Update PPB_VideoDecoder documentation.

ppapi/api/ppb_video_decoder.idl

 /* Copyright (c) 2014 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.
  */
 
 /**
  * This file defines the <code>PPB_VideoDecoder</code> interface.
  */
 
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@@ skipping 73 matching lines @@
    */
   int32_t Initialize(
       [in] PP_Resource video_decoder,
       [in] PP_Resource graphics3d_context,
       [in] PP_VideoProfile profile,
       [in] PP_Bool allow_software_fallback,
       [in] PP_CompletionCallback callback);
 
   /**
    * Decodes a bitstream buffer. Copies |size| bytes of data from the plugin's
-   * |buffer|. The plugin should maintain the buffer and not call Decode() again
-   * until the decoder signals completion by returning PP_OK or by running
-   * |callback|.
+   * |buffer|. The plugin should wait until the decoder signals completion by
+   * returning PP_OK or by running |callback| before calling Decode() again.

[igorc, 2014/05/21 20:34:29]

And another one - why do we need two ways to successfully complete Decode()?
This makes me write more code than feels necessary... I checked with Elijah and
he said this feels a bit unusual.

For simplicity, I would just say that none of the functions that take callbacks
should ever return PP_OK - they either fail, or tell that callback is now
pending.

    *
    * In general, each bitstream buffer should contain a demuxed bitstream frame
    * for the selected video codec. For example, H264 decoders expect to receive
    * one AnnexB NAL unit, including the 4 byte start code prefix, while VP8
    * decoders expect to receive a bitstream frame without the IVF frame header.
    *
    * If the call to Decode() eventually results in a picture, the |decode_id|
    * parameter is copied into the returned picture. The plugin can use this to
    * associate decoded pictures with Decode() calls (e.g. to assign timestamps
    * or frame numbers to pictures.) This value is opaque to the API so the
    * plugin is free to pass any value.
    *
    * @param[in] video_decoder A <code>PP_Resource</code> identifying the video
    * decoder.
    * @param[in] decode_id An optional value, chosen by the plugin, that can be
    * used to associate calls to Decode() with decoded pictures returned by
    * GetPicture().
    * @param[in] size Buffer size in bytes.
    * @param[in] buffer Starting address of buffer.
    * @param[in] callback A <code>PP_CompletionCallback</code> to be called on
    * completion.
    *
    * @return An int32_t containing an error code from <code>pp_errors.h</code>.
+   * Returns PP_ERROR_FAILED if the decoder isn't initialized or if a Flush()
+   * or Reset() call is pending.
+   * Returns PP_ERROR_INPROGRESS if there is another Decode() call pending.
+   * Returns PP_ERROR_NOMEMORY if a bitstream buffer can't be created.
+   * Returns PP_ERROR_ABORTED when Reset() is called while Decode() is pending.
    */
   int32_t Decode(
       [in] PP_Resource video_decoder,
       [in] uint32_t decode_id,
       [in] uint32_t size,
       [in] mem_t buffer,
       [in] PP_CompletionCallback callback);
 
   /**
    * Gets the next picture from the decoder. The picture is valid after the
    * decoder signals completion by returning PP_OK or running |callback|. The
    * plugin can call GetPicture() again after the decoder signals completion.
    * When the plugin is finished using the picture, it should return it to the
    * system by calling RecyclePicture().
    *
    * @param[in] video_decoder A <code>PP_Resource</code> identifying the video
    * decoder.
    * @param[out] picture A <code>PP_VideoPicture</code> to hold the decoded
    * picture.
    * @param[in] callback A <code>PP_CompletionCallback</code> to be called on
    * completion.
    *
    * @return An int32_t containing an error code from <code>pp_errors.h</code>.
-   * Returns PP_OK if a picture is available.
+   * Returns PP_ERROR_FAILED if the decoder isn't initialized or if a Reset()
+   * call is pending.
+   * Returns PP_ERROR_INPROGRESS if there is another GetPicture() call pending.
    * Returns PP_ERROR_ABORTED when Reset() is called, or if a call to Flush()
    * completes while GetPicture() is pending.
    */
   int32_t GetPicture(
       [in] PP_Resource video_decoder,
       [out] PP_VideoPicture picture,
       [in] PP_CompletionCallback callback);
 
