macOS: Optionally flush sub-layers via sub-image copies of the backingstore

When we're flushing the backingstore to sub-views with their own layers
we don't want to pay the cost of uploading the whole backingstore to the
GPU in the case where we're dealing with a discrete GPU.

To work around this we make a copy of the appropriate part of the surfcace.

This results in additional copies of the data, and will need further
investigation to limit these.

Task-number: QTBUG-77447
Change-Id: I318ae80e433dd7b0a55fd5a598b19f114d8bd28e
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>
(cherry picked from commit eaf4911db29a82b3d94826a8ff09afe075a2b636)
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

1 files changed, 19 insertions, 10 deletions

diff --git a/src/plugins/platforms/cocoa/qcocoabackingstore.mm b/src/plugins/platforms/cocoa/qcocoabackingstore.mm
index cb453f7613..2343f16b9f 100644
--- a/src/plugins/platforms/cocoa/qcocoabackingstore.mm
+++ b/src/plugins/platforms/cocoa/qcocoabackingstore.mm
@@ -572,17 +572,26 @@ void QCALayerBackingStore::flush(QWindow *flushedWindow, const QRegion &region,
         flushedView.layer.contents = nil;
     }
 
-    qCInfo(lcQpaBackingStore) << "Flushing" << backBufferSurface
-         << "to" << flushedView.layer << "of" << flushedView;
-
-    flushedView.layer.contents = backBufferSurface;
+    if (flushedView == backingStoreView) {
+        qCInfo(lcQpaBackingStore) << "Flushing" << backBufferSurface
+            << "to" << flushedView.layer << "of" << flushedView;
+        flushedView.layer.contents = backBufferSurface;
+    } else {
+        auto subviewRect = [flushedView convertRect:flushedView.bounds toView:backingStoreView];
+        auto scale = flushedView.layer.contentsScale;
+        subviewRect = CGRectApplyAffineTransform(subviewRect, CGAffineTransformMakeScale(scale, scale));
+
+        // We make a copy of the image data up front, which means we don't
+        // need to mark the IOSurface as being in use. FIXME: Investigate
+        // if there's a cheaper way to get sub-image data to a layer.
+        m_buffers.back()->lock(QPlatformGraphicsBuffer::SWReadAccess);
+        QImage subImage = m_buffers.back()->asImage()->copy(QRectF::fromCGRect(subviewRect).toRect());
+        m_buffers.back()->unlock();
 
-    if (flushedView != backingStoreView) {
-        const CGSize backingStoreSize = backingStoreView.bounds.size;
-        flushedView.layer.contentsRect = CGRectApplyAffineTransform(
-            [flushedView convertRect:flushedView.bounds toView:backingStoreView],
-            // The contentsRect is in unit coordinate system
-            CGAffineTransformMakeScale(1.0 / backingStoreSize.width, 1.0 / backingStoreSize.height));
+        qCInfo(lcQpaBackingStore) << "Flushing" << subImage
+            << "to" << flushedView.layer << "of subview" << flushedView;
+        QCFType<CGImageRef> cgImage = subImage.toCGImage();
+        flushedView.layer.contents = (__bridge id)static_cast<CGImageRef>(cgImage);
     }
 
     // Since we may receive multiple flushes before a new frame is started, we do not