Avoid unnecessary post task during frame delivery.

media/base/video_decoder.h

 // 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.
 
 #ifndef MEDIA_BASE_VIDEO_DECODER_H_
 #define MEDIA_BASE_VIDEO_DECODER_H_
 
 #include <string>
 
 #include "base/callback.h"
@@ skipping 47 matching lines @@
   // satisfy the requirements above.
   //
   // |cdm_context| can be used to handle encrypted buffers. May be null if the
   // stream is not encrypted.
   //
   // Note:
   // 1) The VideoDecoder will be reinitialized if it was initialized before.
   //    Upon reinitialization, all internal buffered frames will be dropped.
   // 2) This method should not be called during pending decode or reset.
   // 3) No VideoDecoder calls should be made before |init_cb| is executed.
+  // 4) VideoDecoders should take care to run |output_cb| as soon as the frame
+  // is ready (i.e. w/o thread trampolining) since it can strongly affect frame
+  // delivery times with high-frame-rate material.

[xhwang, 2016/04/25 21:15:55]

We typically expect callbacks (including the output_cb) to be fired
asynchronously, to avoid reentrancy issues, e.g.

Decode() -> DecodeBuffer() -> output_cb.Run()

Looking at DecoderStream, this may be okay. But it will definitely make our
callstack more complicated. So I wonder how much performance gain does this buy
us? Could you please make it more clear, here and in DecoderStream, that this
means output_cb.Run() could be fired in the same call stack as Decode()?

[DaleCurtis, 2016/04/25 21:31:44]

This definitely buys us a lot of performance gain in the case of
GpuVideoDecoder, i.e. w/o this fix we're unable to play 720p60 content without
constantly underflowing.

[DaleCurtis, 2016/04/25 22:32:22]

Done. Fixed the existing comment too which wasn't true.

   virtual void Initialize(const VideoDecoderConfig& config,
                           bool low_delay,
                           CdmContext* cdm_context,
                           const InitCB& init_cb,
                           const OutputCB& output_cb) = 0;
 
   // Requests a |buffer| to be decoded. The status of the decoder and decoded
   // frame are returned via the provided callback. Some decoders may allow
   // decoding multiple buffers in parallel. Callers should call
   // GetMaxDecodeRequests() to get number of buffers that may be decoded in
@@ skipping 34 matching lines @@
   // Returns maximum number of parallel decode requests.
   virtual int GetMaxDecodeRequests() const;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(VideoDecoder);
 };
 
 }  // namespace media
 
 #endif  // MEDIA_BASE_VIDEO_DECODER_H_