// Copyright (C) 2021 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only

/*!
  \page wayland-and-qt.html
  \title Wayland and Qt
  \brief An overview of the Wayland protocol and how it fits into Qt.
  \ingroup explanations-platforms

  Wayland was developed as an alternative to X11 on Linux. Its main purpose is to manage how
  the content of applications is displayed together on a shared screen, and how a user can
  interact with multiple applications sharing the same input devices.

  This role in an operating system is often called a \e{display server}. The Wayland display
  server may also sometimes be called a \e{compositor} and a \e{window manager}, referring to
  specific tasks it performs as part of its duty.

  In the following, we will give a short introduction to Wayland and its role in Qt. For more
  details and background about Wayland itself, refer to the
  \l{https://wayland.freedesktop.org/docs/html/}{official documentation}.

  \section1 What is a Display Server

  The display server is the part of the operating system which manages the screen real estate and
  other shared resources. On a typical desktop system, you may have many independent applications
  running at the same time, each expecting to be able to render graphics to the screen and receive
  input.

  The display server is a link between the application and shared resources such as a screen and
  input devices. A typical display server on a desktop system will place application content into
  distinct rectangular "windows", which can be moved and resized by the user. The display server
  makes sure that the application content is displayed at the right position on the screen, that the
  active window receives input from the keyboard, that overlapping windows are drawn in the right
  order, and so on.

  On other types of systems, the display server may be more restrictive. If the screen is an
  instrument panel in a car, or the control panel of a fork lift, for instance, then moving and
  resizing windows may not be desirable. Instead, each application may be locked into a pre-defined
  area of the screen and receive input from preassigned devices.

  Either way, as long as there are multiple isolated processes competing for the same resources,
  a display server is useful.

  \section1 The Role of Wayland

  The \l {https://wayland.freedesktop.org/}{Wayland} name may refer to several related items:

  \list
    \li A set of protocols for communicating between a display server and its clients.
    \li A library written in C with functions for inter-process communication, serving as the
        foundation for implementing said protocols.
    \li An XML-based language for extending the protocol, as well as a tool for generating binding
        code in C from such extensions.
  \endlist

  Qt provides implementations for both the client and the server side of the protocol.

  Normal Qt applications can be run as clients on a Wayland display server by selecting the
  "wayland" QPA plugin (this is the default on certain systems). You can do
  this by using the \c {-qpa wayland} option. In addition, the
  \l{Qt Wayland Compositor} module can be used to develop the display server itself.

  Qt also has convenience functionality for easily extending the Wayland protocols with new
  interfaces.

  \section1 Wayland and Other Technology

  On the Linux desktop, Wayland is an alternative to X11 and related extensions. It is a
  compositing display server at its core, and the term "compositor" is often used to describe the
  Wayland server. This means that clients will render content into an off-screen buffer, which
  will later be "composited" with other clients on the screen, allowing window effects such as
  drop shadows, transparency, background blurring, and so on.

  One important design principle of the original X11 protocols is that the display server can be
  running on a thin terminal with only a screen and input devices. Its clients would then be
  running on remote systems with more processing power, communicating with the server over a
  network connection.

  In contrast, Wayland is designed around the observation that, in modern setups, the client and
  display server are usually running on the same hardware. Distributed computing, remote storage
  and remote desktop functionality are usually handled through other mechanisms. Designing this into
  the protocol enables sharing graphics memory between the client and the server: When the
  compositor is placing client content on screen, it can simply copy it from one part of graphics
  memory to another.

  For this to work optimally, the graphics driver must support Wayland. This support is provided
  through an extension to \c EGL which is called \c EXT_platform_wayland.

  \note Qt Wayland also supports compositing on systems where \c EXT_platform_wayland is not
  supported, either through \c XComposite or by copying application content to shared CPU memory.
  But for optimal performance, we recommend systems with driver support.

  X11 has been extended to support features such as composition and direct rendering, but Wayland
  is designed around this use case from the ground up. It also aims to be small and extensible,
  in contrast to the complexity that has developed in X11 over time.

  \section1 Extensibility and Embedded Systems

  Since Wayland has a minimal core and is easily extensible, it is an ideal tool when building
  embedded Linux platforms.

  Desktop-style window system features, for instance, are not part of the core protocol. Instead,
  Wayland has a special category of protocol extensions called "shells" that provide a way for the
  client to manage its surfaces. Desktop-style features are provided through a shell called
  \c{XDG Shell}. For other types of systems, a more specialized (and perhaps more restrictive)
  "shell" can be used. For instance, when making In-Vehicle Infotainment systems, the \c{IVI Shell}
  might be preferable.

