From 22f5665de79513c41fbef8f1b918f44d184daf1f Mon Sep 17 00:00:00 2001
From: Jeffy Chen <jeffy.chen@rock-chips.com>
Date: Tue, 16 Apr 2019 18:46:49 +0800
Subject: [PATCH 17/31] linuxfb: Use triple buffer by default

Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>
---
 .../platforms/linuxfb/qlinuxfbdrmscreen.cpp   | 56 +++++++++++++++----
 1 file changed, 44 insertions(+), 12 deletions(-)

diff --git a/src/plugins/platforms/linuxfb/qlinuxfbdrmscreen.cpp b/src/plugins/platforms/linuxfb/qlinuxfbdrmscreen.cpp
index dcc1ef27..29d146f4 100644
--- a/src/plugins/platforms/linuxfb/qlinuxfbdrmscreen.cpp
+++ b/src/plugins/platforms/linuxfb/qlinuxfbdrmscreen.cpp
@@ -59,7 +59,13 @@ QT_BEGIN_NAMESPACE
 
 Q_LOGGING_CATEGORY(qLcFbDrm, "qt.qpa.fb")
 
+#define TRIPLE_BUFFER
+
+#ifdef TRIPLE_BUFFER
+static const int BUFFER_COUNT = 3;
+#else
 static const int BUFFER_COUNT = 2;
+#endif
 
 class QLinuxFbDevice : public QKmsDevice
 {
@@ -75,12 +81,12 @@ public:
     };
 
     struct Output {
-        Output() : backFb(0), flipped(false) { }
+        Output() : backFb(0), flipPending(false) { }
         QKmsOutput kmsOutput;
         Framebuffer fb[BUFFER_COUNT];
         QRegion dirty[BUFFER_COUNT];
         int backFb;
-        bool flipped;
+        bool flipPending;
         QSize currentRes() const {
             const drmModeModeInfo &modeInfo(kmsOutput.modes[kmsOutput.mode]);
             return QSize(modeInfo.hdisplay, modeInfo.vdisplay);
@@ -97,6 +103,7 @@ public:
     void setMode();
 
     void swapBuffers(Output *output);
+    void waitForFlip(Output *output);
 
     int outputCount() const { return m_outputs.count(); }
     Output *output(int idx) { return &m_outputs[idx]; }
@@ -301,7 +308,7 @@ void QLinuxFbDevice::createFramebuffers()
                 return;
         }
         output.backFb = 0;
-        output.flipped = false;
+        output.flipPending = false;
     }
 }
 
@@ -355,19 +362,18 @@ void QLinuxFbDevice::pageFlipHandler(int fd, unsigned int sequence,
     Q_UNUSED(tv_usec);
 
     Output *output = static_cast<Output *>(user_data);
+
+#ifndef TRIPLE_BUFFER
+    // The next buffer would be available after flipped
     output->backFb = (output->backFb + 1) % BUFFER_COUNT;
+#endif
+
+    output->flipPending = false;
 }
 
-void QLinuxFbDevice::swapBuffers(Output *output)
+void QLinuxFbDevice::waitForFlip(Output *output)
 {
-    Framebuffer &fb(output->fb[output->backFb]);
-    if (drmModePageFlip(fd(), output->kmsOutput.crtc_id, fb.fb, DRM_MODE_PAGE_FLIP_EVENT, output) == -1) {
-        qErrnoWarning(errno, "Page flip failed");
-        return;
-    }
-
-    const int fbIdx = output->backFb;
-    while (output->backFb == fbIdx) {
+    while (output->flipPending) {
         drmEventContext drmEvent;
         memset(&drmEvent, 0, sizeof(drmEvent));
         drmEvent.version = 2;
@@ -379,6 +385,27 @@ void QLinuxFbDevice::swapBuffers(Output *output)
     }
 }
 
+void QLinuxFbDevice::swapBuffers(Output *output)
+{
+#ifdef TRIPLE_BUFFER
+    // Wait flip to make sure last buffer displayed
+    waitForFlip(output);
+#endif
+
+    Framebuffer &fb(output->fb[output->backFb]);
+    if (drmModePageFlip(fd(), output->kmsOutput.crtc_id, fb.fb, DRM_MODE_PAGE_FLIP_EVENT, output) == -1) {
+        qErrnoWarning(errno, "Page flip failed");
+        return;
+    }
+
+    output->flipPending = true;
+
+#ifdef TRIPLE_BUFFER
+    // The next buffer should always available in triple buffer case.
+    output->backFb = (output->backFb + 1) % BUFFER_COUNT;
+#endif
+}
+
 QLinuxFbDrmScreen::QLinuxFbDrmScreen(const QStringList &args)
     : m_screenConfig(nullptr),
       m_device(nullptr)
@@ -436,6 +463,11 @@ QRegion QLinuxFbDrmScreen::doRedraw()
     for (int i = 0; i < BUFFER_COUNT; ++i)
         output->dirty[i] += dirty;
 
+#ifndef TRIPLE_BUFFER
+    // Wait flip before accessing new buffer
+    m_device->waitForFlip(output);
+#endif
+
     if (output->fb[output->backFb].wrapper.isNull())
         return dirty;
 
-- 
2.20.1