ffmpeg_opt: consider HW acceleration method when selecting decoder

Usually a HW decoder is expected when user specifies a HW acceleration
method via -hwaccel option, however the current implementation doesn't
take HW acceleration method into account, it is possible to select a SW
decoder.

For example:
$ ffmpeg -hwaccel vaapi -i av1.mp4 -f null -
$ ffmpeg -hwaccel nvdec -i av1.mp4 -f null -
$ ffmpeg -hwaccel vdpau -i av1.mp4 -f null -
[...]
Stream #0:0 -> #0:0 (av1 (libdav1d) -> wrapped_avframe (native))

libdav1d is selected in this case even if vaapi, nvdec or vdpau is
specified.

After applying this patch, the native av1 decoder (with vaapi, nvdec or
vdpau support) is selected for decoding(libdav1d is still used for
probing format).
$ ffmpeg -hwaccel vaapi -i av1.mp4 -f null -
$ ffmpeg -hwaccel nvdec -i av1.mp4 -f null -
$ ffmpeg -hwaccel vdpau -i av1.mp4 -f null -
[...]
Stream #0:0 -> #0:0 (av1 (native) -> wrapped_avframe (native))

Tested-by: Mario Roy <marioeroy@gmail.com>
Signed-off-by: Haihao Xiang <haihao.xiang@intel.com>
Signed-off-by: Anton Khirnov <anton@khirnov.net>

fftools/ffmpeg_opt.c

@@ -857,7 +857,9 @@ static const AVCodec *find_codec_or_die(const char *name, enum AVMediaType type,
     return codec;
 }
 
-static const AVCodec *choose_decoder(OptionsContext *o, AVFormatContext *s, AVStream *st)
+static const AVCodec *choose_decoder(OptionsContext *o, AVFormatContext *s, AVStream *st,
+                                     enum HWAccelID hwaccel_id, enum AVHWDeviceType hwaccel_device_type)
+
 {
     char *codec_name = NULL;
 
@@ -868,8 +870,32 @@ static const AVCodec *choose_decoder(OptionsContext *o, AVFormatContext *s, AVSt
         if (recast_media && st->codecpar->codec_type != codec->type)
             st->codecpar->codec_type = codec->type;
         return codec;
-    } else
+    } else {
+        if (st->codecpar->codec_type == AVMEDIA_TYPE_VIDEO &&
+            hwaccel_id == HWACCEL_GENERIC &&
+            hwaccel_device_type != AV_HWDEVICE_TYPE_NONE) {
+            const AVCodec *c;
+            void *i = NULL;
+
+            while ((c = av_codec_iterate(&i))) {
+                const AVCodecHWConfig *config;
+
+                if (c->id != st->codecpar->codec_id ||
+                    !av_codec_is_decoder(c))
+                    continue;
+
+                for (int j = 0; config = avcodec_get_hw_config(c, j); j++) {
+                    if (config->device_type == hwaccel_device_type) {
+                        av_log(NULL, AV_LOG_VERBOSE, "Selecting decoder '%s' because of requested hwaccel method %s\n",
+                               c->name, av_hwdevice_get_type_name(hwaccel_device_type));
+                        return c;
+                    }
+                }
+            }
+        }
+
         return avcodec_find_decoder(st->codecpar->codec_id);
+    }
 }
 
 static int guess_input_channel_layout(InputStream *ist)
@@ -1003,7 +1029,7 @@ static void add_input_streams(OptionsContext *o, AVFormatContext *ic)
             ist->hwaccel_pix_fmt = AV_PIX_FMT_NONE;
         }
 
-        ist->dec = choose_decoder(o, ic, st);
+        ist->dec = choose_decoder(o, ic, st, ist->hwaccel_id, ist->hwaccel_device_type);
         ist->decoder_opts = filter_codec_opts(o->g->codec_opts, ist->st->codecpar->codec_id, ic, st, ist->dec);
 
         ist->reinit_filters = -1;
@@ -1309,7 +1335,7 @@ static int open_input_file(OptionsContext *o, const char *filename)
 
     /* apply forced codec ids */
     for (i = 0; i < ic->nb_streams; i++)
-        choose_decoder(o, ic, ic->streams[i]);
+        choose_decoder(o, ic, ic->streams[i], HWACCEL_NONE, AV_HWDEVICE_TYPE_NONE);
 
     if (find_stream_info) {
         AVDictionary **opts = setup_find_stream_info_opts(ic, o->g->codec_opts);