  The Wayland server broadcasts a list of its supported protocols (or "interfaces") when a client
  connects, and the client can bind to the ones it wants to use. This can be any of the standard
  interfaces, but new extensions are also easy to add. Wayland defines an easily understandable XML
  format for defining protocols and the \c waylandscanner tool can be used to generate C code from
  these. (In Qt, we also have \c qtwaylandscanner which generates additional C++ binding code.)

  After a client binds to an interface, it can make "requests" to the server and the server can send
  "events" to the client. The requests and events, as well as their arguments, are defined in the
  XML file describing the protocol.

  For building a platform from scratch, when you control the code of both server and clients,
  adding extensions is an easy and controlled way of adding operating system features.

  \section1 Multi-Process or Single-Process

  When building a simple embedded platform with Qt, a perfectly viable option is to have all parts
  of the UI running in a single process. However, as the system becomes more complex, you may want
  to consider a multi-process system instead. This is where Wayland comes in. With Qt, at any point
  in your development process, you can choose to switch between single-process and multi-process.

  \section2 Benefits of Multi-Process

  The following diagrams illustrate the difference between multi-process and single-process
  systems.

  \image ../images/wayland-multi-process.png
         {Diagram of a multi-process Wayland client architecture}
  \caption Multi-Process Client Architecture

  \image ../images/wayland-single-process-eglfs.png
         {Diagram of a single-process EGLFS client architecture}
  \caption Single Process Client Architecture

  The \l{Qt Wayland Compositor} module is ideal for creating the display server and compositor
  in multi-process systems on embedded Linux. The use of multi-process has the following benefits:

  \list
    \li \l{stability}{Stability}
    \li \l{security}{Security}
    \li \l{performance}{Performance}
    \li \l{interoperability}{Interoperability}
  \endlist

  \target stability
  \table 100%
    \header
      \li {2,1} Stability
    \row
      \li Easier to recover when clients hang or crash
      \li If you have a complex UI, then multi-process is useful because if one
          part of the UI crashes, it doesn't affect the entire system.
          Similarly, the display won't freeze, even when one client freezes.

          \note If your client is mandated by law to render safety-critical
          information, consider using \l{Qt Safe Renderer}.
    \row
      \li Protection against possible memory leaks
      \li In a multi-process system, if one client has a memory leak and
          consumes lots of memory, that memory is recovered when that client
          exits. In contrast with single-process, the memory leak remains until
          the entire system restarts.
  \endtable

  \target security
  \table 100%
    \header
      \li Security
    \row
      \li In a single-process system, all clients can access each other's
          memory. For example, there's no isolation for sensitive data
          transfer; every line of code must be equally trustworthy. This
          isolation is there, by design, in multi-process systems.
  \endtable

  \target performance
  \table 100%
    \header
      \li Performance
    \row
      \li If you have a CPU with multiple cores, a multi-process system can
          help distribute the load evenly across different cores, making more
          efficient use of your CPU.
  \endtable

  \target interoperability
  \table 100%
    \header
      \li Interoperability
    \row
      \li You can interface with non-Qt clients in a multi-process system, as
          long as your clients understand Wayland or X11. For example, if you
          use gstreamer for video or if you want to use a navigation
          application built with another UI toolkit, you can run these clients
          alongside your other Qt-based clients.
  \endtable

  \section2 Trade-offs of Multi-Process

  When going from single-process to multi-process, it is important to be conscious of the following
  trade-offs:

    \list
    \li \l{increased-video-memory}{Increased video memory consumption}
    \li \l{increased-main-memory}{Increased main memory consumption}
    \li \l{repeated-storage}{Repeated storage of graphical resources}
    \li \l{input-latency}{Input latency}
  \endlist

  \target increased-video-memory
  \table 100%
    \header
      \li Increased video memory consumption
    \row
      \li This can be a constraint for embedded devices. In multi-process, each
          client needs to have its own graphics buffer, which it sends to the
          compositor. Consequently, you use more video memory compared to the
          single-process case: where everything is drawn at once and there is
          no need to store the different parts in intermediary buffers.
  \endtable

  \target increased-main-memory
  \table 100%
    \header
      \li Increased main memory consumption
    \row
      \li Apart from some extra overhead at the OS level, running multiple
          clients may also use more main memory as some parts need to be
          duplicated once per client. For example, if you run QML, each client
          requires a separate QML engine. Consequently, if you run a single
          client that uses Qt Quick Controls, it's loaded once. If you then
          split this client into multiple clients, you're loading Qt Quick
          Controls multiple times, resulting in a higher startup cost to
          initialize your clients.
  \endtable

  \target repeated-storage
  \table 100%
    \header
      \li Repeated storage of graphical resources
    \row
      \li In a single-process system, if you're using the same textures,
          background, or icons in many places, those images are only stored
          once. In contrast, if you use these images in a multi-process system,
          then you have to store them multiple times. In this case, one
          solution is to share graphical resource between clients. Qt already
          allows sharing image resources in main memory across processes
          without involving Wayland. Sharing GPU textures across processes, on
          the other hand, requires more intricate solutions. With Qt, such solutions
          can be developed as Wayland extension protocols and with QQuickImageProvider,
          for instance.
  \endtable

