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/*!
    \page configure-linux-device.html
    \title Configure an Embedded Linux Device
    \brief Provides information about how to configure an Embedded Linux device in Qt.

    Cross-compiling Qt for a given device requires a \b toolchain and a \b sysroot. The
    toolchain is expected to contain a version of gcc, or another compiler, and associated
    tools built for cross-compilation. This means these tools run on the \b host system
    (typically x64), while producing binaries for the target architecture (for example, 32
    or 64 bit ARM). The sysroot contains the headers and libraries for the target system,
    allowing compiling and linking libraries and applications on the host.

    This overview page describes to the generic approach, where no distribution building
    systems, such as Yocto or Buildroot, are used. It is always possible to cross-compile
    and deploy Qt onto a device as long as a suitable toolchain and sysroot are available.

    \warning This page can only provide a generic, high-level overview. There are a
    vast number of details that can vary depending on the build environment, the
    target device, and the toolchain. When in doubt, refer to your system
    integrator. For pre-built reference images and SDKs, refer to the
    \l{http://doc.qt.io/QtForDeviceCreation}{Qt for Device Creation} offering.

    When running Qt-based applications without a windowing system, such as X11 or Wayland,
    some devices require vendor-specific adaptation code for EGL and OpenGL ES
    support. This is provided in form of backends for the EGLFS platform plugin. This is
    not relevant for non-accelerated platforms, such as those that use the LinuxFB
    platform plugin, which is meant for software-based rendering only. As of Qt 6, many
    embedded systems use \l{https://en.wikipedia.org/wiki/Direct_Rendering_Manager}{drm}
    to set a video mode, manage display connectors and graphical surfaces. For example, an
    NXP i.MX8-based device or a Raspberry Pi 4 will use this approach, and therefore the
    most commonly used backend for EGLFS is \b eglfs_kms, which enables EGL and OpenGL
    ES based rendering with \c drm, using \c gbm for surface and buffer management. Older
    devices, such as the NXP i.MX6, will continue to use the legacy, GPU vendor-specific
    approach to connect EGL window surfaces to the framebuffer, using dedicated eglfs
    backends, such as \c eglfs_viv.

    \note Be aware that Qt is just one component in the software stack for an embedded
    device. Especially when accelerated graphics are involved, Qt expects a functional
    graphics stack, with an appropriate configuration for the userspace and kernel
    components, such as the display driver. These components are outside of Qt's domain,
    and it is the system integrator's responsibility to ensure the base system is fully
    functional and optimal, including accelerated graphics.

    For further information on graphics and input configuration for Embedded Linux
    systems, refer to \l{Qt for Embedded Linux}.

    \section1 Toolchain Files versus Device Makespecs

    In Qt 5, you would typically use a device spec under the \e qtbase/mkspecs/devices
    directory. These contain the appropriate compiler and linker flags for a certain
    device, also making sure the correct EGL and OpenGL ES libraries are picked up, in
    case they are in a non-standard location in the sysroot.

    For example, you could have configured a Qt 5 build for a Raspberry Pi 2 with a
    configure command like the following:

    \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

    With Qt 6 and CMake, this approach is \b{no longer sufficient} on its own. Rather, a
    \l{https://cmake.org/cmake/help/latest/manual/cmake-toolchains.7.html}{CMake toolchain
    file} must be provided before configuring can happen. It is in this file where
    customization with regards to compiler and linker flags, and toolchain and sysroot
    specific quirks, happens.

    In the below sections we will present a toolchain file that can be used in many cases,
    with minimal customization. It is based on the approach presented in
    \l{https://www.qt.io/blog/standalone-boot2qt-/-yocto-sdk-cmake-toolchain}{this blog
    post}.

    \note The toolchain file presented below is an example, that will often need further
    customizations for a given device. Users and system integrators are also free to
    create their own toolchain files in any way they see fit.

    While CMake is the only supported build system for building Qt itself, applications
    may still be built using \c qmake in Qt 6.0. In order to get a \c qmake setup that is
    functional with cross-compilation, one will need to specify some of the legacy
    arguments to CMake or to configure.

