Don't wait endlessly for flushing the batched queries sent to the GPU driver during H/W decode

content/common/gpu/media/dxva_video_decode_accelerator.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/common/gpu/media/dxva_video_decode_accelerator.h"
 
 #if !defined(OS_WIN)
 #error This file should only be built on Windows.
 #endif   // !defined(OS_WIN)
 
@@ skipping 55 matching lines @@
       StopOnError(error_code);                                      \
       return ret;                                                   \
     }                                                               \
   } while (0)
 
 #define RETURN_AND_NOTIFY_ON_HR_FAILURE(result, log, error_code, ret)  \
   RETURN_AND_NOTIFY_ON_FAILURE(SUCCEEDED(result),                      \
                                log << ", HRESULT: 0x" << std::hex << result, \
                                error_code, ret);
 
+// Maximum number of iterations we allow before aborting the attempt to flush
+// the batched queries to the driver.
+enum { kMaxiterationsForD3dFlush = 10 };

[Ami GONE FROM CHROMIUM, 2012/11/13 23:31:25]

s/xi/xI/

[ananta, 2012/11/14 01:06:47]

Done.

+
 static IMFSample* CreateEmptySample() {
   base::win::ScopedComPtr<IMFSample> sample;
   HRESULT hr = MFCreateSample(sample.Receive());
   RETURN_ON_HR_FAILURE(hr, "MFCreateSample failed", NULL);
   return sample.Detach();
 }
 
 // Creates a Media Foundation sample with one buffer of length |buffer_length|
 // on a |align|-byte boundary. Alignment must be a perfect power of 2 or 0.
 static IMFSample* CreateEmptySampleWithBuffer(int buffer_length, int align) {
@@ skipping 303 matching lines @@
                               NULL,
                               d3d_surface,
                               NULL,
                               D3DTEXF_NONE);
     RETURN_ON_HR_FAILURE(hr, "Colorspace conversion via StretchRect failed",
                          false);
     // Ideally, this should be done immediately before the draw call that uses
     // the texture. Flush it once here though.
     hr = query_->Issue(D3DISSUE_END);
     RETURN_ON_HR_FAILURE(hr, "Failed to issue END", false);
-    do {
-      hr = query_->GetData(NULL, 0, D3DGETDATA_FLUSH);
-      if (hr == S_FALSE)
-        Sleep(1);  // Poor-man's Yield().
-    } while (hr == S_FALSE);
+
+    // The loop below attempts to flush the batched queries to the driver. We
+    // don't want to loop endlessly waiting for the driver to return success.
+    // The current workaround is to allow a max of kMaxiterationsForD3dFlush
+    // iterations.

[Ami GONE FROM CHROMIUM, 2012/11/13 23:31:25]

This comment is useless; it restates in English what the code is clearly doing
in C++.

A useful comment would explain:
a) why a FLUSH is needed/desired
b) why sometimes we need to give up without actually getting a FLUSH
c) why (b) is ok in the presence of (a)

[ananta, 2012/11/14 01:06:47]

Rephrased. There is also some new code which addresses the case where if we
reach the upper limit on iterations, we force a readback from the ANGLE surface
to a surface created in system memory which ensures that the pending operations
are completed.

+    int iterations = 0;
+    while ((query_->GetData(NULL, 0, D3DGETDATA_FLUSH) == S_FALSE) &&
+           ++iterations < kMaxiterationsForD3dFlush) {
+      Sleep(1);  // Poor-man's Yield().
+    }
     eglBindTexImage(
         static_cast<EGLDisplay*>(eglGetDisplay(EGL_DEFAULT_DISPLAY)),
         decoding_surface_,
         EGL_BACK_BUFFER);
   } else {
     scoped_array<char> bits;
     RETURN_ON_FAILURE(GetBitmapFromSurface(DXVAVideoDecodeAccelerator::device_,
                                            dest_surface, &bits),
                       "Failed to get bitmap from surface for rendering",
                       false);
@@ skipping 724 matching lines @@
       return;
   }
 
   MessageLoop::current()->PostTask(FROM_HERE, base::Bind(
       &DXVAVideoDecodeAccelerator::NotifyFlushDone, base::AsWeakPtr(this)));
 
   state_ = kNormal;
 }
 
 }  // namespace content