Ref count buffer usage

The QPlatformBackingStore can get flushed multiple times between paints.

Flush sets the front buffer, but it does not create a new backbuffer.
We can't do so without doing an expensive pre-emptive copy.

This means we send the same front buffer multiple times. This is
somewhat questionable with regards to the Wayland specification, but
seems to work.

If we do send a buffer multiple times we can't consider it free until
the last attached buffer is released; otherwise we end up painting into
a buffer whilst the server is still using it, leading to
flickering.

Change-Id: I8235eed6a85f0d52b37544e7bcb623b16a9dd832
Reviewed-by: Marco Martin <mart@kde.org>
Reviewed-by: Johan Helsing <johan.helsing@qt.io>

2 files changed, 4 insertions, 4 deletions

diff --git a/src/client/qwaylandbuffer.cpp b/src/client/qwaylandbuffer.cpp
index a0fcc532f..076a0d57d 100644
--- a/src/client/qwaylandbuffer.cpp
+++ b/src/client/qwaylandbuffer.cpp
@@ -66,7 +66,7 @@ void QWaylandBuffer::init(wl_buffer *buf)
 
 void QWaylandBuffer::release(void *data, wl_buffer *)
 {
-    static_cast<QWaylandBuffer *>(data)->mBusy = false;
+    static_cast<QWaylandBuffer *>(data)->mBusy--;
 }
 
 const wl_buffer_listener QWaylandBuffer::listener = {
diff --git a/src/client/qwaylandbuffer_p.h b/src/client/qwaylandbuffer_p.h
index 9e8cba2e4..b3513d151 100644
--- a/src/client/qwaylandbuffer_p.h
+++ b/src/client/qwaylandbuffer_p.h
@@ -73,14 +73,14 @@ public:
     virtual QSize size() const = 0;
     virtual int scale() const { return 1; }
 
-    void setBusy() { mBusy = true; }
-    bool busy() const { return mBusy; }
+    void setBusy() { mBusy++; }
+    bool busy() const { return mBusy > 0; }
 
 protected:
     struct wl_buffer *mBuffer;
 
 private:
-    bool mBusy;
+    int mBusy;
 
     static void release(void *data, wl_buffer *);
     static const wl_buffer_listener listener;