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

  Since the Qt 5.0 release, Qt no longer contains its own window system (QWS)
  implementation. For single-process use cases, the \l{Qt Platform Abstraction}
  is a superior solution. Multiple graphical processes can be supported
  through Wayland.

  There are multiple platform plugins that are potentially usable on
  Embedded Linux systems: EGLFS, LinuxFB, DirectFB, Wayland. The
  availability of these depend on the configuration of Qt.  On many
  boards \e eglfs is chosen as the default one. If the default is not
  suitable, the \c QT_QPA_PLATFORM environment variable parameter can
  be used to request another plugin. Alternatively, for quick tests,
  the \c -platform command-line can be used with the same syntax.

  \section1 Configuring a Specific Device

  Building Qt for a given device requires a toolchain and a sysroot.
  Additionally, some devices require vendor-specific adaptation code for EGL
  and OpenGL ES 2.0 support. This is not relevant for non-accelerated
  platforms, for example the ones using the LinuxFB plugin, which is meant for
  software-based rendering only.

  The directory \e qtbase/mkspecs/devices contains configuration and graphics
  adaptation code for a number of devices. For example, the
  \c linux-rasp-pi2-g++ mkspec contains build settings such as the optimal
  compiler and linker flags for the
  \l{http://www.raspberrypi.org}{Raspberry Pi 2} device. The mkspec also
  contains information about either an implementation of the \e eglfs hooks
  (vendor-specific adaptation code), or a reference to the suitable \e eglfs
  device integration plugin. The device is selected through the
  \l{Qt Configure Options}{configure} tool's \c -device parameter. The name
  that follows after this argument must, at least partially,
  match one of the subdirectories under \e devices.

  The following is an example configuration for the Raspberry Pi 2. For most
  Embedded Linux boards, the configure command looks similar:

  \badcode
    ./configure -release -opengl es2 -device linux-rasp-pi2-g++ -device-option CROSS_COMPILE=$TOOLCHAIN/arm-bcm2708/gcc-linaro-arm-linux-gnueabihf-raspbian/bin/arm-linux-gnueabihf- -sysroot $ROOTFS -prefix /usr/local/qt5
  \endcode

  The most important parameters are \c -device and \c -sysroot. By specifying
  \c -sysroot, the include files and libraries used by \c {configure}'s feature
  detection tests, as well as Qt itself, is taken from the specified
  location, instead of the host PC's standard locations. This means that
  installing development packages on the host machine has no relevance. For
  example, to get \c libinput support it is not sufficient or necessary to have
  the \c libinput development headers and libraries installed on the host
  environment. Instead, the headers and the libraries for the target
  architecture (e.g. ARM) must be present in the \c sysroot.

  \c pkg-config is supported also when performing cross-compilation.
  \c configure automatically sets \c PKG_CONFIG_LIBDIR to make \c pkg-config
  report compiler and linker settings based on the \c sysroot instead of the host
  machine. This usually functions well without any further adjustments. However,
  environment variables such as \c PKG_CONFIG_PATH must be unset for the host
  machine before running \c configure. Otherwise, the Qt build may attempt to
  use inappropriate headers and libraries from the host system.

  Specifying \c -sysroot results in automatically setting the \c --sysroot
  argument when invoking the compiler. In some cases this is not desirable and
  can be disabled by passing \c -no-gcc-sysroot to \c configure.

  \c -prefix, \c -extprefix and \c -hostprefix control the intended destination
  directory of the Qt build. In the above example the ARM build of Qt is
  expected to be placed in \c{/usr/local/qt5} on the target device. Note that
  running \c{make install} does not deploy anything to the device. Instead, the
  \c install step targets the directory specified by \c extprefix which defaults
  to \c sysroot + \c prefix and is therefore optional. However, in many cases
  "polluting" the sysroot is not desirable and thus specifying \c -extprefix
  becomes important. Finally, \c -hostprefix allows separating host tools like
  qmake, rcc, uic from the binaries for the target. When given, such tools will
  be installed under the specified directory instead of \c extprefix.