   /**
    * Recycles a picture that the plugin has received from the decoder.
    * The plugin should call this as soon as it has finished using the texture so
    * the decoder can decode more pictures.
    *
    * @param[in] video_decoder A <code>PP_Resource</code> identifying the video
    * decoder.
    * @param[in] picture A <code>PP_VideoPicture</code> to return to
    * the decoder.
    */
   void RecyclePicture(
       [in] PP_Resource video_decoder,
       [in] PP_VideoPicture picture);

[igorc, 2014/05/21 19:44:37]

Could you clarify which fields need to be filled out in PP_VideoPicture? I don't
have the original struct and would prefer to provide just the texture id, and
set everything else to zero.

 
   /**
-   * Flushes the decoder. The plugin should call this when it reaches the end of
-   * its video stream in order to stop cleanly. The decoder will run all pending
-   * calls to completion. The plugin should make no further calls to the decoder
-   * other than GetPicture() and RecyclePicture() until the decoder signals
-   * completion by running the callback. Just before completion, any pending
-   * GetPicture() call will complete by running the callback with result
-   * PP_ERROR_ABORTED to signal that no more pictures are available.
+   * Flushes the decoder. The plugin should call Flush() when it reaches the
+   * end of its video stream in order to stop cleanly. The decoder will run any
+   * pending Decode() call to completion. The plugin should make no further
+   * calls to the decoder other than GetPicture() and RecyclePicture() until

[igorc, 2014/05/21 19:07:05]

Just to clarify - am I not allowed to call Reset() while Flush() is pending? I
would prefer that Reset() is allowed, as the purpose of Reset() is to complete
"as quickly as possible".

+   * the decoder signals completion by running |callback|. Just before
+   * completion, any pending GetPicture() call will complete by running its
+   * callback with result PP_ERROR_ABORTED to signal that no more pictures are
+   * available. The plugin should recycle any pictures it is using before
+   * resuming decoding.
    *
    * @param[in] video_decoder A <code>PP_Resource</code> identifying the video
    * decoder.
    * @param[in] callback A <code>PP_CompletionCallback</code> to be called on

[igorc, 2014/05/21 19:07:05]

As we discussed - could you add a clarification that the callback cannot be
called recursively from a call such as Flush(), GetPicture(), etc?

The reasoning is that most clients will likely use mutexes around calls, and
would have to queue all callbacks to avoid random deadlocks when a callback is
invoked recursively.

    * completion.
    *
    * @return An int32_t containing an error code from <code>pp_errors.h</code>.
+   * Returns PP_ERROR_FAILED if the decoder isn't initialized.
    */
   int32_t Flush(

[igorc, 2014/05/21 20:15:47]

Could you also clarify the difference between PP_OK_COMPLETIONPENDING and PP_OK
for this and other calls? I found PP_OK_COMPLETIONPENDING in your example, but
not all calls are checking results there yet.

       [in] PP_Resource video_decoder,
       [in] PP_CompletionCallback callback);
 
   /**
    * Resets the decoder as quickly as possible. The plugin can call Reset() to
-   * skip to another position in the video stream. Pending calls to Decode() and
-   * GetPicture()) are immediately aborted, causing their callbacks to run with
-   * PP_ERROR_ABORTED. The plugin should not make any further calls to the
-   * decoder until the decoder signals completion by running |callback|.
+   * skip to another position in the video stream. After Reset() returns, any
+   * pending calls to Decode() and GetPicture()) abort, causing their callbacks
+   * to run with PP_ERROR_ABORTED. The plugin should not make further calls to
+   * the decoder until the decoder signals completion by running |callback|.
+   * The pictures in use by the plugin remain valid until decoding is resumed,
+   * but need not be recycled.
    *
    * @param[in] video_decoder A <code>PP_Resource</code> identifying the video
    * decoder.
    * @param[in] callback A <code>PP_CompletionCallback</code> to be called on
    * completion.
    *
    * @return An int32_t containing an error code from <code>pp_errors.h</code>.
+   * Returns PP_ERROR_FAILED if the decoder isn't initialized.
    */
   int32_t Reset(
       [in] PP_Resource video_decoder,
       [in] PP_CompletionCallback callback);
 };