// Copyright (C) 2020 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only

/*!
  \page embedded-linux.html
  \title Qt for Embedded Linux
  \brief Provides information about Embedded Linux support in Qt.
  \ingroup supportedplatform

  \section1 Platform plugins for embedded Linux devices

  On Embedded Linux systems, there are multiple platform plugins that you can
  use: EGLFS, VkKhrDisplay, LinuxFB, or Wayland. The availability of these
  plugins depends on how Qt is configured. Of these, Wayland requires the
  presence of a compositor, and provides a full windowing system supporting
  multiple windows, similarly to X11 or Windows. The others work without any
  windowing system, meaning the Qt application is in full control of the
  rendering and output. They typically support one fullscreen Qt "window" per
  screen.

  EGLFS is the default plugin on many boards. If it's not suitable, use the
  \c QT_QPA_PLATFORM environment variable to request another plugin.
  Alternatively, for quick tests, use the \c -platform command-line argument
  with the same syntax.

  \note As of Qt 5.0, Qt no longer has its own window system (QWS)
  implementation. For single-process use cases, the \l{Qt Platform Abstraction}
  is a superior solution; multi-process use cases are supported through
  \l{Qt Wayland Compositor}{Wayland}.

  See \l{Configure an Embedded Linux Device} for an overview of configuring Qt
  for cross-compilation using an Embedded Linux toolchain.

  \target embedded eglfs
  \section2 EGLFS

  \l {http://www.khronos.org/egl}{EGL} is an interface between OpenGL and the
  native windowing system. Qt can use EGL for context and surface management,
  however the API contains no platform-specifics. Creating a \e {native window},
  which won't necessarily be an actual window on the screen, must still be
  done by platform-specific means. This is why we need the board or GPU-specific
  adaptation code. Typically, these adaptations are provided as:

  \list
    \li \e{EGLFS hooks} -- a single source file compiled into the platform
        plugin
    \li \e{EGL device integration} -- dynamically loaded plugins
  \endlist

  EGLFS is a platform plugin for running Qt applications on top of EGL and
  OpenGL ES 2.0, without an actual windowing system like X11 or Wayland. It is
  the recommended plugin for modern Embedded Linux devices that include a GPU.

  In addition to Qt Quick and native OpenGL applications, EGLFS supports
  software-rendered windows, like QWidget, too. For QWidget, the widgets'
  contents are rendered using the CPU into images, which are then
  uploaded into textures and composited by the plugin.

  EGLFS forces the first top-level window - either a QWidget or a QQuickView
  - to become fullscreen. This window is also chosen to be the \e root widget
  window into which all other top-level widgets are composited. For example,
  dialogs, popup menus, or combo boxes. This behavior is necessary because
  with EGLFS there is always exactly one native window and one EGL window
  surface; these belong to the widget or window that is created first. This
  approach works well when there is a main window that exists for the
  application's lifetime and all other widgets are either not top-levels or are
  created afterward, once the main window is shown.

  There are further restrictions for OpenGL-based windows. EGLFS supports a
  single fullscreen GL window (as of Qt 5.3), like OpenGL-based QWindow,
  a QQuickView, or a QOpenGLWidget. Opening additional OpenGL windows or mixing
  such windows with QWidget-based content isn't supported; Qt terminates the
  application with an error message.

  Furthermore, APIs designed for desktop platforms or environments with a
  windowing system, such as \l{Drag and Drop}, are not supported on EGLFS.

  \section3 Environment variables used by EGLFS
  If necessary, \c eglfs can be configured using the following environment
  variables:

