eglfs_kms: Add headless mode for DRM render nodes

Attempting to switch /dev/dri/cardX to /dev/dri/renderDY is futile
on its own now since many output-related drm operations fail and we
eventually crash.

Add a new headless mode that skips the real screen stuff and registers
a fairly dummy, headless screen, does not bother with the mouse cursor,
and disallows rendering to the screen via a QWindow (while keeping the
actual rendering still fully functional).

Such applications will not need any special privileges and will run even
if there is a DRM master (X11, Wayland compositor) active.

For example the configuration can look like this:

{
    "device": "/dev/dri/renderD128",
    "headless": "1024x768"
}

After this applications have two choices to perform offscreen
rendering:

1. Use an ordinary window (and its default framebuffer, meaning the
gbm_surface), e.g. a QOpenGLWindow subclass

  MyOpenGLWindow w;
  w.show(); // will not actually show on screen
  w.grabFramebuffer().save("output.png");

Note that there is no vsync-based throttling. Also note that windows are
still sized to match the screen size, hence the need for specifying a size
in the headless property.

2. Or the typical offscreen approach with an extra FBO

  QOffscreenSurface s;
  s.setFormat(ctx.format());
  s.create();
  ctx.makeCurrent(&s0;
  QOpenGLFramebufferObject fbo(1024, 768);
  fbo.bind();
  ctx.functions()->glClearColor(1, 0, 0, 1);
  ctx.functions()->glClear(GL_COLOR_BUFFER_BIT);
  fbo.toImage().save("output.png");
  ctx.doneCurrent();

Task-number: QTBUG-62262
Change-Id: Ic1dbfa2b27b223bd5ef8ba36b665f0f61abf4f06
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Andy Nichols <andy.nichols@qt.io>

1 files changed, 3 insertions, 1 deletions

diff --git a/src/plugins/platforms/eglfs/deviceintegration/eglfs_kms_support/qeglfskmsscreen.h b/src/plugins/platforms/eglfs/deviceintegration/eglfs_kms_support/qeglfskmsscreen.h
index a2e3402bf2..aacea90eae 100644
--- a/src/plugins/platforms/eglfs/deviceintegration/eglfs_kms_support/qeglfskmsscreen.h
+++ b/src/plugins/platforms/eglfs/deviceintegration/eglfs_kms_support/qeglfskmsscreen.h
@@ -56,7 +56,7 @@ class QEglFSKmsInterruptHandler;
 class Q_EGLFS_EXPORT QEglFSKmsScreen : public QEglFSScreen
 {
 public:
-    QEglFSKmsScreen(QKmsDevice *device, const QKmsOutput &output);
+    QEglFSKmsScreen(QKmsDevice *device, const QKmsOutput &output, bool headless = false);
     ~QEglFSKmsScreen();
 
     void setVirtualPosition(const QPoint &pos);
@@ -113,6 +113,8 @@ protected:
     PowerState m_powerState;
 
     QEglFSKmsInterruptHandler *m_interruptHandler;
+
+    bool m_headless;
 };
 
 QT_END_NAMESPACE