  See \l {Qt Configure Options} for more information.

  \section1 Platform Plugins for Embedded Linux Devices

  \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: The creation of a \e {native
  window} (which will not necessarily be an actual window on the screen) must
  still be done by platform-specific means. Hence the need for the board or
  GPU-specific adaptation code. Such adaptations are provided either as
  \e {eglfs hooks}, which can be a single source file compiled into the
  platform plugin, or as dynamically loaded \e {EGL device integration}
  plugins.

  \e EGLFS is a platform plugin for running Qt5 applications on top of EGL
  and OpenGL ES 2.0 without an actual windowing system (like X11 or Wayland).
  In addition to Qt Quick 2 and native OpenGL applications, it supports
  software-rendered windows (for example QWidget) too. In the latter case the
  widgets' contents are rendered using the CPU into images, which are then
  uploaded into textures and composited by the plugin.

  This is the recommended plugin for modern Embedded Linux devices that include
  a GPU.

  \e EGLFS forces the first top-level window (be it 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 (for example dialogs,
  popup menus or combobox dropdowns) are composited. This is necessary because
  with \e EGLFS there is always exactly one native window and EGL window surface,
  and 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 entire lifetime of
  the application and all other widgets are either non top-levels or are created
  afterwards, once the main window is shown.

  There are further restrictions for OpenGL-based windows. As of Qt 5.3, \c eglfs
  supports a single fullscreen GL window (for example, an OpenGL-based QWindow,
  a QQuickView or a QGLWidget). Opening additional OpenGL windows or mixing such
  windows with QWidget-based content is not supported and terminates the
  application with an error message.

  If necessary, \c eglfs can be configured using the following environment
  variables:

  \list

  \li \c {QT_QPA_EGLFS_INTEGRATION} - In addition to the compiled-in \e hooks,
  it is also possible to provide device or vendor-specific adaptation in the
  form of dynamically loaded plugins. 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 \c EGLFS_DEVICE_INTEGRATION entry:
  the name of the preferred backend for that particular device. This is
  optional, but very useful to avoid the need to set this environment variable
  in case there are more than one plugins present in 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 that on some boards
  the 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 and so no plugins must be loaded.

  \li \c {QT_QPA_EGLFS_PHYSICAL_WIDTH} and \c
  {QT_QPA_EGLFS_PHYSICAL_HEIGHT} - Physical screen width and height in
  millimeters. On platforms where the value cannot be queried from the
  framebuffer device \e{/dev/fb0} or via other means, a default DPI of
  100 is used. This variable can be used to override any such
  defaults. Setting this is important because QWidget or Qt Quick
  Controls based applications rely on these values. Running with the
  hard-coded settings may result in user interface element sizes
  unsuitable for the display in use.

  \li \c {QT_QPA_EGLFS_ROTATION} - Specifies the rotation applied to
  software-rendered content in QWidget-based applications. Supported values are
  180, 90, and -90. This does not apply to OpenGL-based windows, including Qt
  Quick. Qt Quick applications can apply transformations in their QML scene
  instead. The standard eglfs mouse cursor always takes the value into account,
  with appropriately positioned and rotated pointer image, regardless of the
  application type. The special cursor implementations, such as the KMS/DRM
  backend's hardware cursor, may not support rotation.

  \li \c {QT_QPA_EGLFS_FORCEVSYNC} - When set, \c eglfs requests
  \c FBIO_WAITFORVSYNC on the framebuffer device after each call to
  eglSwapBuffers(). This is only relevant for backends relying on the legacy
  \c fbdev subsystem of Linux. 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.

  \li \c {QT_QPA_EGLFS_FORCE888} - When set, the red, green, and blue color
  channel sizes are ignored whenever creating a new context, window or offscreen
  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 but
  are known not to be ideal, for example, due to banding effects. Instead of
  changing application code, this variable provides an easier shortcut to force
  24 or 32 bpp configurations.

