Dawn readpixels fixes.

The dawn backend was calling GrDawnGpu::flush() in a few places (in
particular, for readpixels).Since GrGpu::submitToGpu() is now
responsible for unmapping the staging buffers prior to command
submission, internally calling GrDawnGpu::flush() is error-prone,
since it requires you to unmapStagingBuffers() manually.

The fix is to get rid of GrDawnGpu::flush(), and put its guts into
GrDawnGpu::onSubmitToGpu(). Then we call submitToGpu(true) where we
used to call flush().

Also, make the backend responsible for moving staging buffers from the
active to the busy list. We do it just before calling mapAsync(),
since the callback may return right away and we want the buffer to be
in the busy state at that point.

Change-Id: I99cfa5e7ea3a454b0383a30483fa93795191347a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/288516
Reviewed-by: Greg Daniel <[email protected]>
Commit-Queue: Stephen White <[email protected]>

Author: SenorBlanco

src/gpu/GrGpu.cpp

@@ -692,12 +692,6 @@ bool GrGpu::submitToGpu(bool syncCpu) {
 
     bool submitted = this->onSubmitToGpu(syncCpu);
 
-    // Move all active staging buffers to the busy list.
-    // TODO: this should probably be handled inside of the onSubmitToGpu by the backends.
-    while (GrStagingBuffer* buffer = fActiveStagingBuffers.head()) {
-        fActiveStagingBuffers.remove(buffer);
-        fBusyStagingBuffers.addToTail(buffer);
-    }
     return submitted;
 }
 

src/gpu/dawn/GrDawnGpu.cpp

@@ -460,21 +460,11 @@ void GrDawnGpu::deleteTestingOnlyBackendRenderTarget(const GrBackendRenderTarget
 }
 
 void GrDawnGpu::testingOnly_flushGpuAndSync() {
-    this->flush();
+    this->submitToGpu(true);
 }
 
 #endif
 
-void GrDawnGpu::flush() {
-    this->flushCopyEncoder();
-    if (!fCommandBuffers.empty()) {
-        fQueue.Submit(fCommandBuffers.size(), &fCommandBuffers.front());
-        fCommandBuffers.clear();
-    }
-    this->mapStagingBuffers();
-    fDevice.Tick();
-}
-
 void GrDawnGpu::addFinishedProc(GrGpuFinishedProc finishedProc,
                                 GrGpuFinishedContext finishedContext) {
     fFinishCallbacks.add(finishedProc, finishedContext);
@@ -485,7 +475,12 @@ static void callback(WGPUFenceCompletionStatus status, void* userData) {
 }
 
 bool GrDawnGpu::onSubmitToGpu(bool syncCpu) {
-    this->flush();
+    this->flushCopyEncoder();
+    if (!fCommandBuffers.empty()) {
+        fQueue.Submit(fCommandBuffers.size(), &fCommandBuffers.front());
+        fCommandBuffers.clear();
+    }
+    this->mapStagingBuffers();
     if (syncCpu) {
         wgpu::FenceDescriptor desc;
         wgpu::Fence fence = fQueue.CreateFence(&desc);
@@ -541,7 +536,7 @@ bool GrDawnGpu::onReadPixels(GrSurface* surface, int left, int top, int width, i
     wgpu::Texture tex = get_dawn_texture_from_surface(surface);
     SkASSERT(tex);
 
-    if (0 == rowBytes) {
+    if (!tex || 0 == rowBytes) {
         return false;
     }
     size_t origRowBytes = rowBytes;
@@ -567,7 +562,7 @@ bool GrDawnGpu::onReadPixels(GrSurface* surface, int left, int top, int width, i
 
     wgpu::Extent3D copySize = {(uint32_t) width, (uint32_t) height, 1};
     this->getCopyEncoder().CopyTextureToBuffer(&srcTexture, &dstBuffer, &copySize);
-    this->flush();
+    this->submitToGpu(true);
 
     const void *readPixelsPtr = nullptr;
     buf.MapReadAsync(callback, &readPixelsPtr);
@@ -721,7 +716,9 @@ void GrDawnGpu::flushCopyEncoder() {
 
 void GrDawnGpu::mapStagingBuffers() {
     // Map all active buffers, so we get a callback when they're done.
-    for (auto buffer : fActiveStagingBuffers) {
+    while (auto buffer = fActiveStagingBuffers.head()) {
+        fActiveStagingBuffers.remove(buffer);
+        fBusyStagingBuffers.addToTail(buffer);
         static_cast<GrDawnStagingBuffer*>(buffer)->mapAsync();
     }
 }

src/gpu/dawn/GrDawnGpu.h

@@ -53,7 +53,6 @@ class GrDawnGpu : public GrGpu {
 
     void testingOnly_flushGpuAndSync() override;
 #endif
-    void flush();
     std::unique_ptr<GrStagingBuffer> createStagingBuffer(size_t size) override;
 
     GrStencilAttachment* createStencilAttachmentForRenderTarget(const GrRenderTarget*,