  \target input-latency
  \table 100%
    \header
      \li Input-to-photon latency
    \row
      \li On a single-process system, the application accesses the main frame buffer directly.
          This means that the latency between input events and reflecting them on screen can be
          minimized in such a setup. On a multi-process system, the application content has to
          be triple-buffered to ensure the client isn't drawing into the buffers while they are
          simultaneously being read by the server, as that would cause tearing. This means that
          there is an implicit latency in a multi-process system.
  \endtable

  \section1 Why Use Wayland Instead of X11 or Custom Solutions

  As described earlier, X11 is not an optimal match for typical system setups today.
  It is quite large and complex, and lacks ability in customization. In fact, it is difficult
  to run a client fluidly with X11, and reach 60 fps without tearing. Wayland, in contrast,
  is easier to implement, has better performance, and contains all the necessary parts to
  run efficiently on modern graphics hardware. For embedded, multi-process systems on Linux,
  Wayland is the standard.

  However, if you are working with old hardware or legacy applications,
  then Wayland may not be a good option. The Wayland protocol is designed with
  security and isolation in mind, and is strict/conservative about what
  information and functionality is available to clients. While this leads to a
  cleaner and more secure interface, some functionality that legacy
  applications expect may no longer be available on Wayland.

  Particularly, there are three common use cases where Wayland may not be the
  best option:
  \list 1
    \li The hardware or platform is old and only supports X11; in which case
        you have no choice.
    \li You have to support legacy applications that depend on features that
        are absent in the Wayland protocol for security and simplicity.
    \li You have to support legacy applications that use a UI toolkit that
        doesn't run on Wayland at all. In some cases, you may be able to work
        around this by running those applications on
        \l{https://wayland.freedesktop.org/docs/html/ch05.html}{XWayland}
        instead.
  \endlist

  Back when X11 was very popular, developers wrote their own custom solutions
  to circumvent X11 issues. Older Qt versions had the Qt Windowing System
  (QWS), which is now discontinued. Today, most of these use cases are covered
  by Wayland, and custom solutions are becoming less and less common.

  \section1 What Qt Wayland Offers

  \b{For Clients}
  \br
  Qt clients can run on any Wayland compositor, including Weston, the reference
  compositor developed as part of the Wayland project.

  Any Qt program can run as a Wayland client (as part of a multi-process system)
  or a standalone client (single-process). This is determined on startup, where
  you can choose between the different backends. During the development process,
  you can develop the client on the desktop first, then test it on the target
  hardware later. You don't need to run your clients on the actual target
  hardware all the time.

  \image ../images/wayland-single-process-develop.png
         {Diagram of a single-process Wayland client development setup}
  \caption Single-Process Client Development

  If you develop on a Linux machine, you can also run the compositor within a
  window on your development machine. This lets you run clients in an
  environment that closely resembles the target device. Without rebuilding the
  client, you can also run it with \c{-platform wayland} to run it inside the
  compositor. If you use \c{-platform xcb} (for X11), you can run the client on
  the desktop. In other words, you can start developing your clients before the
  compositor is ready for use.

  \b{For Servers}
  \br
  The server, or compositor, connects to the display and shows the contents of
  each client on the screen. The compositor handles input and sends input
  events to the corresponding client. In turn, each client connects to the
  compositor and sends the content of its windows. It's up to the compositor to
  decide:

  \list
    \li How and where to show the content
    \li Which content to show
    \li What to do with the different client graphics buffers
  \endlist

  This means, it's up to the compositor to decide what a multi-process system
  is. For instance, the clients could be part of a 3D scene with windows on the
  walls, on a VR system, mapped to a sphere, and so on.

  The \l{Qt Wayland Compositor} is an API for building your own compositor. It
  gives you full freedom to build a custom compositor UI and manage the windows
  of various clients. You can combine both Qt Quick and QML with the Qt Wayland
  Compositor to create impressive, imaginative UIs. For more information, see
  \l{Qt Wayland Compositor}.

  Qt also provides powerful and user-friendly APIs to implement Wayland extensions and use
  them from QML or C++.

  \section2 Related Content
  \list
    \li \l{https://resources.qt.io/videos/qtws17-qt-wayland-compositor-creating-multi-process-user-interface-johan-helsing-the-qt-company}
        {QtWS17 - Qt Wayland Compositor: Creating multi-process user interface}
    \li \l{https://doc.qt.io/QtApplicationManager/introduction.html}{Qt Application Manager}
  \endlist

*/
