Error "Could not open DRM device" running Qt app with platform eglfs in Raspberry Pi

While testing recently cross-compiled Qt libraries (5.15.0) for Raspberry Pi 3 running ArchLinuxARM using a sample application (qopenglwidget) I received error:

Could not open DRM device /dev/dri/card0 (Permission denied)

Cross-compiling full Qt 5.11.x for Raspberry Pi: error with QGeoMap class in Qt Location module

From this post, an error appears when cross-compiling Qt 5.11.x (full from source code tarball, i.e. qt-everywhere-src-5.11.0.tar.xz) and following the steps from this guide. 

The solution I found:

Qt on Raspberry: QFontDatabase error

After deploying a cross-compiled Qt application for the Raspberry Pi boards (by following this guide), this error message appears:

QFontDatabase: Cannot find font directory /usr/local/qt5pi/lib/fonts.
Note that Qt no longer ships fonts. Deploy some (from http://dejavu-fonts.org for example) or switch to fontconfig.

Cross-compiling QtMqtt module for Raspberry Pi

A previous post of mine described the Qt module for MQTT protocol, and how to compile it for the Android platform. That seems a good fit to develop a client application for a dashboard for instance, where you can subscribe to different topics and monitor the evolution of such items.
Ok you'll say, but what about creating MQTT publishers, i.e. devices that may have sensors and will provide the readings with MQTT messages?

Using Qt Creator to cross-compile and debug Raspberry Pi Qt5 apps

Have you ever wonder how to setup Qt Creator in your powerful workstation to develop Qt apps that can be deployed and tested in your Raspberry Pi (reachable from such workstation)?

Raspbian: /opt/qt/bin/qmake: cannot execute binary file: Exec format error

If you're cross-compiling Qt for Raspberry Pi (for instance following this guide), and once everything finished and seems Ok, you may encounter the following error while running qmake:
/opt/qt/bin/qmake: cannot execute binary file: Exec format error