lavc: allow using AVCodecContext.hw_frames_ctx for decoding

For now it will only be used by the default get_buffer2 callback for
allocating hw frames.

doc/APIchanges

@@ -13,6 +13,9 @@ libavutil:     2015-08-28
 
 API changes, most recent first:
 
+2016-xx-xx - xxxxxxx - lavc 57.23.0 - avcodec.h
+  AVCodecContext.hw_frames_ctx now may be used by decoders.
+
 2016-xx-xx - xxxxxxx - lavc 57.20.0 - avcodec.h
   Add FF_PROFILE_H264_MULTIVIEW_HIGH and FF_PROFILE_H264_STEREO_HIGH.
 

libavcodec/avcodec.h

@@ -3078,15 +3078,25 @@ typedef struct AVCodecContext {
     int            nb_coded_side_data;
 
     /**
-     * Encoding only.
-     *
-     * For hardware encoders configured to use a hwaccel pixel format, this
-     * field should be set by the caller to a reference to the AVHWFramesContext
-     * describing input frames. AVHWFramesContext.format must be equal to
+     * A reference to the AVHWFramesContext describing the input (for encoding)
+     * or output (decoding) frames. The reference is set by the caller and
+     * afterwards owned (and freed) by libavcodec.
+     *
+     * - decoding: This field should be set by the caller from the get_format()
+     *             callback. The previous reference (if any) will always be
+     *             unreffed by libavcodec before the get_format() call.
+     *
+     *             If the default get_buffer2() is used with a hwaccel pixel
+     *             format, then this AVHWFramesContext will be used for
+     *             allocating the frame buffers.
+     *
+     * - encoding: For hardware encoders configured to use a hwaccel pixel
+     *             format, this field should be set by the caller to a reference
+     *             to the AVHWFramesContext describing input frames.
+     *             AVHWFramesContext.format must be equal to
      *             AVCodecContext.pix_fmt.
      *
-     * This field should be set before avcodec_open2() is called and is
-     * afterwards owned and managed by libavcodec.
+     *             This field should be set before avcodec_open2() is called.
      */
     AVBufferRef *hw_frames_ctx;
 } AVCodecContext;

libavcodec/utils.c

@@ -513,6 +513,9 @@ int avcodec_default_get_buffer2(AVCodecContext *avctx, AVFrame *frame, int flags
 {
     int ret;
 
+    if (avctx->hw_frames_ctx)
+        return av_hwframe_get_buffer(avctx->hw_frames_ctx, frame, 0);
+
     if ((ret = update_frame_pool(avctx, frame)) < 0)
         return ret;
 
@@ -793,6 +796,8 @@ int ff_get_format(AVCodecContext *avctx, const enum AVPixelFormat *fmt)
         av_freep(&avctx->internal->hwaccel_priv_data);
         avctx->hwaccel = NULL;
 
+        av_buffer_unref(&avctx->hw_frames_ctx);
+
         ret = avctx->get_format(avctx, choices);
 
         desc = av_pix_fmt_desc_get(ret);
@@ -804,6 +809,16 @@ int ff_get_format(AVCodecContext *avctx, const enum AVPixelFormat *fmt)
         if (!(desc->flags & AV_PIX_FMT_FLAG_HWACCEL))
             break;
 
+        if (avctx->hw_frames_ctx) {
+            AVHWFramesContext *hw_frames_ctx = (AVHWFramesContext*)avctx->hw_frames_ctx->data;
+            if (hw_frames_ctx->format != ret) {
+                av_log(avctx, AV_LOG_ERROR, "Format returned from get_buffer() "
+                       "does not match the format of provided AVHWFramesContext\n");
+                ret = AV_PIX_FMT_NONE;
+                break;
+            }
+        }
+
         if (!setup_hwaccel(avctx, ret, desc->name))
             break;
 

libavcodec/version.h

@@ -28,8 +28,8 @@
 #include "libavutil/version.h"
 
 #define LIBAVCODEC_VERSION_MAJOR 57
-#define LIBAVCODEC_VERSION_MINOR 22
-#define LIBAVCODEC_VERSION_MICRO  2
+#define LIBAVCODEC_VERSION_MINOR 23
+#define LIBAVCODEC_VERSION_MICRO  0
 
 #define LIBAVCODEC_VERSION_INT  AV_VERSION_INT(LIBAVCODEC_VERSION_MAJOR, \
                                                LIBAVCODEC_VERSION_MINOR, \