  \table 100%
    \header
      \li Environment Variable
      \li Description
    \row
      \li \c {QT_QPA_EGLFS_INTEGRATION}
      \li In addition to the compiled-in \e hooks, it is also possible to use
          dynamically loaded plugins to provide device or vendor-specific
          adaptation. This environment variable enforces a specific plugin.
          For example, setting it to \e{eglfs_kms} uses the KMS/DRM backend.
          This is only an option when no static or compiled-in hooks were
          specified in the device makespecs. In practice, the traditional
          compiled-in hooks are rarely used, almost all backends are now
          migrated to plugins. The device makespecs still contain a relevant,
          though optional,  \c EGLFS_DEVICE_INTEGRATION entry: the name of
          the preferred backend for that particular device. Avoid setting
          this environment variable if there is more than one plugin present
          on the target system. In a desktop environment, the KMS or X11 backends
          are prioritized, depending on the presence of the \c DISPLAY environment
          variable.
          \note On some boards a special value of \c none is used instead
          of an actual plugin. This indicates that no special integration is
          necessary to use EGL with the framebuffer; no plugins must be loaded.
    \row
      \li \c {QT_QPA_EGLFS_PHYSICAL_WIDTH} and \c {QT_QPA_EGLFS_PHYSICAL_HEIGHT}
      \li Specifies the physical screen's width and height in millimeters. Note
          that since Qt 6 the physical screen size is no longer used to determine
          logical dpi.
    \row
      \li \c {QT_QPA_EGLFS_ROTATION}
      \li Specifies the rotation applied to software-rendered content in
          QWidget-based applications. Supported values are 180, 90, and -90.
          This variable does not apply to OpenGL-based windows, including Qt
          Quick. Qt Quick applications can apply transformations in their QML
          scene instead. The standard \c eglfs mouse cursor always takes the
          value into account, with an appropriately positioned and rotated
          pointer image, regardless of the application type. However, special
          cursor implementations, such as the KMS/DRM backend's hardware
          cursor, may not support rotation.
    \row
      \li \c {QT_QPA_EGLFS_FORCEVSYNC}
      \li When set, \c eglfs requests \c FBIO_WAITFORVSYNC on the framebuffer
          device after each call to eglSwapBuffers(). This variable is only
          relevant for backends relying on the legacy Linux \c fbdev subsystem.
          Normally, with a default swap interval of 1, Qt assumes that calling
          eglSwapBuffers() takes care of vsync; if it doesn't (for example,
          due to driver bugs), try setting \c QT_QPA_EGLFS_FORCEVSYNC to a
          non-zero value.
    \row
      \li \c {QT_QPA_EGLFS_FORCE888}
      \li When set, the red, green, and blue color channel sizes are ignored
          when \c eglfs creates a new context, window or off-screen surface.
          Instead, the plugin requests a configuration with 8 bits per channel.
          This can be helpful on devices where configurations with less than 32
          or 24 bits per pixel (for example, 5-6-5 or 4-4-4) are chosen by
          default despite knowing they are not ideal, for example, due to
          banding effects. Instead of changing application code, this variable
          provides a shortcut to force 24 or 32 bpp configurations.
  \endtable

  Additionally, the following less commonly used variables are available:

  \table 100%
    \header
      \li Environment Variable
      \li Description
    \row
      \li \c {QT_QPA_EGLFS_FB}
      \li Overrides the framebuffer device. The default is \c /dev/fb0. On most
          embedded platforms this variable isn't very relevant because the
          framebuffer is used only to query settings like the display
          dimensions. However, on certain devices, this variable provides the
          ability to specify which display to use in multiple display setups,
          similar to the \c fb parameter in LinuxFB.
    \row
      \li \c {QT_QPA_EGLFS_WIDTH} and \c {QT_QPA_EGLFS_HEIGHT}
      \li Contains the screen's width and height in pixels. While \c eglfs
          tries to determine the dimensions from the framebuffer device
          \e{/dev/fb0}, this doesn't always work. It may be necessary to
          manually specify the sizes.
    \row
      \li \c {QT_QPA_EGLFS_DEPTH}
      \li Overrides the color depth for the screen. On platforms where the
          framebuffer device \e{/dev/fb0} is not available or the query is not
          successful, a default of \c 32 is used. Use this variable to override
          any such defaults.
          \note This variable only affects the color depth value reported by
          QScreen. It has no connection to EGL configurations and the color
          depth used for OpenGL rendering.
    \row
      \li \c {QT_QPA_EGLFS_SWAPINTERVAL}
      \li By default, a swap interval of \c 1 is requested. This variable
          enables synchronizing to the display's vertical refresh. Use this
          variable to override the swap interval's value. For instance, passing
          0 disables blocking on swap, resulting in running as fast as possible
          without any synchronization.
    \row
      \li \c {QT_QPA_EGLFS_DEBUG}
      \li When set, some debugging information is printed on the debug output.
          For example, the input QSurfaceFormat and the properties of the
          chosen EGL configuration are printed while creating a new context.
          When used together with Qt Quick's \c {QSG_INFO} variable, you can
          get useful information for troubleshooting issues related to the EGL
          configuration.
  \endtable

  \section3 Logging

  In addition to \c {QT_QPA_EGLFS_DEBUG}, \c eglfs also supports Qt's modern
  categorized logging system. The following logging categories are available:

  \list

    \li \c qt.qpa.egldeviceintegration – Enables logging for dynamically loaded
        backends. Use this category to check what backend is in use.
    \li \c qt.qpa.input – Enables debug output both from the \c evdev and
        \c libinput input handlers. Use this category to check if a given input
        device was recognized and opened.
    \li \c qt.qpa.eglfs.kms – Enables verbose logging in the KMS/DRM backend.
  \endlist

  After running \c configure, make sure to inspect its output. This is the
  easiest, quickest way to identify whether you have the necessary EGLFS
  backend, libudev, or libinput enabled. In short, if there's an undesired
  "no" in your \c configure output, run:

  \code
  ./configure -v
  \endcode

  to turn on the verbose output, so that you can see the compiler and linker
  invocations for each configure test.

  \note If you encounter errors about missing headers, libraries, or seemingly
  cryptic linker failures, often, they are a sign of an incomplete or broken
  sysroot and isn't related to Qt.

