vulkan,docs: Document vk_object_base

Acked-by: Iago Toral Quiroga <itoral@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15472>

docs/vulkan/base-objs.rst

@@ -0,0 +1,83 @@
+Base object structs
+===================
+
+The Vulkan runtime code provides a set of base object structs which must be
+used if you want your driver to take advantage of any of the runtime code.
+There are other base structs for various things which are not covered here
+but those are optional.  The ones covered here are the bare minimum set
+which form the core of the Vulkan runtime code:
+
+.. contents::
+   :local:
+
+
+vk_object_base
+--------------
+
+The root base struct for all Vulkan objects is
+:cpp:struct:`vk_object_base`.  Every object exposed to the client through
+the Vulkan API *must* inherit from :cpp:struct:`vk_object_base` by having a
+:cpp:struct:`vk_object_base` or some struct that inherits from
+:cpp:struct:`vk_object_base` as the driver struct's first member.  Even
+though we have `container_of()` and use it liberally, the
+:cpp:struct:`vk_object_base` should be the first member as there are a few
+places, particularly in the logging framework, where we use void pointers
+to avoid casting and this only works if the address of the driver struct is
+the same as the address of the :cpp:struct:`vk_object_base`.
+
+The standard pattern for defining a Vulkan object inside a driver looks
+something like this:
+
+.. code-block:: c
+
+   struct drv_sampler {
+      struct vk_object_base base;
+
+      /* Driver fields */
+   };
+
+   VK_DEFINE_NONDISP_HANDLE_CASTS(drv_sampler, base, VkSampler,
+                                  VK_OBJECT_TYPE_SAMPLER);
+
+Then, to the object in a Vulkan entrypoint,
+
+.. code-block:: c
+
+   VKAPI_ATTR void VKAPI_CALL drv_DestroySampler(
+       VkDevice                                    _device,
+       VkSampler                                   _sampler,
+       const VkAllocationCallbacks*                pAllocator)
+   {
+      VK_FROM_HANDLE(drv_device, device, _device);
+      VK_FROM_HANDLE(drv_sampler, sampler, _sampler);
+
+      if (!sampler)
+         return;
+
+      /* Tear down the sampler */
+
+      vk_object_free(&device->vk, pAllocator, sampler);
+   }
+
+The :cpp:any:`VK_DEFINE_NONDISP_HANDLE_CASTS()` macro defines a set of
+type-safe cast functions called ``drv_sampler_from_handle()`` and
+``drv_sampler_to_handle()`` which cast a :cpp:type:`VkSampler` to and from a
+``struct drv_sampler *``.  Because compile-time type checking with Vulkan
+handle types doesn't always work in C, the ``_from_handle()`` helper uses the
+provided :cpp:type:`VkObjectType` to assert at runtime that the provided
+handle is the correct type of object.  Both cast helpers properly handle
+``NULL`` and ``VK_NULL_HANDLE`` as inputs.  The :cpp:any:`VK_FROM_HANDLE()`
+macro provides a convenient way to declare a ``drv_foo`` pointer and
+initialize it from a ``VkFoo`` handle in one smooth motion.
+
+.. doxygenstruct:: vk_object_base
+   :members:
+
+.. doxygenfunction:: vk_object_base_init
+.. doxygenfunction:: vk_object_base_finish
+
+.. doxygendefine:: VK_DEFINE_HANDLE_CASTS
+
+.. doxygendefine:: VK_DEFINE_NONDISP_HANDLE_CASTS
+
+.. doxygendefine:: VK_FROM_HANDLE