Documentation Index
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Getting a Qt application to render on the Denon Prime’s embedded display is more involved than setting QT_QPA_PLATFORM=eglfs. The hardware carries a Mali-T76x GPU at a specific DDK revision that conflicts with the default Buildroot driver, the display controller applies a portrait-to-landscape rotation that the Qt coordinate system must match, and the device’s own Qt installation uses an EGLFS backend that cannot take over the framebuffer. All three issues must be resolved together before MIXXX produces any visible output.
Hardware Overview
| Component | Detail |
|---|
| GPU | Mali-T76x (4 cores, r1p0 0x0750) on Rockchip RK3288 |
| Display connector | DSI-1 (Prime Go) / LVDS-1 (Prime 4) |
| Native framebuffer | 800×1280 portrait |
| Qt logical screen | 1280×800 landscape (display controller rotates the framebuffer) |
| Touchscreen | ILI2117 on /dev/input/event0 |
| KMS/DRM device | /dev/dri/card0 |
DDK Mismatch: The Critical Problem
The stock firmware’s Mali GPU is hardware revision r1p0. Buildroot’s default Mali package ships DDK r0p0. The user-space libmali.so must match the kernel-side ABI exactly — a version mismatch causes EGL initialisation to fail at startup, producing a black screen or an immediate crash.
The fix is to discard the Buildroot-compiled EGL/GLES stubs and symlink them directly to the device’s native driver:
# Symlink bundled EGL/GLES to device's native Mali r1p0 driver
cd /media/az01-internal/mixxx/lib
rm -f libEGL.so libGLESv2.so libGLESv1_CM.so
ln -sf /usr/lib/libmali.so.14.0 libEGL.so
ln -sf /usr/lib/libmali.so.14.0 libGLESv2.so
ln -sf /usr/lib/libmali.so.14.0 libGLESv1_CM.so
libmali.so.14.0 is libmali-r1p0.so selected at boot by /sbin/az01-libmali-setup, which reads the GPU hardware revision from /sys/devices/platform/ffa30000.gpu/gpuinfo and bind-mounts the correct variant. All three EGL/GLES entry-point libraries in the SD card bundle should be symlinks pointing to this path.
Why SD Card Qt 5.15.8 (Not Device Qt 5.15.2)
The device ships Qt 5.15.2 at /usr/qt/lib. It uses the eglfs_emu platform integration, which is designed for emulated environments and cannot take over the display from fbcon. When used for MIXXX, it produces a permanent black screen.
The SD card carries Qt 5.15.8 compiled from the same Buildroot environment as MIXXX, with a custom-built libqeglfs-mali-integration.so (14 KB) placed at:
qt-plugins/egldeviceintegrations/libqeglfs-mali-integration.so
eglfs_mali integration, which correctly initialises the Mali GPU, acquires the DRM plane, and takes over framebuffer rendering. It is the single component that makes display output possible.
Device Qt 5.15.2 is still included in LD_LIBRARY_PATH as a fallback (listed after the SD bundle, before system libs) for any Qt component not bundled on the card.
Required Environment Variables
All display-related variables are set by mixxx_launcher.sh before MIXXX is exec’d:
| Variable | Value | Purpose |
|---|
LD_LIBRARY_PATH | $BUNDLE/lib:/usr/qt/lib:/usr/lib | SD Qt 5.15.8 first; device Qt 5.15.2 fallback; system libs last |
QT_PLUGIN_PATH | $BUNDLE/qt-plugins | Points to the SD card’s plugin directory containing eglfs_mali |
QT_QPA_PLATFORM | eglfs | Full-screen EGLFS compositor — no X11 or Wayland on this device |
QT_QPA_EGLFS_INTEGRATION | eglfs_mali | Selects the custom Mali integration plugin |
QT_QPA_EGLFS_ROTATION | 90 (Prime Go) / 270 (Prime 4) | Rotates Qt’s logical screen to match the display controller |
QT_QPA_FONTDIR | /usr/share/fonts | System fonts from device rootfs (not bundled on SD card) |
QT_QPA_GENERIC_PLUGINS | evdevtouch:/dev/input/event0,evdevkeyboard:/dev/input/event1 | Touchscreen and keyboard input |
QT_QPA_EVDEV_TOUCHSCREEN_PARAMETERS | /dev/input/event0:rotate=90 | Maps ILI2117 physical coordinates to Qt’s logical 1280×800 screen |
HOME | /tmp | MIXXX config/cache in tmpfs; not /root |
QT_QPA_EGLFS_PHYSICAL_WIDTH | 155 | Physical screen width in mm (Prime Go 5″ display) |
QT_QPA_EGLFS_PHYSICAL_HEIGHT | 98 | Physical screen height in mm |
Touchscreen Mapping
The ILI2117 sensor reports coordinates in the physical landscape orientation of the device (the way you hold it). Qt’s evdevtouch plugin must rotate those coordinates to match Qt’s logical 1280×800 screen. The rotate=90 parameter in QT_QPA_EVDEV_TOUCHSCREEN_PARAMETERS performs this mapping.