  As an example, when targeting the Raspberry Pi with the Broadcom proprietary
  graphics drivers, the output should contain something like the following:

  \badcode
    QPA backends:
    EGLFS ................................ yes
    EGLFS details:
      EGLFS i.Mx6 ........................ no
      EGLFS i.Mx6 Wayland ................ no
      EGLFS EGLDevice .................... no
      EGLFS GBM .......................... no
      EGLFS Mali ......................... no
      EGLFS Raspberry Pi ................. yes
      EGL on X11 ......................... no
  \endcode

  If this is not the case, it's not advisable to proceed further with the build
  since accelerated graphics won't be functional without the Raspberry
  Pi-specific backend, even if the rest of Qt compiles successfully.

  \section2 VkKhrDisplay

  While EGLFS only supports OpenGL (ES), VkKhrDisplay is an \b experimental
  platform plugin that supports rendering with the
  \l{https://www.vulkan.org/}{Vulkan} API. To enumerate displays and set up
  rendering, it relies on the
  \l{https://registry.khronos.org/vulkan/specs/1.3-extensions/man/html/VK_KHR_display.html}{VK_KHR_display}
  family of extensions. Note that it is not given that a Vulkan implementation
  within a graphics stack supports this functionality. Currently this platform
  plugin has been verified and tested with
  \l{https://docs.mesa3d.org/drivers/v3d.html}{Mesa and V3DV} running on a
  Raspberry Pi 4.

  This platform plugin does not support OpenGL or any software rendering.
  Attempting to show a QWidget-based user interface will therefore fail. The
  only supported QWindow surface type is QSurface::VulkanSurface. For Qt Quick
  applications this implies that Vulkan-based rendering must be enforced either
  by setting \c{QSG_RHI_BACKEND=vulkan} in the environment, or by calling
  \l{QQuickWindow::setGraphicsApi}(QSGRendererInterface::Vulkan); early on
  before creating a QQuickWindow or QQuickView.

  To use this platform plugin, run the application with \c{-platform vkkhrdisplay}
  or set \c QT_QPA_PLATFORM to \c vkkhrdisplay. The plugin is built only when
  Qt is configured with Vulkan support.

  Advanced EGLFS-style configuration (e.g. the JSON configuration file) or
  outputting to multiple screens from the same application are not currently
  implemented. Applications can however choose the screen to use via environment
  variables.

  To determine the index values, check the logs printed on the debug output by
  the plugin. Currently these logs are uncategorized (printed via qDebug) since
  inspecting them is essential in most cases, in order to ensure the plugin
  picks the appropriate display and mode.

  \list

  \li \c QT_VK_DISPLAY_INDEX - When set, the display with the given index is used.

  \li \c QT_VK_MODE_INDEX - When set, the mode with the given index is used.

  \li \c QT_VK_PHYSICAL_DEVICE_INDEX - When set, the Vulkan physical device with
  the given index is used. This will not be relevant in most cases on embedded.
  Note that this variable is also used by the rest of the Qt graphics stack.

  \endlist

  Input (keyboard, mouse, touch) handling is similar to EGLFS, supporting
  \c evdev, \c libinput, and \c tslib. There is no mouse cursor rendering
  implemented, however. This is because there is no concept of a hardware cursor
  in this environment, and rendering a cursor with Vulkan within the platform
  plugin, similarly to what EGLFS does with OpenGL, is problematic for multiple
  reasons. Therefore this platform plugin is not well suited for mouse-based
  input at the moment.

  The related environment variables are:

  \list

  \li \c QT_QPA_DISABLE_INPUT - Disables keyboard/mouse/touch input.

  \li \c QT_QPA_NO_LIBINPUT - Prefers the evdev-based input handlers even when
  \l{https://www.freedesktop.org/wiki/Software/libinput/}{libinput} is available.

  \li \c QT_QPA_TSLIB - Requests using the legacy
  \l{http://www.tslib.org/}{tslib} library.

  \endlist

  \section2 LinuxFB

  This plugin writes directly to the framebuffer via Linux's fbdev subsystem.
  Only software-rendered content is supported. Note that on some setups the
  display performance is expected to be limited. To use Qt Quick applications
  with this platform plugin, the \c software scenegraph backend must be used,
  either by setting \c{QT_QUICK_BACKEND=software} in the environment, or by
  calling \l{QQuickWindow::}{setGraphicsApi()} with
  QSGRendererInterface::Software. QWidget applications, or QWindow with a
  surface type of QSurface::RasterSurface, are supported, but this does not
  include special widgets such as QOpenGLWidget.

  As fbdev is being deprecated in the Linux kernel, DRM dumb
  buffer support is also available. To use it, set the
  \c QT_QPA_FB_DRM environment variable to a non-zero value. When set, provided
  that dumb buffers are supported by your system, legacy framebuffer devices
  like \c{/dev/fb0} won't be accessed. Instead, the rendering is set up via the
  DRM APIs, similar to the \c{eglfs_kms} backend in EGLFS. The output is
  double-buffered and page flipped, providing proper vsync for
  software-rendered content as well.

