Make sure to configure with -device imx6 (which is the same as -device linux-imx6-g+). Without this the build may still succeed but eglfs will not be functional due to not having the Vivante-specific hooks compiled in from qtbase/mkspecs/devices/linux-imx6-g+. Both linuxfb and eglfs should work. Note however that in 5.2.0 linuxfb is broken. This is corrected in 5.2.1.
Multiple screens
When having multiple displays connected (e.g. the touch LCD panel and HDMI), there are two framebuffer devices: /dev/fb0 and /dev/fb2. Note that fb1 and fb3 are overlays for fb0 and fb2 and are not useful for our purposes here. Both eglfs and linuxfb uses /dev/fb0 by default. To change this, linuxfb can be configued with the 'fb' parameter: just launch the application with "-platform linuxfb:fb=/dev/fb2". eglfs has no way to configure this before 5.3.0 so apps using eglfs will always show up on the display belonging to fb0. In 5.3.0 a new environment variable is introduced: QT_QPA_EGLFS_FB. Setting this to /dev/fb2 before launching the app will make the EGL/GLES content show up on the second display. Update: It might be possible to launch eglfs apps on the other display with 5.2 too, by setting FB_FRAMEBUFFER_0 to /dev/fb2. There is currently no way to display content on multiple displays from the same application. The workaround is to launch two separate applications and use some sort of IPC mechanism to communicate between them.
Screen configuration
Both linuxfb and eglfs will try to query the display dimensions (both physical and pixels) from the framebuffer device. This may not always lead to good looking results. For eglfs the screen size can be set explicitly by setting QT_QPA_EGLFS_WIDTH and QT_QPA_EGLFS_HEIGHT to the desired width/height (in pixels) before launching the application. linuxfb has a size=…x… plugin parameter (for example -platform linuxfb:fb=/dev/fb2:size=1920x1080).
Multiple top-level widgets (QWidget)
linuxfb, which supports sw-rendered widget apps only, has full support for multiple top-level widgets. It is nonetheless advisable to have one widget that covers the entire screen (e.g. showFullScreen()) since the unpainted areas of the screen are not cleared. Starting with Qt 5.2.0 eglfs also has a certain level of support for multiple top-level widgets. Opening new top-levels, e.g. a menu or a combobox dropdown, will not lead to creating a new native window and EGL surface. Instead, all widget windows are uploaded into textures and composited into a single fullscreen window that is backed by a native window and EGL surface. Before 5.2 having multiple top-levels displayed correctly was not possible at all. eglfs forces the first top-level to become fullscreen. This is because something has to cover the entire surface, we cannot have "empty" areas. This window is also chosen to be the "root" widget window, into which all other top-level widgets are composited. This works well for applications that have a single main window that exists for the entire lifetime of the application, and all other widgets are either non-top-levels or are created later on. It presents a potential issue for other type of applications, though, and can lead to unexpected results.
