RPi4 and Yocto Updates

I recently obtained a brand new Raspberry Pi4 device and took the free days around x-mas to play a little bit around with it. And I must say, I am very pleased with this device!

Raspberry Pi 4 B

The important updates for me, comparing to the older Pis are:

  • Two displays can be connected! Either two HDMI or one DSI plus one HDMI.
  • It has a VideoCore VI GPU (very different to the IV from RPi3), which is driven by the MESA V3D driver.

My goal was to get a Yocto-built multi-display plasma-mobile environment on the device. Except two magic lines for the /boot/config.txt configuration that enabled multi-display output for me, it nearly worked out of the box.

RPi4 Multi-Display Setup

The important configuration step, compared to the default configuration as provided by meta-raspberry, are the following two lines that I had to add to the /boot/config.txt boot configuration:


Without these lines, the second screen always displayed just the Raspberry’s rainbow boot screen but it was never detected. I tested with both DSI+HDMI and HMDI+HDMI and both screens were always correctly detected at boot with these configuration.

Running Qt on Yocto-Build Image

Having the above configuration added, I was able to run a simple multi-screen QtWayland Compositor on the device. Note that I built Qt with

PACKAGECONFIG_append = " gbm kms eglfs"

and QtWayland with

PACKAGECONFIG_append_raspberrypi4 = " wayland-drm-egl-server-buffer"

With these options and having all requirements installed, the compositor runs via

export XDG_RUNTIME_DIR=/var/run/
export GALLIUM_HUD=fps # gives nice profiling information about fps
export QT_WAYLAND_CLIENT_BUFFER_INTEGRATION=linux-dmabuf-unstable-v1
qmlscene Compositor.qml -platform eglfs

It is important to note that qmlscene internally sets the Qt::AA_ShareOpenGLContexts attribute which you have to do yourself when running a compositor with your own main file.

Having this compositor running, I could run a simple spinning rectangle application via

export XDG_RUNTIME_DIR=/var/run/
qmlscene SpinningRectangle.qml -platform wayland-egl

Plasma Mobile

The final step though was to get our KDE Demo Setup running. Since there were no live conferences this year, some parts were slightly outdated. So, this was a good opportunity to update our meta layers:

  • meta-kf5 is now updated to KDE Frameworks 5.77.0. Note that we also cleaned up the license statements a few months ago, which was only possible due to much better license information via the SPDX/REUSE conversion of frameworks.
  • meta-kde also gained an update to the latest Plasma release and to the latest KDE Applications release. The number of provided applications — additional to Plasma — is still small, but I also used the opportunity to add some more KDE Edu applications (Blinken, Marble, Kanagram, KHangman, GCompris).

Final Result

Plasma mobile running with two screens \o/

PS: My whole test configuration if available by my (quick and dirty) umbrella test repository, in which I have all used meta layers integrated as submodules.