  \note When dumb buffers are in use, none of the options described below are
  applicable since properties like physical and logical screen sizes are all
  queried automatically.

  \section3 Specifying additional settings

  The \c linuxfb plugin allows you to specify additional settings via the
  \c QT_QPA_PLATFORM environment variable or \c -platform command-line option.
  For example, \c {QT_QPA_PLATFORM=linuxfb:fb=/dev/fb1} specifies that the
  framebuffer device \c /dev/fb1 must be used instead of the default \c fb0.
  To specify multiple settings, separate the mwith a colon (:).

  \table
    \header
      \li Settings
      \li Description
    \row
      \li \c {fb=/dev/fbN}
      \li Specifies the framebuffer devices. On multiple display setups,
          this setting allows you to run the application on different displays.
          Currently, there's no way to use multiple framebuffers from one Qt
          application.
    \row
      \li \c{size=}\e{<width>}\c{x}\e{<height>}
      \li Specifies the screen size in pixels. The plugin tries to query the
          display dimensions, both physical and logical, from the framebuffer
          device. However, this query may not always lead to proper results;
          it may be necessary to specify the values explicitly.
    \row
      \li \c{mmsize=}\e{<width>}\c{x}\e{<height>}
      \li Specifies the physical width and height in millimeters.
    \row
      \li \c{offset=}\e{<width>}\c{x}\e{<height>}
      \li Specifies the top-left corner of the screen offset in pixels. The
          default position is at \c{(0, 0)}.
    \row
      \li \c {nographicsmodeswitch}
      \li Specifies not to switch the virtual terminal to graphics mode
          (\c KD_GRAPHICS). Typically, \e enabling graphics mode disables the
          blinking cursor and screen blanking. However, when this parameter is
          set, those two features are also skipped.
    \row
      \li \c {tty=/dev/ttyN}
      \li Overrides the virtual console. Only used when \c{nographicsmodeswitch}
          isn't set.
  \endtable

  As of Qt 5.9, the behavior of EGLFS and LinuxFB has been synchronized, with
  regards to the window sizing policy: the first top-level window is forced to
  cover the entire screen, with both platform plugins. If this is not desired,
  set the \c{QT_QPA_FB_FORCE_FULLSCREEN} environment variable to \c 0 to
  restore the behavior from earlier Qt versions.

  \section1 Display output

  The level of support to target one or more displays from one single Qt application
  varies between the platform plugins. Support often depends on the device and its
  graphics stack.

  \section2 EGLFS with the eglfs_kms backend

  When the KMS/DRM backend is in use, EGLFS reports all available screens in
  QGuiApplication::screens(). Applications can target different screens with
  different windows via QWindow::setScreen().

  \note The restriction of one single fullscreen window per screen still
  applies. Changing screens after making the QWindow visible isn't supported
  either. Therefore, it's essential that embedded applications make all the
  necessary QWindow::setScreen() calls before calling QWindow::show().

  When you start developing on a given embedded device, often it's necessary to
  verify the behavior of the device and drivers, and that the connected displays
  are working as they should. One easy way is to use the \b{hellowindow} example.
  Launching it with the \c{-platform eglfs --multiscreen --timeout} arguments
  shows a rotating Qt logo on each connected screen for a few seconds.

  \section3 Custom configuration

  The KMS/DRM backend also supports custom configurations via a JSON file. To
  enable this, set the \c QT_QPA_EGLFS_KMS_CONFIG environment variable to the
  name of the file. You can also embed this file into the application via the Qt
  resource system.

  Most of these configuration options apply to all KMS/DRM-based backends,
  regardless of the buffer management technology (GBM or EGLStreams).

  Here's an example configuration:

  \badcode
    {
      "device": "/dev/dri/card1",
      "hwcursor": false,
      "pbuffers": true,
      "outputs": [
        {
          "name": "VGA1",
          "mode": "off"
        },
        {
          "name": "HDMI1",
          "mode": "1024x768"
        }
      ]
    }
  \endcode

  Here we configure the specified device so that:

  \list
    \li It won't use the hardware cursor (falls back to rendering the mouse
        cursor via OpenGL; by default hardware cursors are enabled as they're
        more efficient).
    \li It backs QOffscreenSurface with standard EGL pbuffer surfaces (by
        default this is disabled and a gbm surface is used instead).
    \li Output on the VGA connector is disabled, while HDMI is active with a
        resolution of 1024x768.
  \endlist

  Additionally, such a configuration also disables looking for a device via
  \c libudev; instead the specified device is used.

  When \c mode is not defined, the system's preferred mode is chosen. The
  accepted values for \c mode are: \c off, \c current, \c preferred, \c skip,
  width\c{x}height, width\c{x}height\c{@}vrefresh, or a modeline string.

  Specifying \c current chooses a mode with a resolution that matches the
  current one. Because mode-setting is done only when the desired mode is
  actually different from the active one (unless forced via the
  \c QT_QPA_EGLFS_ALWAYS_SET_MODE environment variable), this value is useful to
  preserve the current mode and any content in the planes not touched by Qt.