    \section1 Host Tools

    Cross-compiling Qt requires a host build of Qt being available. During the build,
    tools such as \c moc, \c rcc, \c qmlcachegen, \c qsb, and others, are invoked from
    there. For example, if one cross-compiles for ARM on an x64 machine, a local x64 build
    of the same Qt version must be made available first. The path to this Qt build will be
    passed to configure or cmake.

    \section1 Configuring Qt

    Let's assume that the following are available:

    \list
    \li a toolchain and sysroot under \c{$HOME/rpi-sdk},
    \li a checkout of Qt, at minimum the qtbase module, under \c{$HOME/qt-cross},
    \li a host build of Qt in \c{$HOME/qt-host}.
    \endlist

    In addition, the following must be decided before configuring:

    \list

    \li Where will the Qt build be installed on the local system once the build completes?
    In the example we will use \c{$HOME/qt6-rpi}.

    \li Where will the Qt build be deployed on the device? In the example we will use
    \c{/usr/local/qt6}.

    \endlist

    In the example we are going to use a Raspberry Pi 4 SDK (toolchain+sysroot) generated
    via Yocto, but the instructions here are completely generic, with no dependency on
    Yocto. The steps are the same with any other toolchain and sysroot, once the toolchain
    file is updated with the correct cross compiler and other paths.

    After creating and switching to a \c build directory:

    \badcode
    $HOME/qt-cross/qtbase/configure -release -opengl es2 -nomake examples -nomake tests \
      -qt-host-path $HOME/qt-host                              \
      -extprefix $HOME/qt6-rpi                                 \
      -prefix /usr/local/qt6                                   \
      -- -DCMAKE_TOOLCHAIN_FILE=$HOME/qt-cross/toolchain.cmake
    \endcode

    In practice this configure command is equivalent to the following direct CMake call:

    \badcode
    cmake -GNinja -DCMAKE_BUILD_TYPE=Release -DINPUT_opengl=es2 -DQT_BUILD_EXAMPLES=OFF -DQT_BUILD_TESTS=OFF \
      -DQT_HOST_PATH=$HOME/qt-host                           \
      -DCMAKE_STAGING_PREFIX=$HOME/qt6-rpi                   \
      -DCMAKE_INSTALL_PREFIX=/usr/local/qt6                  \
      -DCMAKE_TOOLCHAIN_FILE=$HOME/qt-cross/toolchain.cmake  \
      $HOME/qt-cross/qtbase
    \endcode

    Given the appropriate toolchain file, this is sufficient to generate a Qt build that
    then allows applications to be built using CMake. To enable applications to be built
    with \c qmake as well, the Qt 5 style device spec and device options must be
    specified, in addition to all arguments shown above:

    \badcode
    $HOME/qt-cross/qtbase/configure ...
      ...
      -device linux-rasp-pi4-v3d-g++ \
      -device-option CROSS_COMPILE=$HOME/rpi_sdk/sysroots/x86_64-pokysdk-linux/usr/bin/arm-poky-linux-gnueabi/arm-poky-linux-gnueabi- \
      -device-option DISTRO_OPTS="hard-float" \
      ...
    \endcode

    By default, when cross-compiling, only the Qt libraries and tools that are
    supposed to run on the target device are built. Build-related tools like \c
    moc and \c uic are not built. Building such tools can be enabled by setting
    \c{QT_BUILD_TOOLS_WHEN_CROSSCOMPILING} to \c{ON}.

    \note When \c{QT_BUILD_TOOLS_WHEN_CROSSCOMPILING} is enabled, target binaries of tools
    like \c qmake will get installed to the staging location. Therefore, if \c qmake is
    used to build applications, call the \c{host-qmake} script instead.

    Once configuration completes without errors, run \c{cmake --build . --parallel} to
    build. Once built, run \c{cmake --install .} to install the results to
    \c{$HOME/qt6-rpi}. From there the Qt build can be deployed to the device by using
    rsync, scp, or another method.

