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, 9 insertions, 2 deletions

diff --git a/src/plugins/platforms/eglfs/deviceintegration/eglfs_kms_support/qeglfskmsscreen.cpp b/src/plugins/platforms/eglfs/deviceintegration/eglfs_kms_support/qeglfskmsscreen.cpp
index 533695ea0f..cc70ab5a3c 100644
--- a/src/plugins/platforms/eglfs/deviceintegration/eglfs_kms_support/qeglfskmsscreen.cpp
+++ b/src/plugins/platforms/eglfs/deviceintegration/eglfs_kms_support/qeglfskmsscreen.cpp
@@ -68,12 +68,13 @@ private:
     QEglFSKmsScreen *m_screen;
 };
 
-QEglFSKmsScreen::QEglFSKmsScreen(QKmsDevice *device, const QKmsOutput &output)
+QEglFSKmsScreen::QEglFSKmsScreen(QKmsDevice *device, const QKmsOutput &output, bool headless)
     : QEglFSScreen(static_cast<QEglFSIntegration *>(QGuiApplicationPrivate::platformIntegration())->display())
     , m_device(device)
     , m_output(output)
     , m_powerState(PowerStateOn)
     , m_interruptHandler(new QEglFSKmsInterruptHandler(this))
+    , m_headless(headless)
 {
     m_siblings << this; // gets overridden later
 
@@ -109,6 +110,9 @@ void QEglFSKmsScreen::setVirtualPosition(const QPoint &pos)
 // geometry() calls rawGeometry() and may apply additional transforms.
 QRect QEglFSKmsScreen::rawGeometry() const
 {
+    if (m_headless)
+        return QRect(QPoint(0, 0), m_device->screenConfig()->headlessSize());
+
     const int mode = m_output.mode;
     return QRect(m_pos.x(), m_pos.y(),
                  m_output.modes[mode].hdisplay,
@@ -177,7 +181,7 @@ Qt::ScreenOrientation QEglFSKmsScreen::orientation() const
 
 QString QEglFSKmsScreen::name() const
 {
-    return m_output.name;
+    return !m_headless ? m_output.name : QStringLiteral("qt_Headless");
 }
 
 QString QEglFSKmsScreen::manufacturer() const
@@ -214,6 +218,9 @@ void QEglFSKmsScreen::restoreMode()
 
 qreal QEglFSKmsScreen::refreshRate() const
 {
+    if (m_headless)
+        return 60;
+
     quint32 refresh = m_output.modes[m_output.mode].vrefresh;
     return refresh > 0 ? refresh : 60;
 }