have a wrapper function to call it easily:
    }
 
 Interfaces
-==========
+----------
 
 Interfaces allow a limited form of multiple inheritance.  Instances are
 similar to normal types except for the fact that are only defined by
@@ -182,7 +206,7 @@ an argument to a method on its corresponding SomethingIfClass, or to
 dynamically cast it to an object that implements the interface.
 
 Methods
-=======
+-------
 
 A *method* is a function within the namespace scope of
 a class. It usually operates on the object instance by passing it as a
@@ -275,8 +299,8 @@ Alternatively, object_class_by_name() can be used to obtain the class and
 its non-overridden methods for a specific type. This would correspond to
 ``MyClass::method(...)`` in C++.
 
-The first example of such a QOM method was #CPUClass.reset,
+One example of such methods is ``DeviceClass.reset``. More examples
-another example is #DeviceClass.realize.
+can be found at :ref:`device-life-cycle`.
 
 Standard type declaration and definition macros
 ===============================================
@@ -382,10 +406,32 @@ OBJECT_DEFINE_ABSTRACT_TYPE() macro can be used instead:
    OBJECT_DEFINE_ABSTRACT_TYPE(MyDevice, my_device,
                                MY_DEVICE, DEVICE)
 
+.. _device-life-cycle:
 
+Device Life-cycle
+=================
+
+As class initialisation cannot fail devices have an two additional
+methods to handle the creation of dynamic devices. The ``realize``
+function is called with ``Error **`` pointer which should be set if
+the device cannot complete its setup. Otherwise on successful
+completion of the ``realize`` method the device object is added to the
+QOM tree and made visible to the guest.
+
+The reverse function is ``unrealize`` and should be were clean-up
+code lives to tidy up after the system is done with the device.
+
+All devices can be instantiated by C code, however only some can
+created dynamically via the command line or monitor.
+
+Likewise only some can be unplugged after creation and need an
+explicit ``unrealize`` implementation. This is determined by the
+``user_creatable`` variable in the root ``DeviceClass`` structure.
+Devices can only be unplugged if their ``parent_bus`` has a registered
+``HotplugHandler``.
 
 API Reference
--------------
+=============
 
 See the :ref:`QOM API<qom-api>` and :ref:`QDEV API<qdev-api>`
 documents for the complete API description.