Avoid crash with EGLFS

[daschuer]

With EGLFS we can use either QWidgets or OpenGL, not both at the same time.
This is a band aid that shall avoid the fatal abort reported in #15863.

A better fix would be to use only GELS.
This type of refactoring is probably not beneficial because we are already working in a QML GUI which likely works with EGLFS. We need to investigate with this.

[JoergAtGithub]

As we've two implementations for the waveforms (OpenGL and QQuick) would it help to switch from QOpenGLWindow to doc.qt.io/qt-6/qquickwidget.html using the QQuick waveform implementation we use for QML already?
This would also solve the problem that Apple deprecated OpenGL at all and support might be removed with an upcoming macOS release.

[daschuer]

I don't think so. EGLFS is used with a frame buffer device, where no windows manager is available. See https://doc.qt.io/qt-6/embedded-linux.html.
We either use QWidget, where Qt handles the off-screen rendering and screen updates or render everything with GLES. Since we currently use both only QWidget waveforms are available.

I can't test it though. Hope @mirko can confirm.

[mirko]

fb(dev) is legacy from kernel side by now and AFAIK is - if still used - routed via DRM/KMS.
I'm going the direct way via DRM/KMS. I only ever dealt with OpenGLES2 and Qt, so unfortunately I don't know about the OpenGL stuff. But as far as I remember, for Qt5 you could compile qt5gui either with OpenGL or OpenGLES support (see e.g. https://packages.debian.org/trixie/libqt5gui5-gles), whereas with Qt6 that's interchangeable at runtime. Probably not answering any of your questions, though :)

[JoergAtGithub]

See: https://doc.qt.io/qt-6/embedded-linux.html

[mirko]

The environment I use for reference:

export QT_QPA_PLATFORM=eglfs
export QT_QPA_EGLFS_INTEGRATION=eglfs_kms
export QT_QPA_EGLFS_KMS_CONFIG=/etc/qt_eglfs_kms.json
mixxx --qml --developer

whereas /etc/qt_eglfs_kms.json contains:

{
  "hwcursor": false,
  "outputs": [
    {
      "name": "HDMI1",
      "mode": "1920x1080"
    }
  ]
}