Allow multiple concurrent Decode() requests in VideoDecoder interface.

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 "base/callback.h"
 #include "base/memory/ref_counted.h"
 #include "media/base/media_export.h"
@@ skipping 28 matching lines @@
   // 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, reset or stop.
   // 3) No VideoDecoder calls except for Stop() should be made before
   //    |status_cb| is executed.
   virtual void Initialize(const VideoDecoderConfig& config,
                           const PipelineStatusCB& status_cb) = 0;
 
   // Requests a |buffer| to be decoded. The status of the decoder and decoded
-  // frame are returned via the provided callback. Only one decode may be in
-  // flight at any given time.
+  // frame are returned via the provided callback. Some decoders may allow
+  // decoding multiple frames in parallel. Callers should call
+  // GetMaxDecodeRequests() to get number of frames that may be decoded in
+  // parallel.

[Ami GONE FROM CHROMIUM, 2014/04/16 01:00:00]

An alternative approach that may work better is to return true/false depending
on whether the decoder was able to accept the request.
(better for the client who doesn't have to track the # of outstanding requests?)

[Sergey Ulanov, 2014/04/16 01:44:14]

Isn't DecodeStream the only client of this interface?
Also I think GetMaxDecodeRequests() would need to be exposed anyway, so that
DecodeStream knows how many output buffers it may need to buffer.

   //
   // Implementations guarantee that the callback will not be called from within
   // this method.
   //
   // If the returned status is kOk:
   // - Non-EOS (end of stream) frame contains decoded video data.
   // - EOS frame indicates the end of the stream.
   // Otherwise the returned frame must be NULL.
   typedef base::Callback<void(Status,
                               const scoped_refptr<VideoFrame>&)> DecodeCB;
@@ skipping 25 matching lines @@
 
   // Returns true if the decoder needs bitstream conversion before decoding.
   virtual bool NeedsBitstreamConversion() const;
 
   // Returns true if the decoder currently has the ability to decode and return
   // a VideoFrame. Most implementations can allocate a new VideoFrame and hence
   // this will always return true. Override and return false for decoders that
   // use a fixed set of VideoFrames for decoding.
   virtual bool CanReadWithoutStalling() const;
 
+  // Returns how many frames that may be decoded in parallel.
+  virtual int GetMaxDecodeRequests() const;
+
  private:
   DISALLOW_COPY_AND_ASSIGN(VideoDecoder);
 };
 
 }  // namespace media
 
 #endif  // MEDIA_BASE_VIDEO_DECODER_H_