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 29 matching lines @@
   // 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,
                           bool low_delay,
                           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 buffers in parallel. Callers should call
+  // GetMaxDecodeRequests() to get number of buffers that may be decoded in
+  // parallel. Decoder must call |decode_cb| in the same order in which Decode()
+  // is called.
   //
   // Implementations guarantee that the callback will not be called from within
-  // this method.
+  // this method and that |decode_cb| will not be blocked on the following
+  // Decode() calls (i.e. |decode_cb| will be called even Decode() is never
+  // called again).
   //
   // 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;
   virtual void Decode(const scoped_refptr<DecoderBuffer>& buffer,
                       const DecodeCB& decode_cb) = 0;
 
   // Some VideoDecoders may queue up multiple VideoFrames from a single
   // DecoderBuffer, if we have any such queued frames this will return the next
   // one. Otherwise we return a NULL VideoFrame.
+  //
+  // TODO(xhwang): Revisit this method.
   virtual scoped_refptr<VideoFrame> GetDecodeOutput();
 
   // Resets decoder state, fulfilling all pending DecodeCB and dropping extra
   // queued decoded data. After this call, the decoder is back to an initialized
   // clean state.
   // Note: No VideoDecoder calls should be made before |closure| is executed.
   virtual void Reset(const base::Closure& closure) = 0;
 
   // Stops decoder, fires any pending callbacks and sets the decoder to an
   // uninitialized state. A VideoDecoder cannot be re-initialized after it has
   // been stopped.
   // Note that if Initialize() is pending or has finished successfully, Stop()
   // must be called before destructing the decoder.
   virtual void Stop() = 0;
 
   // 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 maximum number of parallel decode requests.
+  virtual int GetMaxDecodeRequests() const;
+
  private:
   DISALLOW_COPY_AND_ASSIGN(VideoDecoder);
 };
 
 }  // namespace media
 
 #endif  // MEDIA_BASE_VIDEO_DECODER_H_