  \c skip causes the connector for the output to be ignored as if it were
  disconnected. \c off is similar, but it changes the mode and turns off the
  display.

  \section3 Default behavior

  By default, all screens reported by the DRM layer are treated as one big virtual
  desktop. The mouse cursor implementation takes this into account and moves
  across the screens as expected. Although not recommended, you can disable the
  virtual desktop by setting \c separateScreens to \c false in the configuration.

  By default, the virtual desktop is formed left to right, based on the order of
  connectors as reported by the system. To change this, set \c virtualIndex to a
  value starting from 0.

  For example, the following configuration uses the preferred resolution but ensures
  that the left side of the virtual desktop is the screen connected to the HDMI port;
  while the right side is the screen connected to the DisplayPort:

  \badcode
    {
      "device": "drm-nvdc",
      "outputs": [
        {
          "name": "HDMI1",
          "virtualIndex": 0
        },
        {
          "name": "DP1",
          "virtualIndex": 1
        }
      ]
    }
  \endcode

  The order of elements in the array is not relevant. Outputs with unspecified
  virtual indices are placed after the others, with the original order in the
  DRM connector list preserved.

  To create a vertical desktop space (that is, to stack top to bottom instead of
  left to right), add a \c virtualDesktopLayout property after \c device
  with the value of \c vertical.

  \warning It's recommended that all screens in the virtual desktop use the same
  resolution, otherwise elements like the mouse cursor may behave in unexpected
  ways when entering areas that only exist on one given screen.

  When \c virtualIndex is not sufficient, the \c virtualPos property can be used
  to explicitly specify the top-left position of the screen in question. Taking
  the previous example and assuming a resolution of 1080p for HDMI1, the
  following code snippet places a second HDMI-based screen below the first one:

  \badcode
    {
       ...
      "outputs": [
        ...
        {
          "name": "HDMI2",
          "virtualPos": "0, 1080"
        }
      ]
    }
  \endcode

  \note Avoid such configurations when mouse support is desired. The mouse
  cursor's behavior may be unexpected with non-linear layouts. Touch should
  present no issues.

  \section3 Automatic physic screen size querying

  In some cases, the automatic querying of the physical screen size via DRM may
  fail. Normally the \c QT_QPA_EGLFS_PHYSICAL_WIDTH and
  \c QT_QPA_EGLFS_PHYSICAL_HEIGHT environment variable would be used to provide the
  missing values. This is not suitable anymore when multiple screens are
  present. Instead, use the \c physicalWidth and \c physicalHeight properties
  in the \c outputs list to specify the sizes in millimeters.

  \note Different physical sizes and thus differing logical DPIs are discouraged
  because it may lead to unexpected issues due to some graphics stack components
  not knowing about multiple screens and relying solely on the first screen's
  values.

  \section3 Active outputs and QScreen instances

  Each active output from the \c outputs array corresponds to one QScreen
  instance reported from QGuiApplication::screens(). By default, the primary screen
  that QGuiApplication::primaryScreen() reports is the screen that is registered
  first. If you're not using \c virtualIndex, this means the decision is based on
  the DRM connector order. To override this, set the \c primary property to \c true
  on the desired entry in the \c outputs list.

  For example, to ensure the screen corresponding to the VGA output is the primary even
  when the system happens to report the HDMI one first, do the following:

  \badcode
    {
      "device": "/dev/dri/card0",
      "outputs": [
          { "name": "HDMI1" },
          { "name": "VGA1", "mode": "1280x720", "primary": true },
          { "name": "LVDS1", "mode": "off" }
      ]
    }
  \endcode

  For troubleshooting, it might be useful to enable debug logs from the KMS/DRM
  backend. To do this, enable the \c qt.qpa.eglfs.kms categorized logging rule.

  \note In an embedded environment, virtual desktops are more limited compared to
  a full windowing system. Windows overlapping multiple screens, non-fullscreen
  windows, and moving windows between screens, should be avoided and may not
  function as expected.

  \section3 A common use case

  The most common and best supported use case for a multi-screen setup is to
  open a dedicated QQuickWindow or QQuickView for each screen. With the default
  \c threaded render loop of the Qt Quick scenegraph, each of these windows will
  get its own dedicated render thread. This is good because the threads can be
  throttled independently based on vsync, and will not interfere with each
  other. With the \c basic loop, this can get problematic, causing animations to
  degrade.