  \endlist

  In addition, the following - less commonly used - variables are
  available as well:

  \list

  \li \c {QT_QPA_EGLFS_FB} - Overrides the framebuffer device. The default is
  \c /dev/fb0. On most embedded platforms this is not very relevant because the
  framebuffer is used only for querying settings like the display dimensions.
  On certain devices however, this parameter provides the ability to specify
  the display to be used in multiple display setups, similarly to the \c fb
  parameter in LinuxFB.

  \li \c {QT_QPA_EGLFS_WIDTH} and \c {QT_QPA_EGLFS_HEIGHT} - Contain the screen
  width and height in pixels. While \c eglfs tries to determine the dimensions
  from the framebuffer device \e{/dev/fb0}, but this does not work always and
  manually specifying the sizes may become necessary.

  \li \c {QT_QPA_EGLFS_DEPTH} - 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, the default of \c 32 is used. This variable can be used
  to override any such defaults. Note that this affects only the color depth
  value reported by QScreen. It has no connection to EGL configurations and the
  color depth used for OpenGL rendering.

  \li \c {QT_QPA_EGLFS_SWAPINTERVAL} - By default a swap interval of \c 1 will
  be requested. This enables synchronizing to the displays vertical refresh. The
  value can be overridden with this environment variable. For instance, passing
  0 will disable blocking on swap, resulting in running as fast as possible
  without any synchronization.

  \li \c {QT_QPA_EGLFS_DEBUG} - 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. Together with Qt Quick's \c {QSG_INFO} variable, this can provide
  useful information for troubleshooting issues related to the EGL
  configuration.

  \endlist

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

  \list

    \li \c qt.qpa.egldeviceintegration – Enables logging for dynamically loaded
    backends. Very useful 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. Very useful 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 the output of
  it. Not having the necessary eglfs backend, libudev, or libinput
  enabled due to the corresponding configure tests failing are fairly
  common issues that can be easily and quickly recognized this
  way. When there is an undesired "no" result, run \c configure with
  \c{-v} to turn on verbose output in order to see the compiler and
  linker invocations for each configure test.

  \note Errors about missing headers, libraries or seemingly cryptic
  linker failures are often a sign of an incomplete or broken sysroot
  and have nothing to do with and cannot be solved by Qt.

  As an example, when targeting the Raspberry Pi with the Broadcom
  proprietary graphics drivers, the output should contain something
  like the following. If this is not the case, there is no point in
  proceeding further with the build since accelerated graphics will
  not be functional without the Raspberry Pi-specific backend, even if
  the rest of Qt compiles successfully.

  \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 Rasberry Pi .................. yes
      EGL on X11 ......................... no
  \endcode

  \section2 LinuxFB

  This plugin writes directly to the framebuffer via the fbdev subsystem of
  Linux. Only software-rendered content is supported. Note that on some setups
  the display performance is expected to be limited.

  As of Qt 5.9, DRM dumb buffer support is also available due to fbdev being
  deprecated in the Linux kernel. This must be requested by setting the
  \c QT_QPA_FB_DRM environment variable to a non-zero value. When set, provided
  that dumb buffers are supported by the system, legacy framebuffer devices like
  \c{/dev/fb0} are never accessed. Instead, rendering is set up via the DRM
  APIs, similarly to the \c{eglfs_kms} backend of eglfs. The output will be
  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.

  The \c linuxfb plugin allows specifying additional settings by passing them
  in 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. Multiple settings can be specified by separating them with a colon.

  \list

  \li \c {fb=/dev/fbN} - Specifies the framebuffer devices. On multiple display
  setups this typically allows running the application on different
  displays. For the time being there is no way to use multiple framebuffers from
  one Qt application.

