From 86cdd53a6d69070ff26edb6d10e8f2097a430a59 Mon Sep 17 00:00:00 2001
From: Philip Schuchardt <vpicaver@gmail.com>
Date: Sat, 25 Nov 2023 21:44:19 -0600
Subject: [PATCH] Synchronize read buffers on non-Apple Silicon in RHI

Read buffers are not automatically synchronized when the
buffer uses MTLResourceStorageModeManaged. This commit adds
GPU to CPU synchronization for managed buffers, ensuring
proper data transfer for reading operations on non-Apple
Silicon devices.

Fixes: QTBUG-119447
Pick-to: 6.6
Change-Id: I6398f4f27d00950c7dceae918b04e0054d4b5647
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
(cherry picked from commit f21af7f88b2184a420293967d07bf6552541ddc9)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
---
 src/gui/rhi/qrhimetal.mm | 25 ++++++++++++++++---------
 1 file changed, 16 insertions(+), 9 deletions(-)

diff --git a/src/gui/rhi/qrhimetal.mm b/src/gui/rhi/qrhimetal.mm
index 56bb4815b93..e02ab9a2780 100644
--- a/src/gui/rhi/qrhimetal.mm
+++ b/src/gui/rhi/qrhimetal.mm
@@ -2682,6 +2682,15 @@ void QRhiMetal::enqueueResourceUpdates(QRhiCommandBuffer *cb, QRhiResourceUpdate
     QMetalCommandBuffer *cbD = QRHI_RES(QMetalCommandBuffer, cb);
     QRhiResourceUpdateBatchPrivate *ud = QRhiResourceUpdateBatchPrivate::get(resourceUpdates);
 
+    id<MTLBlitCommandEncoder> blitEnc = nil;
+    auto ensureBlit = [&blitEnc, cbD, this]() {
+        if (!blitEnc) {
+            blitEnc = [cbD->d->cb blitCommandEncoder];
+            if (debugMarkers)
+                [blitEnc pushDebugGroup: @"Texture upload/copy"];
+        }
+    };
+
     for (int opIdx = 0; opIdx < ud->activeBufferOpCount; ++opIdx) {
         const QRhiResourceUpdateBatchPrivate::BufferOp &u(ud->bufferOps[opIdx]);
         if (u.type == QRhiResourceUpdateBatchPrivate::BufferOp::DynamicUpdate) {
@@ -2720,19 +2729,17 @@ void QRhiMetal::enqueueResourceUpdates(QRhiCommandBuffer *cb, QRhiResourceUpdate
                 readback.readSize = u.readSize;
                 readback.result = u.result;
                 d->activeBufferReadbacks.append(readback);
+#ifdef Q_OS_MACOS
+                if (bufD->d->managed) {
+                    // On non-Apple Silicon, manually synchronize memory from GPU to CPU
+                    ensureBlit();
+                    [blitEnc synchronizeResource:readback.buf];
+                }
+#endif
             }
         }
     }
 
-    id<MTLBlitCommandEncoder> blitEnc = nil;
-    auto ensureBlit = [&blitEnc, cbD, this] {
-        if (!blitEnc) {
-            blitEnc = [cbD->d->cb blitCommandEncoder];
-            if (debugMarkers)
-                [blitEnc pushDebugGroup: @"Texture upload/copy"];
-        }
-    };
-
     for (int opIdx = 0; opIdx < ud->activeTextureOpCount; ++opIdx) {
         const QRhiResourceUpdateBatchPrivate::TextureOp &u(ud->textureOps[opIdx]);
         if (u.type == QRhiResourceUpdateBatchPrivate::TextureOp::Upload) {