  For example, discovering all connected screens and creating a QQuickView for
  each of them can be done like this:

  \badcode
    int main(int argc, char **argv)
    {
        QGuiApplication app(argc, argv);

        QVector<QQuickView *> views;
        for (QScreen *screen : app.screens()) {
            QQuickView *view = new QQuickView;
            view->setScreen(screen);
            view->setResizeMode(QQuickView::SizeRootObjectToView);
            view->setSource(QUrl("qrc:/main.qml"));
            QObject::connect(view->engine(), &QQmlEngine::quit, qGuiApp, &QCoreApplication::quit);
            views.append(view);
            view->showFullScreen();
        }

        int result = app.exec();

        qDeleteAll(views);
        return result;
    }
  \endcode

  \section2 Advanced eglfs_kms features

  \section3 Cloning (mirroring)

  Screen cloning (mirroring) is supported. This is enabled via
  the \c clones property:

  \badcode
    {
      "device": "/dev/dri/card0",
      "outputs": [
          { "name": "HDMI1", "mode": "1920x1080" },
          { "name": "DP1", "mode": "1920x1080", "clones": "HDMI1" }
     ]
    }
  \endcode

  In this case, the content on the display connected via DisplayPort will be the
  same as on the HDMI one. This is ensured by scanning out the same buffer on
  both.

  However, this feature can only work if the resolutions are the same, there are no
  incompatibilities when it comes to accepted buffer formats, and the application
  doesn't have any output on the QScreen associated with a clone destination. In
  practice, the latter means that no QWindow associated with the QScreen in question
  - DP1 in the example - must ever perform a QOpenGLContext::swapBuffers() operation.
  It's up to the configuration and the application to ensure these.

  \section3 Headless mode using DRM render

  Headless mode via DRM render nodes is supported. This allows
  performing GPU compute (OpenGL compute shaders, OpenCL) or off-screen OpenGL
  rendering without needing DRM master privileges. In this mode, applications can
  function even when there is already another process outputting to the screen.

  Just switching \c device from \c{/dev/dri/card0} to \c{/dev/dri/renderD128} is
  futile on its own since there are a number of operations that cannot be
  performed in headless mode. Therefore, this must be combined with a
  \c headless property, for example:

  \badcode
    {
        "device": "/dev/dri/renderD128",
        "headless": "1024x768"
    }
  \endcode

  Keep in mind that windows are still sized to match the - now virtual -
  screen size, hence the need for specifying a size in the \c headless
  property. There is also a lack of vsync-based throttling.

  Once enabled, applications have two typical choices to perform off-screen
  rendering in headless mode:

  Use an ordinary window, such as a QOpenGLWindow subclass, targeting the
  window's default framebuffer, meaning a \c{gbm_surface} in practice:

  \badcode
      MyOpenGLWindow w;
      w.show(); // will not actually show up on screen
      w.grabFramebuffer().save("output.png");
  \endcode

  Or the typical offscreen approach with an extra FBO:

  \badcode
     QOffscreenSurface s;
     s.setFormat(ctx.format());
     s.create();
     ctx.makeCurrent(&s);
     QOpenGLFramebufferObject fbo(1024, 768);
     fbo.bind();
     ctx.functions()->glClearColor(1, 0, 0, 1);
     ctx.functions()->glClear(GL_COLOR_BUFFER_BIT);
     fbo.toImage().save("output.png");
     ctx.doneCurrent();
  \endcode

  \section3 DRM API selection

  KMS/DRM can be used with two different DRM APIs which are \e legacy and \e atomic.
  The main benefit of DRM atomic API is to allow several DRM plane updates
  within the same renderloop, whereas legacy API would require one plane update
  per vsync.

  Atomic API is useful when your application needs to blend content into overlays
  keeping all the updates within the same vsync. Still not all devices
  support this API and it could be unavailable on some older devices.
  KMS backend will by default use the legacy API, but you can enable the DRM
  atomic API with \c QT_QPA_EGLFS_KMS_ATOMIC environment variable set to 1.

  Using a smaller framebuffer than screen resolution can also be useful.
  This is possible with DRM atomic using the \c size parameter in the JSON file.
  The example below uses a 1280x720 framebuffer on a 3840x2160 videomode :

  \badcode
    {
      "device": "/dev/dri/card0",
      "outputs": [
        { "name": "HDMI1", "mode": "3840x2160", "size": "1280x720", "format": "argb8888" }
      ]
    }
  \endcode

  \section2 EGLFS with an eglfs_kms_egldevice backend

  This backend, typically used on Tegra devices, is similar to the KMS/DRM
  backend mentioned above, except that it relies on the EGLDevice and EGLStream
  extensions instead of GBM.

  For technical details about this approach, check out
  \l {https://wiki.qt.io/Qt_for_Embedded_Linux/XDC2014RitgerEGLNonMesa}{this presentation}.

  As of Qt 5.7 this backend shares many of its internal implementation with the
  GBM-based backend. This means that multiple screens and the advanced
  configuration via \c QT_QPA_EGLFS_KMS_CONFIG are supported. Some settings,
  such as \c hwcursor and \c pbuffers are not applicable however.

  By default, the backend will automatically choose the correct EGL layer for the
  default plane of each output. When necessary, this can be overridden by
  setting the \c QT_QPA_EGLFS_LAYER_INDEX environment variable to the index of
  the desired layer. This approach does not currently support multiple outputs,
  so its usage should be limited to systems with a single screen. To see which
  layers are available, and to debug potential startup issues, enable the
  logging category \c qt.qpa.eglfs.kms.