  \li \c{size=}\e{<width>}\c{x}\e{<height>} - Specifies the screen size in
  pixels. The plugin tries to query the display dimensions, both physical and
  logical, from the framebuffer device. This may not always lead to proper
  results however, and therefore it may become necessary to explicitly specify
  the values.

  \li \c{mmsize=}\e{<width>}\c{x}\e{<height>} - Physical width and height in
  millimeters.

  \li \c{offset=}\e{<width>}\c{x}\e{<height>} - Offset in pixels specifying the
  top-left corner of the screen. The default position is at \c{(0, 0)}.

  \li \c {nographicsmodeswitch} - Do not switch the virtual terminal to graphics
  mode (\c KD_GRAPHICS). In addition to switching to graphics mode, the blinking
  cursor and screen blanking are normally disabled too. When this parameter is
  set, these are also skipped.

  \li \c {tty=/dev/ttyN} - Overrides the virtual console. Only used when
  \c{nographicsmodeswitch} is not set.

  \endlist

  As of Qt 5.9, the behavior of eglfs and linuxfb has been synchronized when it
  comes 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 environment variable \c{QT_QPA_FB_FORCE_FULLSCREEN} to \c 0 in order
  to restore the behavior of earlier Qt versions.

  \section1 Input

  When no windowing system is present, the mouse, keyboard, and touch input are
  read directly via \c evdev or using helper libraries such as \c libinput or
  \c tslib.  Note that this requires that device nodes \c {/dev/input/event*} are
  readable by the user. \c eglfs and \c linuxfb have all the input handling code
  compiled-in.

  \section2 Using libinput

  \l{http://www.freedesktop.org/wiki/Software/libinput}{libinput} is a library
  to handle input devices. It offers an alternative to the Qt's own \c evdev
  input support. To enable using \c libinput, make sure the development files
  for \c libudev and \c libinput are available when configuring and building
  Qt. \c xkbcommon is also necessary if keyboard support is desired. With
  \c eglfs and \c linuxfb no further actions are necessary as these plugins use
  \c libinput by default. If \c libinput support is not available or the
  environment variable \c QT_QPA_EGLFS_NO_LIBINPUT is set, Qt's own evdev
  handlers come in to play.

  \section2 Input on eglfs and linuxfb without libinput

  Parameters like the device node name can be set in the environment variables
  \c QT_QPA_EVDEV_MOUSE_PARAMETERS, \c QT_QPA_EVDEV_KEYBOARD_PARAMETERS and
  \c QT_QPA_EVDEV_TOUCHSCREEN_PARAMETERS. Separate the entries with colons.  These
  parameters act as an alternative to passing the settings in the \c{-plugin}
  command-line argument, and with some backends they are essential: eglfs and
  linuxfb use built-in input handlers so there is no separate \c {-plugin}
  argument in use.

  Additionally, the built-in input handlers can be disabled by setting
  \c QT_QPA_EGLFS_DISABLE_INPUT or \c QT_QPA_FB_DISABLE_INPUT to \c 1.

  \section2 Mouse

  The mouse cursor shows up whenever \c QT_QPA_EGLFS_HIDECURSOR (for eglfs)
  or \c QT_QPA_FB_HIDECURSOR (for linuxfb) is not set and Qt's libudev-based
  device discovery reports that at least one mouse is available. When \c libudev
  support is not present, the mouse cursor always show up unless explicitly
  disabled via the environment variable.

  Hot plugging is supported, but only if Qt was configured with \c libudev support
  (that is, if the \e libudev development headers are present in the sysroot at
  configure time). This allows connecting or disconnecting an input device while
  the application is running.

  The \e evdev mouse handler supports the following extra parameters:

  \list

  \li \c {/dev/input/...} - Specifies the name of the input device. When not
  given, Qt looks for a suitable device either via \e libudev or by walking
  through the available nodes.

  \li \c nocompress - By default, input events that do not lead to changing the
  position compared to the last Qt mouse event are compressed; a new Qt mouse
  event is sent only after a change in the position or button state.  This can
  be disabled by setting the \c nocompress parameter.

