Vulkan-Docs/doc/specs/vulkan/style/extensions.txt

// Copyright (c) 2015-2016 Khronos Group. This work is licensed under a
// Creative Commons Attribution 4.0 International License; see
// http://creativecommons.org/licenses/by/4.0/

[[extensions]]
= Layers & Extensions

This chapter describes required and recommended processes for writing formal
extensions and layers for the Vulkan API.
It is concerned with processes and registration, while fine-grained naming
conventions are included in the <<naming,API Naming Conventions chapter>>.
Prior to revision 1.0.19 of the <<vulkan-spec,Vulkan API Specification>>,
most of the content in this chapter existed as part of Appendix C of that
document.

[NOTE]
.Note
====
The mechanism and process of specifying extensions is subject to change, as
we receive feedback from authors and further requirements of documentation
tooling.
This document will be updated as changes are made.
====


== Introduction

The Khronos extension registries and extension naming conventions serve
several purposes:

  * Avoiding naming collisions between extensions developed by mutually
    unaware parties, both in the extension names themselves, as well as
    their token, command, and type names.
  * Allocating enumerant values for tokens added by extensions
  * Creating a defined order between extensions.
    Extensions with higher numbers may have dependencies upon extensions
    with lower numbers, and must define any relevant interactions with
    lower-numbered extensions.
  * Provides a central repository for documentation and header changes
    associated with extensions

Vulkan's design and general software development trends introduces two new
paradigms that require rethinking the existing mechanisms:

  * Layers, and with them a focus on a more open ecosystem where non-Khronos
    members are expected to extend a Khronos API using the Layer mechanism.
  * Namespaced constants (enumerations) that do not necessarily draw from a
    single global set of token values.


== General Rules/Guidelines

Some general rules to simplify the specific rules below:

  * Extensions and layers must each have a globally unique name.
  * All commands and tokens must have a globally unique name.
  * Extensions can expose new commands, types, and/or tokens, but layers
    must not.
  ** However, layers can expose their own extensions, which in turn are
     allowed to expose new commands and tokens.
  * All extensions must be registered with Khronos.
  * Extensions must be strictly additive and backwards-compatible.
    That is, extensions must not remove existing functionality, or change
    existing default behaviors.
    A Vulkan implementation may support any combination of extensions, but
    applications written using only the core API, or a subset of the
    supported extensions, must continue to work in such an implementation
    without changes in behavior.

[[extensions-naming-conventions]]
== Extension and Layer Naming Conventions

Extensions and layers have formal _names_.
These names are used in a variety of places:

  * When specifying extensions and layers to enable in the API.
  * As a preprocessor symbol in the +vulkan.h+ header file indicating that
    an extension interface is defined at compile time.
  * To control building the Vulkan Specification from asciidoc source
    containing many extension, by explicitly enabling inclusion of one or
    more extensions.

[NOTE]
.Note
====
In version 1.0.24 and earlier versions of the Vulkan Specification,
extensions were maintained in separate git branches from the core
Specification source.
This proved to be unwieldy and difficult to maintain as the core was
repeatedly updated, and as of version 1.0.25, we have moved to a
``single-branch'' model in which most extensions can optionally be included
or not from a single set of source documents.

It is possible that some vendors will choose to continue maintaining their
extension branches, and the existing extension branches will not be removed
from GitHub.
====

There is a rigid syntax for these names:

  * Extensions are named with the syntax: +VK_AUTHOR_<name>+.
  * Layers are named with the syntax: +VK_LAYER_{AUTHOR|FQDN}_<name>+.

Both extensions and layer names include a +VK_+ prefix, as described in the
<<naming-preprocessor,Preprocessor Defines>> section above.
In addition, layers add a +LAYER_+ prefix.

Extension and layer names must both be valid C language identifiers.


[[extensions-naming-conventions-name-strings]]
=== Extension and Layer Name Strings

The +<name>+ portion of extension and layer names is a concise name
describing the purpose or functionality of the extension or layer.
The underscore (`_`) character is used as a delimiter between words.
Every character of the name must be in lower case.


=== Author IDs

Extension and layer names also contain an _author ID_, indicated by +AUTHOR+
above, identifying the author of the extension/layer.
This ID is a short, capitalized string identifying an author, such as a
Khronos member developing Vulkan implementations for their devices, or a
non-Khronos developer creating Vulkan layers.
Author IDs must be registered with Khronos.