  In some cases, it may be necessary to perform a video mode set on application
  startup even when the screen reports that the desired resolution is already
  set. This is normally optimized away, but if the screen stays powered down,
  try setting the environment variable \c QT_QPA_EGLFS_ALWAYS_SET_MODE to a
  non-zero value and relaunch the application.

  To configure the behavior of the EGLStream object used by the backend, use the
  \c QT_QPA_EGLFS_STREAM_FIFO_LENGTH environment variable. This assumes that
  \c KHR_stream_fifo is supported by the target system. By default, the stream
  operates in mailbox mode. To switch to FIFO mode, set a value of 1 or
  greater. The value specifies the maximum number of frames the stream can hold.

  On some systems it may become necessary to target a specific overlay plane
  through a pre-defined connector. Just forcing a layer index via
  \c QT_QPA_EGLFS_LAYER_INDEX does not perform plane configuration and is therefore
  not suitable in itself. Instead, in such special scenarios use the
  \c QT_QPA_EGLFS_KMS_CONNECTOR_INDEX and \c QT_QPA_EGLFS_KMS_PLANE_INDEX
  environment variables. When these are set, only the specified connector and
  plane will be in use, all other outputs will get ignored. The backend will
  take care of picking the EGL layer that corresponds to the desired plane, and
  configuring the plane.

  \section2 Touch input in systems with multiple screens on KMS/DRM

  Touchscreens require additional considerations in multi-display systems
  because touch events have to be routed to the correct virtual screen, and this
  requires a correct mapping between touchscreens and display outputs.

  The mapping is done via the JSON configuration file specified in
  \c QT_QPA_EGLFS_KMS_CONFIG and described in the previous sections. When a
  \c touchDevice property is present in an element of the \c outputs array, the
  value is treated as a device node and the touch device is associated with the
  display output in question.

  For example, assuming our touchscreen has a device node of /dev/input/event5
  and is a touchscreen integrated into the monitor connected via HDMI as the
  secondary screen, the following configuration ensures correct touch (and
  synthesized mouse) event translation:

  \badcode
     {
        "device": "drm-nvdc",
        "outputs": [
          {
            "name": "HDMI1",
            "touchDevice": "/dev/input/event5",
            "virtualIndex": 1
          },
          {
            "name": "DP1",
            "virtualIndex": 0
          }
        ]
    }
  \endcode

  \note When in doubt, enable logging from both the graphics and input
  subsystems by setting the environment variable
  \c{QT_LOGGING_RULES=qt.qpa.*=true} before launching the application. This will
  help identify the correct input device nodes and may uncover output
  configuration issues that can be difficult to debug otherwise.

  \note As of Qt 5.14, the above is only supported for the evdevtouch and
  libinput backends. Other variants will continue to route events to the
  primary screen. To force the usage of evdevtouch on systems where multiple
  input backends are available, set the environment variable
  \c QT_QPA_EGLFS_NO_LIBINPUT to \c 1.

  \section2 EGLFS with other backends

  Other backends, which are typically based on targeting the framebuffer or a
  composition API directly via the vendor's EGL implementation, usually provide
  limited or no support for multiple displays. On i.MX6-based boards with
  Vivante GPUs the \c{QT_QPA_EGLFS_FB} environment variable can be used to
  specify the framebuffer to target, similarly to linuxfb. On the Raspberry Pi
  the \c{QT_QPA_EGLFS_DISPMANX_ID} environment variable can be used to specify
  the screen to output to. The value corresponds to one of the \c{DISPMANX_ID_}
  constants, refer to the Dispmanx documentation. Note that these approaches,
  unlike KMS/DRM, will not typically allow output to multiple screens from
  the same application. Alternatively, driver-specific environment variables or
  kernel parameters may also be available as well to control the used
  framebuffer. Refer to the embedded board's documentation.

  \section1 Video memory

  Systems with a fixed amount of dedicated video memory may need extra care
  before running Qt applications based on Qt Quick or classes like
  QOpenGLWidget. The default setting may be insufficient for such applications,
  especially when they are displayed on a high resolution (for example, full HD)
  screen. In this case, they may start failing in unexpected ways. It is
  recommended to ensure that there is at least 128 MB of GPU memory available.
  For systems that do not have a fixed amount of memory reserved for
  the GPU, this is not an issue.

  \section2 linuxfb

  Use the \c fb plugin parameter to specify the framebuffer device to use.

  \section1 Unix signal handlers

  The console-oriented platform plugins like eglfs and linuxfb install signal
  handlers by default to capture interrupt (\c SIGINT), suspend and continue
  (\c SIGTSTP, \c SIGCONT) and termination (\c SIGTERM). This way the keyboard,
  terminal cursor, and possibly other graphics state can be restored when the
  application terminates or gets suspended due to \c kill, or \c{Ctrl+C} or
  \c{Ctrl+Z}. (although terminating or suspending via the keyboard is only
  possible when \c{QT_QPA_ENABLE_TERMINAL_KEYBOARD} is set, as outlined above in
  the Input section). However, in some cases capturing \c SIGINT can be
  undesirable as it may conflict with remote debugging for instance. Therefore,
  the environment variable \c{QT_QPA_NO_SIGNAL_HANDLER} is provided to opt-out
  from all built-in signal handling.