    If building individual Qt modules, one can use the \c qt-configure-module script from
    the \c bin directory of the staging location (\c{$HOME/qt6-rpi} in the example) to
    configure additional modules, such as qtdeclarative, qtquick3d, and so on. They can
    then be built using \c{cmake --build .} and installed to the staging location by
    running \c{cmake --install .}

    \note Before starting the build, always inspect the output of the configuration step
    carefully: does it have all the expected features enabled? Making a build and
    deploying it to the device is futile if essential features are not enabled at
    configuration time.

    For example, when accelerated graphics via OpenGL is desired, pay extra attention to
    the following features:

    \badcode
    EGL .................................... yes
    OpenGL:
      Desktop OpenGL ....................... no
      OpenGL ES 2.0 ........................ yes
      OpenGL ES 3.0 ........................ yes
    ...
    evdev .................................. yes
    libinput ............................... yes
    ...
    EGLFS .................................. yes
    EGLFS details:
      EGLFS OpenWFD ........................ no
      EGLFS i.Mx6 .......................... no
      EGLFS i.Mx6 Wayland .................. no
      EGLFS RCAR ........................... no
      EGLFS EGLDevice ...................... yes
      EGLFS GBM ............................ yes
      EGLFS VSP2 ........................... no
      EGLFS Mali ........................... no
      EGLFS Raspberry Pi ................... no
      EGLFS X11 ............................ no
    LinuxFB ................................ yes
    \endcode

    With the Raspberry Pi 4 example, we expect that EGL, OpenGL ES and \c{EGLFS GBM} are
    all reported as \c yes, otherwise the EGLFS platform plugin and its \b eglfs_kms
    backend will not be functional on the device. For getting function mouse, keyboard,
    and touch input, either \c evdev or \c libinput must be enabled.

    Similarly, if X11 is planned to be used as the (or one of the) windowing systems on
    the device, then ensure the xcb and X11 related features are marked as \c yes.

    \section1 An Example Toolchain File

    We will assume there is a sysroot and toolchain available under \c{$HOME/rpi-sdk}. \c
    TARGET_SYSROOT and \c CROSS_COMPILER must be adjusted to the toolchain and sysroot in
    use. The example here is only suitable for one specific, Yocto-generated SDK. The same
    is true for \c CMAKE_C_COMPILER and \c CMAKE_CXX_COMPILER.

    We do not rely on any wrapper scripts that would provide environment variables such as
    PKG_CONFIG_*. Rather, the path to the .pc files is specified in the toolchain file. It
    is likely that another sysroot will need adjustments in \c PKG_CONFIG_LIBDIR. For
    example, with a sysroot generated from a Raspberry Pi OS (formerly Raspbian) image one
    would use \c{/usr/lib/arm-gnueabihf/pkgconfig} instead.

    The compiler and linker flags are not necessary optimal in the example. Adjust them as
    necessary for the target device.

    For further information on the CMake specifics in the example toolchain file, refer to
    \l{https://www.qt.io/blog/standalone-boot2qt-/-yocto-sdk-cmake-toolchain}{this blog
    post} and \l{https://cmake.org/cmake/help/latest/manual/cmake-toolchains.7.html}{the
    CMake documentation}.

    \badcode
cmake_minimum_required(VERSION 3.18)
include_guard(GLOBAL)

set(CMAKE_SYSTEM_NAME Linux)
set(CMAKE_SYSTEM_PROCESSOR arm)

set(TARGET_SYSROOT /home/user/rpi-sdk/sysroots/cortexa7t2hf-neon-vfpv4-poky-linux-gnueabi)
set(CROSS_COMPILER /home/user/rpi-sdk/sysroots/x86_64-pokysdk-linux/usr/bin/arm-poky-linux-gnueabi)

set(CMAKE_SYSROOT ${TARGET_SYSROOT})

