rhi: Add support for buffer readbacks

This also marks the beginnings of significantly extending autotesting
of the resource and rendering functionality in QRhi.

Also involves fixing up the buffer operation lists like we did
for textures before. This is to ensure updates and reads on the
same batch execute in the correct order. So just have two lists:
one with buffer, one with texture operations.

Also simplify the struct layouts. No need for those inner structs
with many duplicate members. This reduces the size even, since using a
union was never an option here. Also switch to a VLA, the size is around
253 KB per batch.

The Null backend now keeps track of the QRhiBuffer data so it can return
valid results in readbacks.

Task-number: QTBUG-78984
Task-number: QTBUG-78986
Task-number: QTBUG-78971
Task-number: QTBUG-78883
Change-Id: I9694bd7fec523a4e71cf8a5c77c828123ebbb3bd
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>

1 files changed, 43 insertions, 12 deletions

diff --git a/src/gui/rhi/qrhi.cpp b/src/gui/rhi/qrhi.cpp
index c8ec8c7410..8ec8ae9208 100644
--- a/src/gui/rhi/qrhi.cpp
+++ b/src/gui/rhi/qrhi.cpp
@@ -566,6 +566,13 @@ Q_LOGGING_CATEGORY(QRHI_LOG_INFO, "qt.rhi.general")
 
     \value TriangleFanTopology Indicates that QRhiGraphicsPipeline::setTopology()
     supports QRhiGraphicsPipeline::TriangleFan.
+
+    \value ReadBackNonUniformBuffer Indicates that
+    {QRhiResourceUpdateBatch::readBackBuffer()}{reading buffer contents} is
+    supported for QRhiBuffer instances with a usage different than
+    UniformBuffer. While this is supported in the majority of cases, it can be
+    unsupported, for example, on OpenGL ES 2.0 implementations without the
+    MapBuffer extension.
 */
 
 /*!
@@ -4175,7 +4182,7 @@ void QRhiResourceUpdateBatch::merge(QRhiResourceUpdateBatch *other)
 void QRhiResourceUpdateBatch::updateDynamicBuffer(QRhiBuffer *buf, int offset, int size, const void *data)
 {
     if (size > 0)
-        d->dynamicBufferUpdates.append({ buf, offset, size, data });
+        d->bufferOps.append(QRhiResourceUpdateBatchPrivate::BufferOp::dynamicUpdate(buf, offset, size, data));
 }
 
 /*!
@@ -4189,7 +4196,7 @@ void QRhiResourceUpdateBatch::updateDynamicBuffer(QRhiBuffer *buf, int offset, i
 void QRhiResourceUpdateBatch::uploadStaticBuffer(QRhiBuffer *buf, int offset, int size, const void *data)
 {
     if (size > 0)
-        d->staticBufferUploads.append({ buf, offset, size, data });
+        d->bufferOps.append(QRhiResourceUpdateBatchPrivate::BufferOp::staticUpload(buf, offset, size, data));
 }
 
 /*!
@@ -4199,7 +4206,28 @@ void QRhiResourceUpdateBatch::uploadStaticBuffer(QRhiBuffer *buf, int offset, in
 void QRhiResourceUpdateBatch::uploadStaticBuffer(QRhiBuffer *buf, const void *data)
 {
     if (buf->size() > 0)
-        d->staticBufferUploads.append({ buf, 0, 0, data });
+        d->bufferOps.append(QRhiResourceUpdateBatchPrivate::BufferOp::staticUpload(buf, 0, 0, data));
+}
+
+/*!
+    Enqueues reading back a region of the QRhiBuffer \a buf. The size of the
+    region is specified by \a size in bytes, \a offset is the offset in bytes
+    to start reading from.
+
+    A readback is asynchronous. \a result contains a callback that is invoked
+    when the operation has completed. The data is provided in
+    QRhiBufferReadbackResult::data. Upon successful completion that QByteArray
+    will have a size equal to \a size. On failure the QByteArray will be empty.
+
+    \note Reading buffers with a usage different than QRhiBuffer::UniformBuffer
+    is supported only when the QRhi::ReadBackNonUniformBuffer feature is
+    reported as supported.
+
+    \a readBackTexture(), QRhi::isFeatureSupported()
+ */
+void QRhiResourceUpdateBatch::readBackBuffer(QRhiBuffer *buf, int offset, int size, QRhiBufferReadbackResult *result)
+{
+    d->bufferOps.append(QRhiResourceUpdateBatchPrivate::BufferOp::read(buf, offset, size, result));
 }
 