Some authors have platform communities they wish to distinguish between, and
can register additional author IDs for that purpose.
For example, Google has separate Android and Chrome communities.

Details on how to register an author ID are provided below.
Layer authors not wishing to register an author ID with Khronos can instead
use a fully-qualified domain name (FQDN) as the ID.
The FQDN should be a domain name owned by the author.
FQDNs cannot be used for extensions, only for layers.

  * The following are examples of extension and layer names, demonstrating
    the above syntax:
  ** Extension names all use the base prefix +VK_+.
  ** Khronos-ratified extensions add the reserved author ID +KHR+, and will
     use the prefix +VK_KHR_+.
  ** The following author IDs are reserved and must not be used:
  *** +VK+ - To avoid confusion with the top-level +VK_+ prefix.
  *** +VULKAN+ - To avoid confusion with the name of the Vulkan API.
  *** +LAYER+ - To avoid confusion with the higher-level ``LAYER'' prefix.
  *** +KHRONOS+ - To avoid confusion with the Khronos organization.
  ** Multi-author extensions that have not been ratified by Khronos (those
     developed via cooperation between, and intended to be supported by two
     or more registered authors) add the special author ID +EXT+ to the base
     prefix, and will use the prefix +VK_EXT_+.
  ** Traditional author-specific extensions developed by one author (or one
     author in cooperation with non-authors) add the author ID to the base
     prefix.
     For example, NVIDIA will use the prefix +VK_NV_+, and Valve will use
     the prefix +VK_VALVE_+.
     Some authors can have additional registered author IDs for special
     purposes.
     For example, an Android extension developed by Google - but part of an
     Android open-source community project, and so not a proprietary Google
     extension - will use the author ID +ANDROID+.
  ** Layer names follow the same conventions as extensions, but use the base
     prefix +VK_LAYER_+.
  ** Because layers need not be registered with Khronos, an alternative
     mechanism is needed to allow creating unique layer names without
     registering an author ID.
     Layer authors that prefer not to register an author ID can instead use
     a fully-qualified domain name (FQDN) in reverse-order as an author ID,
     replacing +.+ (period) with `_` (underscore) characters.
     The restriction that layer names must be valid C identifiers means that
     some FQDNs cannot be used as part of layer names.

[source, c]
.Example
----
// Khronos extension name
VK_KHR_mirror_clamp_to_edge

// Multivendor extension name
VK_EXT_debug_marker

// Vendor extension name using author ID NV
VK_NV_glsl_shader

// Vendor layer name using author ID LUNARG
VK_LAYER_LUNARG_vktrace

// Layer name using the FQDN www.3dxcl.invalid instead of an author ID
VK_LAYER_invalid_3dxcl_www
----

[NOTE]
.Note
====
To avoid linking to a nonexistent domain, the reserved TLD +.invalid+ is
used in the example above.
====


[[extensions-naming]]
== Extension Command, Type, and Token Naming Conventions

Extensions may add new commands, types, and tokens, or collectively
``objects'', to the Vulkan API.
These objects are given globally unique names by appending the author ID
defined above for the extension name as described in the
<<naming-extension-identifiers, Extension Identifier Naming Conventions>>
section above.


[[extensions-api-registry]]
== The Vulkan Registry

The canonical definition of the Vulkan APIs is kept in an XML file known as
the *Vulkan registry*.
The registry is kept in +src/spec/vk.xml+ in the branch of the vulkan
project containing the most recently released core API specification.
The registry contains reserved author IDs, core and extension interface
definitions, definitions of individual commands and structures, and other
information which must be agreed on by all implementations.
The registry is used to maintain a single, consistent global namespace for
the registered entities, to generate the Khronos-supplied +vulkan.h+, and to
create a variety of related documentation used in generating the API
specification and reference pages.


[[extensions-author-ID]]
== Registering an Author ID with Khronos

Previous Khronos APIs could only officially be modified by Khronos members.
In an effort to build a more flexible platform, Vulkan allows non-Khronos
developers to extend and modify the API via layers and extensions in the
same manner as Khronos members.
However, extensions must still be registered with Khronos.
A mechanism for non-members to register layers and extensions is provided.

Extension authors will be able to create an account on GitHub and register
an author ID with Khronos through the
+<<vulkan-docs,KhronosGroup/Vulkan-Docs>>+ project.
The author ID must be used for any extensions that author registers.
The same mechanism will be used to request registration of extensions or
layers with Khronos, as described below.

