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)

Qt Creator: Project ERROR: You need to set the ANDROID_NDK_ROOT environment variable

I've just installed Qt 5.12.7 with offline installer, and having previously setup the Android SDK and NDK, along with Java 8, when starting Qt Creator for the first time the following messages appeared under tab "6 General Messages":


Project ERROR: You need to set the ANDROID_NDK_ROOT environment variable to point to your Android NDK.
Could not read qmake configuration file /opt/Qt5.12.7/5.12.7/android_armv7/mkspecs/android-clang/qmake.conf.

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.

Qt Test or not Qt Test

Are you convinced that unit tests are a great tool for great software development? I really am. Having a (good) set of unit tests allow you to be sure that changes to existing code (i.e. refactoring) may not break it, and if you are lean to Test Driven Development (TDD) they allows you to think about the "contract" or responsibilities of a class before actual implementation.

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?

Cross-compiling QtMqtt module for Android

As the MQTT potocol continues to get traction in the IoT world, more and more people seems to rely on Qt MQTT module to implement clients applications that can communicate over the MQ telemetry transport (MQTT) protocol. And given the device-based nature of such clients, it's very likely that you may need the module available for different platforms/architectures.