Add support for forced VSYNC using the EGLFS platform plugin.

Before a buffer swap the new QEglFSHooks::waitForVSync method is
called which looks at QT_QPA_EGLFS_FORCEVSYNC and - if that is set
and non-null - calls ioctl with the FBIO_WAITFORVSYNC request on
the framebuffer device.

This is required on some embedded platforms where the driver does not
support VSYNC yet the Kernel provides a generic implementation.

I tested this using QML_RENDER_TIMING=1 which proofs that the frame
rate for an example of mine drops from >125fps to a straight ~60fps
with a few frames that take ~33ms (i.e. 30fps) as expected for VSYNC.

To prevent excessive open/close calls on the frame buffer device
per frame, the file descriptor is now cached. To keep the QEglFSHooks
interface as clean as possible this is done via a global static in
qeglfshooks_stub.cpp and initialized and freed in platformInit and
platformDestroy.

Change-Id: I4d31b227c65ff22aa089db0fbc62c89a59cbb6c7
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>

src/plugins/platforms/eglfs/qeglfscontext.cpp

@@ -79,6 +79,7 @@ void QEglFSContext::swapBuffers(QPlatformSurface *surface)
             cursor->paintOnScreen();
     }
 
+    hooks->waitForVSync();
     QEGLPlatformContext::swapBuffers(surface);
 }
 

src/plugins/platforms/eglfs/qeglfshooks.h

@@ -69,6 +69,7 @@ public:
     virtual bool hasCapability(QPlatformIntegration::Capability cap) const;
     virtual QEglFSCursor *createCursor(QEglFSScreen *screen) const;
     virtual bool filterConfig(EGLDisplay display, EGLConfig config) const;
+    virtual void waitForVSync() const;
 
     virtual const char *fbDeviceName() const;
 };

src/plugins/platforms/eglfs/qeglfshooks_stub.cpp

@@ -47,9 +47,14 @@
 #include <sys/ioctl.h>
 
 #include <private/qmath_p.h>
+#include <private/qcore_unix_p.h>
 
 QT_BEGIN_NAMESPACE
 
+// file descriptor for the frame buffer
+// this is a global static to keep the QEglFSHooks interface as clean as possible
+static int framebuffer = -1;
+
 const char *QEglFSHooks::fbDeviceName() const
 {
     return "/dev/fb0";
@@ -58,10 +63,17 @@ const char *QEglFSHooks::fbDeviceName() const
 void QEglFSHooks::platformInit()
 {
     Q_UNUSED(hooks);
+
+    framebuffer = qt_safe_open(fbDeviceName(), O_RDONLY);
+
+    if (framebuffer == -1)
+        qWarning("EGLFS: Failed to open %s", fbDeviceName());
 }
 
 void QEglFSHooks::platformDestroy()
 {
+    if (framebuffer != -1)
+        close(framebuffer);
 }
 
 EGLNativeDisplayType QEglFSHooks::platformDisplay() const
@@ -86,22 +98,16 @@ QSizeF QEglFSHooks::physicalScreenSize() const
         }
 
         struct fb_var_screeninfo vinfo;
-        int fd = open(fbDeviceName(), O_RDONLY);
-
         int w = -1;
         int h = -1;
 
-        if (fd != -1) {
+        if (framebuffer != -1) {
-            if (ioctl(fd, FBIOGET_VSCREENINFO, &vinfo) == -1) {
+            if (ioctl(framebuffer, FBIOGET_VSCREENINFO, &vinfo) == -1) {
                 qWarning("EGLFS: Could not query variable screen info.");
             } else {
                 w = vinfo.width;
                 h = vinfo.height;
             }
-
-            close(fd);
-        } else {
-            qWarning("EGLFS: Failed to open %s to detect physical screen size.", fbDeviceName());
         }
 
         const int defaultPhysicalDpi = 100;
@@ -140,22 +146,17 @@ QSize QEglFSHooks::screenSize() const
         }
 
         struct fb_var_screeninfo vinfo;
-        int fd = open(fbDeviceName(), O_RDONLY);
 
         int xres = -1;
         int yres = -1;
 
-        if (fd != -1) {
+        if (framebuffer != -1) {
-            if (ioctl(fd, FBIOGET_VSCREENINFO, &vinfo) == -1) {
+            if (ioctl(framebuffer, FBIOGET_VSCREENINFO, &vinfo) == -1) {
                 qWarning("EGLFS: Could not query variable screen info.");
             } else {
                 xres = vinfo.xres;
                 yres = vinfo.yres;
             }
-
-            close(fd);
-        } else {
-            qWarning("EGLFS: Failed to open %s to detect screen resolution.", fbDeviceName());
         }
 
         const int defaultWidth = 800;
@@ -185,17 +186,12 @@ int QEglFSHooks::screenDepth() const
 
     if (depth == 0) {
         struct fb_var_screeninfo vinfo;
-        int fd = open(fbDeviceName(), O_RDONLY);
 
-        if (fd != -1) {
+        if (framebuffer != -1) {
-            if (ioctl(fd, FBIOGET_VSCREENINFO, &vinfo) == -1)
+            if (ioctl(framebuffer, FBIOGET_VSCREENINFO, &vinfo) == -1)
                 qWarning("EGLFS: Could not query variable screen info.");
             else
                 depth = vinfo.bits_per_pixel;
-
-            close(fd);
-        } else {
-            qWarning("EGLFS: Failed to open %s to detect screen depth.", fbDeviceName());
         }
 
         const int defaultDepth = 32;
@@ -250,6 +246,16 @@ QEglFSCursor *QEglFSHooks::createCursor(QEglFSScreen *screen) const
     return 0;
 }
 
+void QEglFSHooks::waitForVSync() const
+{
+    static const bool forceSync = qgetenv("QT_QPA_EGLFS_FORCEVSYNC").toInt();
+    if (forceSync && framebuffer != -1) {
+        int arg = 0;
+        if (ioctl(framebuffer, FBIO_WAITFORVSYNC, &arg) == -1)
+            qWarning("Could not wait for vsync.");
+    }
+}
+
 #ifndef EGLFS_PLATFORM_HOOKS
 QEglFSHooks stubHooks;
 #endif