diff --git a/ffmpeg_opt.c b/ffmpeg_opt.c
index ff80c8ab44..c87bf01100 100644
--- a/ffmpeg_opt.c
+++ b/ffmpeg_opt.c
@@ -1719,7 +1719,6 @@ static int open_output_file(OptionsContext *o, const char *filename)
     } else {
         for (i = 0; i < o->nb_stream_maps; i++) {
             StreamMap *map = &o->stream_maps[i];
-            int src_idx = input_files[map->file_index]->ist_index + map->stream_index;
 
             if (map->disabled)
                 continue;
@@ -1747,6 +1746,8 @@ loop_end:
                 }
                 init_output_filter(ofilter, o, oc);
             } else {
+                int src_idx = input_files[map->file_index]->ist_index + map->stream_index;
+
                 ist = input_streams[input_files[map->file_index]->ist_index + map->stream_index];
                 if(o->subtitle_disable && ist->st->codec->codec_type == AVMEDIA_TYPE_SUBTITLE)
                     continue;
diff --git a/libavfilter/avfiltergraph.c b/libavfilter/avfiltergraph.c
index 582a87023d..c5a9e3b59d 100644
--- a/libavfilter/avfiltergraph.c
+++ b/libavfilter/avfiltergraph.c
@@ -316,12 +316,24 @@ static int formats_declared(AVFilterContext *f)
     return 1;
 }
 
+/**
+ * Perform one round of query_formats() and merging formats lists on the
+ * filter graph.
+ * @return  >=0 if all links formats lists could be queried and merged;
+ *          AVERROR(EAGAIN) some progress was made in the queries or merging
+ *          and a later call may succeed;
+ *          AVERROR(EIO) (may be changed) plus a log message if no progress
+ *          was made and the negotiation is stuck;
+ *          a negative error code if some other error happened
+ */
 static int query_formats(AVFilterGraph *graph, AVClass *log_ctx)
 {
     int i, j, ret;
     int scaler_count = 0, resampler_count = 0;
-    int count_queried = 0, count_merged = 0, count_already_merged = 0,
-        count_delayed = 0;
+    int count_queried = 0;        /* successful calls to query_formats() */
+    int count_merged = 0;         /* successful merge of formats lists */
+    int count_already_merged = 0; /* lists already merged */
+    int count_delayed = 0;        /* lists that need to be merged later */
 
     for (i = 0; i < graph->nb_filters; i++) {
         AVFilterContext *f = graph->filters[i];
@@ -333,7 +345,8 @@ static int query_formats(AVFilterGraph *graph, AVClass *log_ctx)
             ret = ff_default_query_formats(f);
         if (ret < 0 && ret != AVERROR(EAGAIN))
             return ret;
-        count_queried++;
+        /* note: EAGAIN could indicate a partial success, not counted yet */
+        count_queried += ret >= 0;
     }
 
     /* go through and merge as many format lists as possible */
@@ -463,6 +476,11 @@ static int query_formats(AVFilterGraph *graph, AVClass *log_ctx)
     if (count_delayed) {
         AVBPrint bp;
 
+        /* if count_queried > 0, one filter at least did set its formats,
+           that will give additional information to its neighbour;
+           if count_merged > 0, one pair of formats lists at least was merged,
+           that will give additional information to all connected filters;
+           in both cases, progress was made and a new round must be done */
         if (count_queried || count_merged)
             return AVERROR(EAGAIN);
         av_bprint_init(&bp, 0, AV_BPRINT_SIZE_AUTOMATIC);