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, 18 insertions, 13 deletions

diff --git a/src/platformsupport/kmsconvenience/qkmsdevice_p.h b/src/platformsupport/kmsconvenience/qkmsdevice_p.h
index 390cc52e4d..bbc03b8e2a 100644
--- a/src/platformsupport/kmsconvenience/qkmsdevice_p.h
+++ b/src/platformsupport/kmsconvenience/qkmsdevice_p.h
@@ -77,6 +77,8 @@ public:
 
     QString devicePath() const { return m_devicePath; }
 
+    bool headless() const { return m_headless; }
+    QSize headlessSize() const { return m_headlessSize; }
     bool hwCursor() const { return m_hwCursor; }
     bool separateScreens() const { return m_separateScreens; }
     bool supportsPBuffers() const { return m_pbuffers; }
@@ -88,6 +90,8 @@ private:
     void loadConfig();
 
     QString m_devicePath;
+    bool m_headless;
+    QSize m_headlessSize;
     bool m_hwCursor;
     bool m_separateScreens;
     bool m_pbuffers;
@@ -98,21 +102,21 @@ private:
 struct QKmsOutput
 {
     QString name;
-    uint32_t connector_id;
-    uint32_t crtc_id;
+    uint32_t connector_id = 0;
+    uint32_t crtc_id = 0;
     QSizeF physical_size;
-    int preferred_mode; // index of preferred mode in list below
-    int mode; // index of selected mode in list below
-    bool mode_set;
-    drmModeCrtcPtr saved_crtc;
+    int preferred_mode = -1; // index of preferred mode in list below
+    int mode = -1; // index of selected mode in list below
+    bool mode_set = false;
+    drmModeCrtcPtr saved_crtc = nullptr;
     QList<drmModeModeInfo> modes;
-    int subpixel;
-    drmModePropertyPtr dpms_prop;
-    drmModePropertyBlobPtr edid_blob;
-    bool wants_plane;
-    uint32_t plane_id;
-    bool plane_set;
-    uint32_t drm_format;
+    int subpixel = DRM_MODE_SUBPIXEL_UNKNOWN;
+    drmModePropertyPtr dpms_prop = nullptr;
+    drmModePropertyBlobPtr edid_blob = nullptr;
+    bool wants_plane = false;
+    uint32_t plane_id = 0;
+    bool plane_set = false;
+    uint32_t drm_format = DRM_FORMAT_XRGB8888;
     QString clone_source;
 
     void restoreMode(QKmsDevice *device);
@@ -147,6 +151,7 @@ public:
 
 protected:
     virtual QPlatformScreen *createScreen(const QKmsOutput &output) = 0;
+    virtual QPlatformScreen *createHeadlessScreen();
     virtual void registerScreenCloning(QPlatformScreen *screen,
                                        QPlatformScreen *screenThisScreenClones,
                                        const QVector<QPlatformScreen *> &screensCloningThisScreen);