  \li \c dejitter - Specifies a jitter limit. By default dejittering is disabled.

  \li \c grab - When 1, Qt will grab the device for exclusive use.

  \li \c abs - Some touchscreens report absolute coordinates and cannot be
  differentiated from touchpads.  In this special situation pass \c abs to
  indicate that the device is using absolute events.

  \endlist

  \section2 Keyboard

  The \e evdev keyboard handler supports the following extra parameters:

  \list

  \li \c {/dev/input/...} - Specifies the name of the input device. When not
  given, Qt looks for a suitable device either via \e libudev or by walking
  through the available nodes.
  \li \c {grab} - Enables grabbing the input device.
  \li \c {keymap} - Specifies the name of a custom keyboard map file.
  \li \c {enable-compose} - Enables compositing.
  \li \c {repeat-delay} - Sets a custom key repeat delay.
  \li \c {repeat-rate} - Sets a custom key repeat rate.
  \endlist

  On Embedded Linux systems that do not have their terminal sessions disabled,
  the behavior on a key press can be confusing as input event is processed by
  the Qt application and the tty. To overcome this, the following options are
  available:

  \list

  \li \e EGLFS and \e LinuxFB attempt to disable the terminal keyboard on
  application startup by setting the tty's keyboard mode to \c K_OFF. This
  prevents keystrokes from going to the terminal. If the standard behavior needs
  to be restored for some reason, set the environment variable
  \c QT_QPA_ENABLE_TERMINAL_KEYBOARD to \c 1. Note that this works only when the
  application is launched from a remote console (for example, via \c ssh) and
  the terminal keyboard input remains enabled.

  \li An alternative approach is to use the \e evdev keyboard handler's \c grab
  parameter by passing \e{grab=1} in \c QT_QPA_EVDEV_KEYBOARD_PARAMETERS. This
  results in trying to get a grab on the input device. If the \c grab is
  successful, no other components in the system receive events from it as long as
  the Qt application is running. This approach is more suitable for applications
  started remotely as it does not need access to the tty device.

  \li Finally, for many specialized Embedded Linux images it does not make sense
  to have the standard terminal sessions enabled in the first place. Refer to
  your build environment's documentation on how to disable them. For example,
  when generating images using the \l {http://www.yoctoproject.org}{Yocto
  Project}, unsetting \c SYSVINIT_ENABLED_GETTYS results in having no
  \c getty process running, and thus no input, on any of the virtual terminals.

  \endlist

  If the default built-in keymap is not sufficient, a different one can be
  specified either via the \c keymap parameter or by using the eglfs-specific
  \l{QEglFSFunctions::loadKeymap()}{loadKeymap()} function. The latter allows
  switching the keymap at runtime. Note however that this requires using eglfs'
  built-in keyboard handler; it is not supported when the keyboard handler is
  loaded via the \c -plugin command-line parameter.

  \note Special system key combinations, such as console switching
  (\e{Ctrl+Alt+Fx}) or zap (\e{Ctrl+Alt+Backspace}) are not currently supported
  and are ignored.

  To generate a custom keymap, the \e kmap2qmap utility can be used. This can be
  found in the \e qttools module. The source files have to be in standard Linux
  \c kmap format, which is understood by the kernel's \c loadkeys command.
  This means one can use the following sources to generate \c qmap files:

  \list
  \li The \l {http://lct.sourceforge.net/}{Linux Console Tools (LCT)} project.
  \li \l {http://www.x.org/}{Xorg} X11 keymaps can be converted to the
  \c kmap format with the \c ckbcomp utility.
  \li As \c kmap files are plain-text files, they can also be hand crafted.
  \endlist

  \c kmap2qmap is a command line program, that needs at least 2 files as
  parameters.  The last one is the generated \c .qmap file, while all the
  others are parsed as input \c .kmap files.  For example:

  \badcode
  kmap2qmap i386/qwertz/de-latin1-nodeadkeys.kmap include/compose.latin1.inc de-latin1-nodeadkeys.qmap
  \endcode

  \note \c kmap2qmap does not support all the (pseudo) symbols that the Linux
  kernel supports.  When converting a standard keymap, a number of warnings will
  be shown regarding \c Show_Registers, \c Hex_A, and so on; these messages can
  safely be ignored.

