[PATCH 5/5] docs/devel: introduce some key concepts for QOM development

["Alex Bennée" <alex.bennee@linaro.org>]

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

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
---
 docs/devel/qom.rst | 47 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 47 insertions(+)

diff --git a/docs/devel/qom.rst b/docs/devel/qom.rst
index 98a4f178d5..53633fbd35 100644
--- a/docs/devel/qom.rst
+++ b/docs/devel/qom.rst
@@ -13,6 +13,53 @@ 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 Device Class
+================
+
+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.
+
+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`` and ``hotpluggable`` variables in the root
+``DeviceClass`` structure.
+
+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 minimal device
+=========================
+
+A simple minimal device implementation may look something like bellow:
 
 .. code-block:: c
    :caption: Creating a minimal type
-- 
2.39.2