 /*!
@@ -4212,7 +4240,7 @@ void QRhiResourceUpdateBatch::uploadStaticBuffer(QRhiBuffer *buf, const void *da
 void QRhiResourceUpdateBatch::uploadTexture(QRhiTexture *tex, const QRhiTextureUploadDescription &desc)
 {
     if (desc.cbeginEntries() != desc.cendEntries())
-        d->textureOps.append(QRhiResourceUpdateBatchPrivate::TextureOp::textureUpload(tex, desc));
+        d->textureOps.append(QRhiResourceUpdateBatchPrivate::TextureOp::upload(tex, desc));
 }
 
 /*!
@@ -4237,7 +4265,7 @@ void QRhiResourceUpdateBatch::uploadTexture(QRhiTexture *tex, const QImage &imag
  */
 void QRhiResourceUpdateBatch::copyTexture(QRhiTexture *dst, QRhiTexture *src, const QRhiTextureCopyDescription &desc)
 {
-    d->textureOps.append(QRhiResourceUpdateBatchPrivate::TextureOp::textureCopy(dst, src, desc));
+    d->textureOps.append(QRhiResourceUpdateBatchPrivate::TextureOp::copy(dst, src, desc));
 }
 
 /*!
@@ -4289,7 +4317,7 @@ void QRhiResourceUpdateBatch::copyTexture(QRhiTexture *dst, QRhiTexture *src, co
  */
 void QRhiResourceUpdateBatch::readBackTexture(const QRhiReadbackDescription &rb, QRhiReadbackResult *result)
 {
-    d->textureOps.append(QRhiResourceUpdateBatchPrivate::TextureOp::textureRead(rb, result));
+    d->textureOps.append(QRhiResourceUpdateBatchPrivate::TextureOp::read(rb, result));
 }
 
 /*!
@@ -4301,7 +4329,7 @@ void QRhiResourceUpdateBatch::readBackTexture(const QRhiReadbackDescription &rb,
  */
 void QRhiResourceUpdateBatch::generateMips(QRhiTexture *tex, int layer)
 {
-    d->textureOps.append(QRhiResourceUpdateBatchPrivate::TextureOp::textureMipGen(tex, layer));
+    d->textureOps.append(QRhiResourceUpdateBatchPrivate::TextureOp::genMips(tex, layer));
 }
 
 /*!
@@ -4350,8 +4378,7 @@ void QRhiResourceUpdateBatchPrivate::free()
 {
     Q_ASSERT(poolIndex >= 0 && rhi->resUpdPool[poolIndex] == q);
 
-    dynamicBufferUpdates.clear();
-    staticBufferUploads.clear();
+    bufferOps.clear();
     textureOps.clear();
 
     rhi->resUpdPoolMap.clearBit(poolIndex);
@@ -4360,9 +4387,13 @@ void QRhiResourceUpdateBatchPrivate::free()
 
 void QRhiResourceUpdateBatchPrivate::merge(QRhiResourceUpdateBatchPrivate *other)
 {
-    dynamicBufferUpdates += other->dynamicBufferUpdates;
-    staticBufferUploads += other->staticBufferUploads;
-    textureOps += other->textureOps;
+    bufferOps.reserve(bufferOps.size() + other->bufferOps.size());
+    for (const BufferOp &op : qAsConst(other->bufferOps))
+        bufferOps.append(op);
+
+    textureOps.reserve(textureOps.size() + other->textureOps.size());
+    for (const TextureOp &op : qAsConst(other->textureOps))
+        textureOps.append(op);
 }
 
 /*!