  \section2 Touch

  For some resistive, single-touch touch screens it may be necessary to fall
  back to using \c tslib instead of relying on the Linux multi-touch protocol and
  the event devices. For modern touch screens this is not necessary. \c tslib
  support can be enabled by setting the environment variable
  \c QT_QPA_EGLFS_TSLIB or \c QT_QPA_FB_TSLIB to 1. To change the device, set the
  environment variable \c TSLIB_TSDEVICE or pass the device name on the
  command-line. Note that the \c tslib input handler generates mouse events and
  supports single touch only, as opposed to \c evdevtouch which generates true
  multi-touch QTouchEvent events too.

  The \e evdev touch handler supports the following extra parameters:

  \list

  \li \c {/dev/input/...} - Specifies the name of the input device. When not
  given, Qt looks for a suitable device either via \e libudev or by walking
  through the available nodes.

  \li \c rotate - On some touch screens the coordinates must be rotated, which
  is done by setting \c rotate to 90, 180, or 270.

  \li \c invertx and \c inverty - To invert the X or Y coordinates in the input
  events, pass \c invertx or \c inverty.

  \endlist

  For example, doing \c{export
  QT_QPA_EVDEV_TOUCHSCREEN_PARAMETERS=/dev/input/event5:rotate=180} before
  launching applications results in an explicitly specified touch device and
  flipping the coordinates - useful when the orientation of the actual
  screen and the touch screen do not match.

  \section2 Pen-based tablets

  The \c evdevtablet plugin provides basic support for Wacom and similar,
  pen-based tablets. It generates QTabletEvent events only. To enable it,
  pass \c {QT_QPA_GENERIC_PLUGINS=evdevtablet} in the environment or,
  alternatively, pass \c {-plugin evdevtablet} argument on the command-line.
  The plugin can take a device node parameter, for example
  \c{QT_QPA_GENERIC_PLUGINS=evdevtablet:/dev/event1}, in case the Qt's automatic
  device discovery (based either on \e libudev or a walkthrough of
  \c{/dev/input/event*}) is not functional or misbehaving.

  \section2 Debugging Input Devices

  It is possible to print some information to the debug output by enabling
  the \c qt.qpa.input logging rule, for example by setting the \c QT_LOGGING_RULES
  environment variable to \c{qt.qpa.input=true}. This is useful for detecting
  which device is being used, or to troubleshoot device discovery issues.

  \section2 Using Custom Mouse Cursor Images

  \c eglfs comes with its own set of 32x32 sized mouse cursor images. If these are
  not sufficient, a custom cursor atlas can be provided by setting the \c
  QT_QPA_EGLFS_CURSOR environment variable to the name of a JSON file. The file
  can also be embedded into the application via Qt's resource system.

  For example, an embedded cursor atlas with 8 cursor images per row can be
  specified like the following:

  \badcode
    {
      "image": ":/cursor-atlas.png",
      "cursorsPerRow": 8,
      "hotSpots": [
          [7, 2],
          [12, 3],
          [12, 12],
          ...
      ]
    }
  \endcode

  Note that the images are expected to be tightly packed in the atlas: the
  width and height of the cursors are decided based on the total image size and
  the \c cursorsPerRow setting. Atlases have to provide an image for all the
  supported cursors.

  \section1 Display Output

  When having multiple displays connected, the level of support for targeting
  one or more of these from one single Qt application varies between the
  platform plugins and often depends on the device and its graphics stack.