The display rotation and the touchscreen rotation must match. A mismatch causes touch events to land at incorrect positions relative to what is visible on screen (rotated, mirrored, or inverted).
KMS/DRM
The GPU drives the built-in display via /dev/dri/card0. The DRM connector is reported as “connected” and the Mali integration plugin outputs through the hardware overlay plane. fbcon may still hold DRM plane-4 at boot, but the Mali EGLFS integration acquires a separate overlay plane and renders there, so fbcon’s plane does not interfere with MIXXX’s output.
The TKGL bootstrap module sets the GPU to performance mode before MIXXX launches, matching what Engine OS does at startup:
echo performance > /sys/devices/platform/ffa30000.gpu/devfreq/ffa30000.gpu/governor
cat /sys/devices/platform/ffa30000.gpu/gpuinfo
ondemand governor causes visible frame rate drops during waveform scrolling because the GPU clock ramps up too slowly after idle periods.
Prime 4 Differences
The Prime 4 uses an LVDS-1 display connector rather than the DSI-1 connector on the Prime Go. Both devices share the same Mali-T76x r1p0 GPU, Rockchip RK3288 SoC, and 800×1280 portrait framebuffer, but the LVDS-1 controller applies rotation in the opposite direction and introduces a vsync issue.
Display Hardware
| Property | Prime Go | Prime 4 |
|---|
| Connector | DSI-1 | LVDS-1 |
| DRM sysfs path | card0-DSI-1 | card0-LVDS-1 |
| Framebuffer driver | rockchipdrmfb | rockchipdrmfb |
| Native resolution | 800×1280 portrait | 800×1280 portrait |
| Screen tearing | Not observed | Present without vsync |
Rotation
| Setting | Prime Go (DSI-1) | Prime 4 (LVDS-1) |
|---|
QT_QPA_EGLFS_ROTATION | 90 | 270 |
QT_QPA_EVDEV_TOUCHSCREEN_PARAMETERS rotate | 90 | 270 |
Screen Tearing Fix (Prime 4)
The LVDS-1 DRM path handles page-flip timing differently from DSI-1 and exhibits visible tearing by default. Enable vsync:
export QT_QPA_EGLFS_SWAPINTERVAL=1
Device Auto-Detection
The launcher detects the device model by checking the DRM connector status:
# Prime Go = DSI-1 (rotation 90, no vsync needed)
# Prime 4 = LVDS-1 (rotation 270, needs SWAPINTERVAL=1)
if [ -f /sys/class/drm/card0-DSI-1/status ] && \
grep -q connected /sys/class/drm/card0-DSI-1/status 2>/dev/null; then
DEVICE="primego"
ROTATION=90
SWAPINTERVAL=0
else
DEVICE="prime4"
ROTATION=270
SWAPINTERVAL=1
fi
Warnings and Known Issues
Do not set QT_QPA_EGLFS_KMS_ATOMIC=1. This flag enables the DRM atomic modesetting path, which conflicts with the Mali integration plugin and breaks display output. It is not needed for Mali-T76x and must remain unset.
Only lib/bin/mixxx (the ~10 MB stripped binary) works with EGLFS. The mixxx.real binary (~17 MB, unstripped) crashes immediately with EGLFS: OpenGL windows cannot be mixed with others. The bin/mixxx symlink must point to ../lib/bin/mixxx — do not change it to point to mixxx.real.
nodialog.so is present at lib/nodialog.so in the SD card bundle and can be used as an LD_PRELOAD to suppress Qt dialog boxes in other launcher configurations. It is not used in the default TKGL bootstrap launcher path. Do not add it to LD_PRELOAD unless you are troubleshooting a secondary launcher.
Startup Display Stall
MIXXX takes approximately 5 seconds after the process starts before anything appears on screen. This is not a crash or a hang — it is the Mali GPU initialising the EGLFS context and painting all Qt widgets to the framebuffer for the first time. The 60% CPU usage visible during this period is normal Mali rendering activity. If the screen remains black after 15 seconds, that indicates an EGL initialisation failure, most likely from a DDK mismatch or a missing eglfs_mali plugin.