To reserve an author ID, propose a merge request against
<<extensions-api-registry,+vk.xml+>> in the +1.0+ branch.
The merge must add a +<tag>+ XML tag and fill in the +name+, +author+ and
+contact+ attributes with the requested author ID, the author's formal name
(e.g. company or project name), and contact email address, respectively.
The author ID will only be reserved once this merge request is accepted.

Please do not try to reserve author IDs which clearly belong to another
existing company or software project which may wish to develop Vulkan
extensions or layers in the future, as a matter of courtesy and respect.
Khronos may decline to register author IDs that are not requested in good
faith.


[[extensions-vendor-id]]
== Registering a Vendor ID with Khronos

Vulkan implementers must report a valid vendor ID for their implementation
when queried by fname:vkGetPhysicalDeviceProperties, as described in the
``Devices and Queues'' section of the <<vulkan-spec,Vulkan API
Specification>>.
If there is no valid PCI vendor ID defined for the physical device,
implementations must obtain a Khronos vendor ID.

Khronos vendor IDs are reserved in a similar fashion to
<<extensions-author-ID,author IDs>>.
While vendor IDs are not directly related to API extensions, the reservation
process is very similar and so is described in this section.

To reserve an Khronos vendor ID, you must first have a Khronos author ID.
Propose a merge request against <<extensions-api-registry,+vk.xml+>> in the
+1.0+ branch.
The merge must add a +<vendorid>+ tag and fill in the +name+ and +id+
attributes.
The +name+ attribute must be set to the author ID.
The +id+ attribute must be the first sequentially available ID in the list
of +<vendorid>+ tags.
The vendor ID will be reserved only once this merge request has been
accepted.

Please do not try to reserve vendor IDs unless you are making a good faith
effort to develop a Vulkan implementation and require one for that purpose.


== Registering Extensions and Layers

Extensions must be registered with Khronos.
Layers may be registered, and registration is strongly recommended.
Registration means:

  * Receiving an extension number.
  * Adding the extension or layer name to the list in +vk.xml+ and appearing
    on the Khronos registry website, which will link to associated
    documentation hosted on Khronos.
  * For extensions which add to the Vulkan API, including definitions of
    those additions to +vk.xml+.

Registration for Khronos members is handled by filing a merge request in the
internal gitlab repository against the branch containing the core
specification against which the extension or layer will be written.
The merge must modify +vk.xml+ to define extension names, API interfaces,
and related information.
Registration is not complete until the registry maintainer has validated and
accepted the merge.

Since this process could in principle be completely automated, this suggests
a scalable mechanism for accepting registration of non-Khronos extensions.
Non-Khronos members who want to create extensions must register with Khronos
by creating a GitHub account, and registering their author ID and/or FQDNs
to that account.
They can then submit new extension registration requests by proposing merges
to +vk.xml+.
On acceptance of the merge, the extension will be registered, though its
specification need not be checked into the Khronos GitHub repository at that
point.

The registration process can be split into several steps to accommodate
extension number assignment prior to extension publication:

  * Acquire an extension number.
    This is done by proposing a merge request against +vk.xml+ similarly to
    how <<extensions-author-ID,author IDs are reserved>>.
    The merge should add a new +<extension>+ tag at the end of the file with
    attributes specifying the proposed extension +name+, the next unused
    sequential extension +number+, the +author+ and +contact+ information
    (if different than that already specified for the author ID used in the
    extension name), and finally, specifying +supported="disabled"+.
    The extension number will be reserved only once this merge request is
    accepted into the +1.0+ branch.
  * Develop and test the extension using the registered extension number.
  * Publish the extension to Khronos using the previously registered
    extension number, by submitting merge requests to the +1.0+ branch
    defining the changes specific to the extension.
    Changes to both the specification source, and to +vk.xml+ will be
    needed.
  ** Extension changes to the specification source must be protected by
     asciidoc conditionals as described in the
     <<extensions-documenting,Documenting Extensions>> section.
  ** Changes to +vk.xml+ must define the extension interfaces in the
     +<extension>+ block, and must also change the +supported+ attribute
     value of the +<extension>+ to +supported="vulkan"+.
  ** When publishing an extension, mark it as enabled by proposing a merge
     to the +1.0+ branch changing the +supported+ attribute value of the
     +<extension>+ to +supported="vulkan"+.
     This should be completely automated and under the control of the
     publishers, to allow them to align publication on Khronos with product
     releases.
     However, complete automation might be difficult, since steps such as
     regenerating and validating +vulkan.h+ are involved.
     Once the merge is accepted and the corresponding updated header with
     the new extension interface is committed to the +1.0+ branch,
     publication is complete.
  ** Publishing on the <<vulkan-docs,Khronos public GitHub repository>> is
     preferred whenever possible.
     Khronos members may instead create branches on Khronos' internal gitlab
     server, but those branches will eventually be mirrored to GitHub.
  ** Once the merge request defining an extension has been accepted into the
     +1.0+ branch, publication is complete - although it may not be visible
     on GitHub until the next regular core Specification update is pushed
     out.
  * It is still possible to publish a separate branch of the repository with
    appropriate changes relative to the core Vulkan API branch instead, but
    this approach is deprecated and discouraged.
    If this is done, all changes to +vk.xml+ must still be made in the +1.0+
    branch.