  \section1 Fonts

  Qt normally uses \c fontconfig to provide access to system fonts. If
  \c fontconfig is not available, Qt will fall back to using
  \c QBasicFontDatabase. In this case, Qt applications will look for fonts in Qt's
  \c lib/fonts directory. Qt will automatically detect pre-rendered fonts and
  TrueType fonts. This directory can be overridden by setting the \c
  QT_QPA_FONTDIR environment variable.

  For more information on the supported formats, see \l{Qt for Embedded Linux Fonts}.

  \note Qt no longer ships any fonts in the \c lib/fonts directory. This
  means that it is up to the platform (the system image) to provide the
  necessary fonts.

  \section1 Platform plugins for windowing systems on embedded Linux devices

  \section2 XCB

  This is the X11 plugin used on regular desktop Linux platforms. In some
  embedded environments, that provide X and the necessary development files for
  \l {http://xcb.freedesktop.org}{xcb}, this plugin functions just like it
  does on a regular PC desktop.

  \note On some devices there is no EGL and OpenGL support available under X
  because the EGL implementation is not compatible with Xlib. In this case the
  XCB plugin is built without EGL support, meaning that Qt Quick 2 or other
  OpenGL-based applications do not work with this platform plugin. It can
  still be used, however, to run software-rendered applications (based on QWidget
  for example).

  As a general rule, the usage of XCB on embedded devices is not
  advisable. Plugins like eglfs are likely to provide better performance and
  hardware acceleration.

  \section2 Wayland

  \l{http://wayland.freedesktop.org/}{Wayland} is a light-weight windowing
  system; or more precisely, it is a protocol for clients to talk to a display
  server.

  Qt Wayland provides a \c wayland platform plugin that allows Qt applications
  to connect to a Wayland compositor.

  For more details, see \l{Wayland and Qt}.

  \section1 Performance enhancement guidelines

  \section2 Use hardware rendering where possible
  When performance is critical to your application, avoid the use of Qt modules
  that rely on software rendering, such as \l{Qt Charts}. Prefer modules that rely on
  hardware rendering instead, where possible.

  \section2 Follow the best practices for Qt Quick

  Follow the \l{Best Practices for QML and Qt Quick}, especially with regards to
  including the \l{Building a QML application}{QML CMake API}, so that \l{qmllint},
  \l{QML script compiler} (qmlsc), and the \l{QML type compiler} (qmltc) are available.
  In addition, it's preferable to write declarative QML and minimize Javascript.
  For more information on how using excessive JavaScript may impact performance see
  \l{QML Performance Considerations And Suggestions}.

  \section2 Use images/textures and shader effects instead of the Canvas QML type

  For drawing custom UI elements, use images/textures and \l{Shader Effects}. Don't
  use the QML \l[QML]{Canvas} type. Shaders require hardware acceleration (GPU).

  \section2 Use Qt Quick instead of Qt Widgets

  With Qt Quick, it's possible to use either hardware-accelerated or software-rendered backends.
  For complex Uls, using Qt Widgets on embedded targets is not recommended, as it will always use
  a software backend.

  There are trade-offs here:
  \list
       \li Using the QML engine and Qt Quick comes with initial overhead.
       \li If your UI is very simple and repaints rarely, it may perform faster when implemented
       using Widgets instead of QML.
       \li If your UI benefits from \l{Animation and Transitions in Qt Quick}{animations},
       \l[QML]{Flickable}{smooth scrolling},and \l[QML]{Item::scale}{scaling},
       \l[QML]{ShaderEffect}{rendering effects}, or \l{Qt Quick 3D}{3D} you need to have GPU
       acceleration, and thus Qt Quick.
  \endlist

  \section2 Choose a resolution appropriate for your UI size

  You need to be cautious with higher resolutions. Resolutions of 720p and higher
  may reduce performance.

  \section2 Use a QML Window type as your application's root element

  Use a \l[QML]{Window} as your application's root element with the application's background
  \l[QML]{Window::color}{color}.

  The reasoning for this is that a Window component has a color property that has
  the effect of being a buffer clear. Rendering the background using a full-screen
  \l[QML]{Rectangle} as an application's root \l[QML]{Item} would cause an additional
  draw call. For some RHI backends this may be the same thing, but there is a difference
  between a \l{QOpenGLFunctions::glClear}{glClear} call and drawing a quad. In most cases,
  a single, opaque image might not have a large performance impact but if you were to
  use an alpha value in that item's color, you might see a significant performance impact.

  \section1 Related topics

  \list
   \li \l{Boot to Qt}
   \li \l{Configure an Embedded Linux Device}
   \li \l{Inputs on an Embedded Linux Device}
  \endlist

*/