set(ENV{PKG_CONFIG_PATH} "")
set(ENV{PKG_CONFIG_LIBDIR} ${CMAKE_SYSROOT}/usr/lib/pkgconfig:${CMAKE_SYSROOT}/usr/share/pkgconfig)
set(ENV{PKG_CONFIG_SYSROOT_DIR} ${CMAKE_SYSROOT})

set(CMAKE_C_COMPILER ${CROSS_COMPILER}/arm-poky-linux-gnueabi-gcc)
set(CMAKE_CXX_COMPILER ${CROSS_COMPILER}/arm-poky-linux-gnueabi-g++)

set(QT_COMPILER_FLAGS "-march=armv7-a -mfpu=neon -mfloat-abi=hard")
set(QT_COMPILER_FLAGS_RELEASE "-O2 -pipe")
set(QT_LINKER_FLAGS "-Wl,-O1 -Wl,--hash-style=gnu -Wl,--as-needed")

set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
set(CMAKE_FIND_ROOT_PATH_MODE_PACKAGE ONLY)

include(CMakeInitializeConfigs)

function(cmake_initialize_per_config_variable _PREFIX _DOCSTRING)
  if (_PREFIX MATCHES "CMAKE_(C|CXX|ASM)_FLAGS")
    set(CMAKE_${CMAKE_MATCH_1}_FLAGS_INIT "${QT_COMPILER_FLAGS}")

    foreach (config DEBUG RELEASE MINSIZEREL RELWITHDEBINFO)
      if (DEFINED QT_COMPILER_FLAGS_${config})
        set(CMAKE_${CMAKE_MATCH_1}_FLAGS_${config}_INIT "${QT_COMPILER_FLAGS_${config}}")
      endif()
    endforeach()
  endif()

  if (_PREFIX MATCHES "CMAKE_(SHARED|MODULE|EXE)_LINKER_FLAGS")
    foreach (config SHARED MODULE EXE)
      set(CMAKE_${config}_LINKER_FLAGS_INIT "${QT_LINKER_FLAGS}")
    endforeach()
  endif()

  _cmake_initialize_per_config_variable(${ARGV})
endfunction()
    \endcode

    \section1 Building Applications for the Target Device

    Once the Qt build is done and installed to the staging location, examples or
    applications can be built.

    With CMake, use the generated \c qt-cmake script in the \c bin directory of the
    staging location (\c{$HOME/qt6-rpi} in the example) to configure, then run \c
    ninja. For example:

    \badcode
    $HOME/qt6-rpi/bin/qt-cmake .
    cmake --build .
    \endcode

    The resulting application binary can then be deployed to the device. Using the \c
    qt-cmake helper script is convenient, because the script ensures the toolchain file
    that was used for building Qt is loaded, so there is no need to repeatedly specify it
    for each application.

    Unlike for Qt itself, building applications with qmake is still supported in Qt 6.0,
    as long as a suitable device spec is available, and the appropriate legacy arguments
    were passed to CMake or configure when configuring Qt. If this is all true, then
    running \c qmake and \c make will also generate an application binary for the target
    device.

    \section1 Defaults for Platform Plugins and EGLFS

    Once configured, a default platform plugin is chosen. This is used when launching an
    application without the \c{-platform} argument and without having the \c
    QT_QPA_PLATFORM environment variable set.

    Similarly, the EGLFS platform plugin has multiple backends. The default is chosen
    based on availability and a pre-defined priority order. If drm and gbm are available,
    the default will be the \b eglfs_kms backend. This can always be overridden at runtime
    by setting the \c QT_QPA_EGLFS_INTEGRATION environment variable.

    To change these defaults for the build, without having to force a specific value at
    run time, the following to CMake cache variables are available once CMake has been run
    once:

    \list
    \li \c QT_QPA_DEFAULT_PLATFORM (\c STRING) - The name of the default platform plugin.
    \li \c QT_QPA_DEFAULT_EGLFS_INTEGRATION (\c STRING) - The default EGLFS backend.
    \endlist

    For more information on configuring Qt, see \l {Qt Configure Options}.
*/