ifdef::editing-notes[]
[NOTE]
.editing-note
====
TODO: This section is subject to change and not complete yet, but in broad
is how we expect extension registration and specifications to work.
The process will be refined as members and outside authors define further
extensions.
====
endif::editing-notes[]


[[extensions-documenting]]
== Documenting Extensions

Extensions are documented as modifications to the Vulkan specification.
Changes specific to an extension are protected by asciidoc conditionals.
The changes are only visible in generated documentation when the
Specification is built with an asciidoc attribute of that name defined.

[NOTE]
.Note
====
This is referred to as the ``single-branch'' model, in contrast to the
earlier model where each extension lived in a separate branch from the +1.0+
core Specification source.
====

For example, the +VK_KHR_surface+ extension is now documented in the +1.0+
branch of the GitHub +<<vulkan-docs,KhronosGroup/Vulkan-Docs>>+ project.
However, specifications generated from this branch will only include the
extension when the Makefile is invoked appropriately.

Most language defining extensions can be localized either into a small
number of asciidoc include files which are conditionally included in the
core specification chapters and appendices, or into parts of +vk.xml+
defining the extension interfaces.

[NOTE]
.Note
====
We do not yet fully document an example of including an extension in the
single-branch model.
Instead, authors should refer to the +1.0+ branch and search for the names
of existing extensions, such as +VK_KHR_surface+, as examples.
Some aspects of the changes for this example extension are described below.
====

The WSI extensions were used to help prototype what changes need to be made
for extensions, which include (but may not be limited to) the following:

  * All extensions should add an appendix to the Vulkan specification.
    This can be modeled after what was done for the +VK_KHR_surface+
    extension in +doc/specs/vulkan/appendices/VK_KHR_surface/wsi.txt+, which
    contains metainformation about the extension (as well as code examples,
    and revision history).
    This example is complicated because +VK_KHR_surface+ is a base extension
    on which many window system-specific extensions are layered.
  * Each extension's appendix file is included by adding an +include+
    statement to the +doc/specs/vulkan/vkspec.txt+ file.
    The +include+ statement must be protected by appropriate asciidoc
    conditionals.
    For example:
+
--
.Example Markup
----
\ifdef::VK_KHR_surface[]
\include::appendices/VK_KHR_surface/wsi.txt[]
\endif::VK_KHR_surface[]
----
--
  * If an extension is more of an addition to the Vulkan specification, the
    extension should add a chapter to the Vulkan specification.
    For example, in the case of the +VK_KHR_surface+ example a new chapter
    is added in +vkspec.txt+ by including at the end of the core
    Specification chapters:
+
--
.Example Markup
----
\ifdef::VK_KHR_surface[]
\include::chapters/VK_KHR_surface/wsi.txt[]
\endif::VK_KHR_surface[]
----
--
  * In every other place where the extension modifies the core
    Specification, modify the core files and protect the modifications with
    the same asciidoc conditionals.
    For example, +VK_KHR_surface+ adds new error codes to Vulkan.
    These are added to +chapters/fundamentals.txt+ in the ``Return Codes''
    section as follows:
+
--
.Example Markup
----
... list of existing error codes
\ifdef::VK_KHR_surface[]
\include::VK_KHR_surface/VkResultErrorDescriptions_surface.txt[]
\endif::VK_KHR_surface[]
----
--
  * For vendor extensions, changes made to existing core Specification
    source files and to +vk.xml+ all fall under the Contributor License
    Agreement.
    Vendors may use their own copyright on new files they add to the +1.0+
    branch, although that copyright must be compatible with the
    Specification copyright.