  \section2 eglfs with 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 is not supported
  either. Therefore, it is essential that embedded applications make all the
  necessary QWindow::setScreen() calls before calling QWindow::show().

  When getting started with developing on a given embedded device, it is often
  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
  hellowindow example. Launching it with \c{-platform eglfs --multiscreen
  --timeout} arguments shows a rotating Qt logo on each connected screen for a few
  seconds.

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

  The KMS/DRM backend also supports custom configurations via a JSON file.  Set
  the environment variable \c QT_QPA_EGLFS_KMS_CONFIG to the name of the file to
  enable this. The file can also be embedded into the application via the Qt
  resource system. An example configuration is below:

  \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 will not use the hardware cursor (falls back to rendering the mouse
  cursor via OpenGL; by default hardware cursors are enabled as they are more
  efficient),

  \li it will back 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 and instead the specified device is used.

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

  Specifying \c current will choose a mode with a resolution matching the
  current one. Due to the fact that modesetting 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
  keep the current mode and any content in the planes not touched by Qt.

  All screens reported by the DRM layer will be treated as one big virtual
  desktop by default. The mouse cursor implementation will take this into
  account and move across the screens as expected. Although not recommended, the
  virtual desktop mode can be disabled by setting \c separateScreens to \c false
  in the configuration, if desired.

  By default, the virtual desktop is formed left to right, based on the order of
  connectors as reported by the system. This can be changed by setting
  \c virtualIndex to a value starting from 0. For example, the following
  configuration uses the preferred resolution but ensures that the left side in
  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 will be 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.

  \note It is 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 property \c virtualPos 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 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 however.

  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, however 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.

  Each active output from the \c outputs array corresponds to one QScreen
  instance reported from QGuiApplication::screens(). The primary screen reported
  by QGuiApplication::primaryScreen() is by default the screen that gets
  registered first. When not using \c virtualIndex, this means the decision is
  based on the DRM connector order. To override this, set the property
  \c primary to \c true on the desired entry in the \c outputs list. For example,
  to ensure the screen corresponding to the VGA output will be the primary even
  when the system happens to report the HDMI one first, one can 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 categorized logging rule, \c qt.qpa.eglfs.kms.

  \note In an embedded environment virtual desktops are more limited than with 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.

  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 and animations may
  degrade as a result.

  As an 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

  Screen cloning (mirroring) is supported as of Qt 5.11. It can be enabled by
  the \c clones property:

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

  Here the content on the display connected via DisplayPort will be the same as
  on the HDMI one. This is ensured by simply scanning out the same buffer on
  both.

  \note This can only work if the resolutions are the same, there are no
  incompatibilities when it comes to accepted buffer formats, and the
  application does not 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 is up to the configuration and the
  application to ensure these.

  Headless mode via DRM render nodes is supported as of Qt 5.11. This allows
  performing GPU compute (OpenGL compute shaders, OpenCL) or offscreen 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 offscreen
  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

  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 you 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 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
  the configuring of 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 identifying the correct input device nodes and may uncover output
  configuration issues that can be difficult to debug otherwise.

  \note As of Qt 5.8, the above is only supported for the evdevtouch input
  backend. Other variants, such as the libinput-based one, 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, that 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 to 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.

  \section2 Video Memory

  Systems with a fixed amount of dedicated video memory may need extra care
  before running Qt application 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 does 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.

  The Qt Wayland module provides a \c wayland platform plugin that allows Qt
  application to connect to a Wayland compositor.

  \note You may experience issues with touch screen input while using
  the \l{http://wayland.freedesktop.org/}{Weston} reference compositor.
  Refer to the \l{https://wiki.qt.io/WestonTouchScreenIssues}{Qt Wiki} for
  further information.

  \section1 Related Topics

  \list
   \li \l{Qt for Device Creation}
   \li \l Emulator
   \li \l{Qt Virtual Keyboard}
   \li \l{Qt Quick WebGL}
  \endlist

*/
