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

Index: content/common/gpu/media/dxva_video_decode_accelerator.cc
===================================================================
--- content/common/gpu/media/dxva_video_decode_accelerator.cc	(revision 167469)
+++ content/common/gpu/media/dxva_video_decode_accelerator.cc	(working copy)
@@ -73,6 +73,10 @@
                                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());
@@ -396,11 +400,16 @@
     // 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_,