== Assigning Extension Token Values

Extensions can define their own enumeration types and assign any values to
their enumerants that they like.
Each enumeration has a private namespace, so collisions are not a problem.
However, when extending existing enumeration objects with new values, care
must be taken to preserve global uniqueness of values.
Enumerations which define new bits in a bitmask are treated specially as
described in <<extensions-reserving-bitmask-values,Reserving Bitmask
Values>> below.

Each extension is assigned a range of values that can be used to create
globally-unique enum values.
Most values will be negative numbers, but positive numbers are also
reserved.
The ability to create both positive and negative extension values is
necessary to enable extending enumerations such as etext:VkResult that
assign special meaning to negative and positive values.
Therefore, 1000 positive and 1000 negative values are reserved for each
extension.
Extensions must not define enum values outside their reserved range without
explicit permission from the owner of those values (e.g. from the author of
another extension whose range is infringed on, or from the Khronos Registrar
if the values do not belong to any extension's range).

[NOTE]
.Note
====
Typically, extensions use a unique offset for each enumeration constant they
add, yielding 1000 distinct token values per extension.
Since each enumeration object has its own namespace, if an extension needs
to add many enumeration constant values, it can reuse offsets on a per-type
basis.
====

The information needed to add new values to the XML are as follows:

  * The **extension name** (e.g. +VK_KHR_swapchain+) that is adding the new
    enumeration constant.
  * The existing enumeration **type** being extended (e.g.
    stext:VkStructureType).
  * The name of the new enumeration **token** being added (e.g.
    etext:VK_STRUCTURE_TYPE_SWAPCHAIN_CREATE_INFO_KHR).
  * The **offset**, which is an integer between 0 and 999 relative to the
    base being used for the extension.
  * The **direction** may be specified to indicate a negative value
    (+dir="-"+) when needed for negative etext:VkResult values indicating
    errors, like etext:VK_ERROR_SURFACE_LOST_KHR.
    The default direction is positive, if not specified.

Implicit is the registered number of an extension, which is used to create a
range of unused values offset against a global extension base value.
Individual enumerant values are calculated as offsets in that range.
Values are calculated as follows:

  * [eq]#base_value = 1000000000#
  * [eq]#range_size = 1000#
  * [eq]#enum_offset(extension_number,offset) = base_value {plus}
    (extension_number - 1) {times} range_size + offset#
  * Positive values: [eq]#enum_offset(extension_number,offset)#
  * Negative values: [eq]#-enum_offset(extension_number,offset)#

The exact syntax for specifying extension enumerant values is defined in the
+readme.pdf+ specifying the format of +vk.xml+, and extension authors can
also refer to existing extensions for examples.


[[extensions-reserving-bitmask-values]]
=== Reserving Bitmask Values

Enumerants which define bitmask values are a special case, since there are
only a small number of unused bits available for extensions.
For core Vulkan API and KHR extension bitmask types, reservations must be
approved by a vote of the Vulkan Working Group.
For EXT and vendor extension bitmask types, reservations must be approved by
the listed contact of the extension.
Bits are not reserved, and must not be used in a published implementation or
specification until the reservation is merged into
<<extensions-api-registry,+vk.xml+>> by the registry maintainer.


== Required Extension Tokens

In addition to any tokens specific to the functionality of an extension, all
extensions must define two additional tokens.

  * +VK_EXTNAME_SPEC_VERSION+ is an integer constant which is the revision
    of the extension named +VK_extname+ (+EXTNAME+ is all upper-case, while
    extname is the capitalization of the actual extension name) in
    +vulkan.h+.
    This value begins at 1 with the initial version of an extension
    specification, and is incremented when significant changes (bugfixes or
    added functionality) are made.
    Note that the revision of an extension defined in +vulkan.h+ and the
    revision supported by the Vulkan implementation (the pname:specVersion
    field of the slink:VkExtensionProperties structure corresponding to the
    extension and returned by one of the
    <<extended-functionality-extensions,extension queries>>) may differ.
    In such cases, only the functionality and behavior of the
    lowest-numbered revision can be used.
  * +VK_EXTNAME_EXTENSION_NAME+ is a string constant which is the name of
    the extension.

For example, for the WSI extension +VK_KHR_surface+, at the time of writing
the following definitions were in effect:

[source,c]
----
#define VK_KHR_SURFACE_SPEC_VERSION 24
#define VK_KHR_SURFACE_EXTENSION_NAME "VK_KHR_surface"
----


== Extension Objects, Enums, and Typedefs

Expanding on previous discussion, extensions can add values to existing
enums; and can add their own commands, enums, typedefs, etc.
This is done by adding to <<extensions-api-registry,+vk.xml+>>.
All such additions will be included in the +vulkan.h+ header supplied by
Khronos.

[NOTE]
.Note
====
Application developers are encouraged to be careful when using +switch+
statements with Vulkan API enums.
This is because extensions can add new values to existing enums.
The use of a +default:+ statement, within a +switch+, may avoid future
compilation issues.
====


[[extension-function_prototypes]]
== Extension Function Prototypes

Function pointer declarations and function prototypes for all core Vulkan
API commands are included in the +vulkan.h+ file.
These come from the official XML specification of the Vulkan API hosted by
Khronos.

Function pointer declarations are also included in the +vulkan.h+ file for
all commands defined by registered extensions.
Function prototypes for extensions may be included in +vulkan.h+.
Extension commands that are part of the Vulkan ABI must be flagged in the
XML.
Function prototypes will be included in +vulkan.h+ for all extension
commands that are part of the Vulkan ABI.

An extension can be considered platform specific, in which case its
interfaces in +vulkan.h+ are protected by #ifdefs.
This is orthogonal to whether an extension command is considered to be part
of the Vulkan ABI.

The initial set of WSI extension commands (i.e. for +VK_KHR_surface+,
+VK_KHR_swapchain+, and +VK_KHR_*_surface+) are considered to be part of the
Vulkan ABI.
Function prototypes for these WSI commands are included in the +vulkan.h+
provided by Khronos, though the platform-specific portions of +vulkan.h+ are
protected by #ifdefs.

[NOTE]
.Note
====
Based on feedback from implementers, Khronos expects that the Android,
Linux, and Windows Vulkan SDKs will include our +vulkan.h+ and export the
supported WSI functions for those platforms from their loader libraries.
Other implementations can make different choices for their headers and
loader libraries, but are encouraged to be consistent with these
implementations.
====


== Accessing Extension Functions from Programs

flink:vkGetInstanceProcAddr and flink:vkGetDeviceProcAddr can be used in
order to obtain function pointer addresses for core and extension commands
(per the description in the ``Command Function Pointers'' section of the
<<vulkan-spec,Vulkan API Specification>>).
Different Vulkan API loaders can choose to statically export functions for
some or all of the core Vulkan API commands, and can statically export
functions for some or all extension commands.
If a loader statically exports a function, an application can link against
that function without needing to call one of the ftext:vkGet*ProcAddr
commands.

[NOTE]
.Note
====
The Vulkan API loader for Android, Linux, and Windows exports functions for
all core Vulkan API commands, and for a set of WSI extension commands that
are applicable to those operating systems (see Vulkan loader documentation
for the relevant platform/OS for details).
The WSI functions are considered special, because they are required for many
applications.
====


[[extensions-interactions]]
== Extension Interactions

Extensions modifying the behavior of existing commands should provide
additional parameters by using the pname:pNext field of an existing
structure, pointing to a new structure defined by the extension, as
described in the ``Valid Usage'' section of the <<vulkan-spec,Vulkan API
Specification>>.
Extension structures defined by multiple extensions affecting the same
structure can be chained together in this fashion.
Any structure which can be chained in this fashion must begin with the
following two members:

["source","{basebackend@docbook:c++:cpp}",title=""]
----
VkStructureType        sType;
const void*            pNext;
----

It is in principle possible for extensions to provide additional parameters
through alternate means, such as passing a handle parameter to a structure
with a pname:sType defined by the extension, but this approach is
discouraged and should not be used.

When chaining multiple extensions to a structure, the implementation will
process the chain starting with the base parameter and proceeding through
each successive chained structure in turn.
Extensions should be defined to accept any order of chaining, and must
define their interactions with other extensions such that the results are
deterministic.
If an extension needs a specific ordering of its extension structure with
respect to other extensions in a chain to provide deterministic results, it
must define the required ordering and expected behavior as part of its
specification.