v4l2: videodec: Don't forcibly drain on resolution changes

Let the driver detects the change and reconfigure the capture side
transparently from there. This avoid reallocation of the output buffers,
and eliminates the need to stop and restart the capture task. This is
only happening if the driver have support for this, otherwise the old
behaviour is maintained.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/4525>

subprojects/gst-plugins-good/sys/v4l2/gstv4l2videodec.c

@@ -259,13 +259,15 @@ static gboolean
 gst_v4l2_video_dec_set_format (GstVideoDecoder * decoder,
     GstVideoCodecState * state)
 {
+  GstV4l2VideoDec *self = GST_V4L2_VIDEO_DEC (decoder);
   GstV4l2Error error = GST_V4L2_ERROR_INIT;
   gboolean ret = TRUE;
-  GstV4l2VideoDec *self = GST_V4L2_VIDEO_DEC (decoder);
+  gboolean dyn_resolution = self->v4l2output->fmtdesc &&
+      (self->v4l2output->fmtdesc->flags & V4L2_FMT_FLAG_DYN_RESOLUTION);
 
   GST_DEBUG_OBJECT (self, "Setting format: %" GST_PTR_FORMAT, state->caps);
 
-  if (self->input_state) {
+  if (self->input_state && !dyn_resolution) {
     if (compatible_caps (self, state->caps)) {
       GST_DEBUG_OBJECT (self, "Compatible caps");
       goto done;
@@ -303,7 +305,10 @@ gst_v4l2_video_dec_set_format (GstVideoDecoder * decoder,
     self->output_flow = GST_FLOW_OK;
   }
 
-  ret = gst_v4l2_object_set_format (self->v4l2output, state->caps, &error);
+  /* No V4L2_FMT_FLAG_DYN_RESOLUTION or no fmtdesc set yet */
+  if (!dyn_resolution)
+    ret = gst_v4l2_object_set_format (self->v4l2output, state->caps, &error);
+
   if (ret)
     self->input_state = gst_video_codec_state_ref (state);
   else
@@ -414,6 +419,9 @@ gst_v4l2_video_dec_finish (GstVideoDecoder * decoder)
 
   GST_VIDEO_DECODER_STREAM_UNLOCK (decoder);
 
+  /* If we are in the middle of a source change, cancel it */
+  self->draining = FALSE;
+
   if (gst_v4l2_decoder_cmd (self->v4l2output, V4L2_DEC_CMD_STOP, 0)) {
     GstTask *task;
 
@@ -771,11 +779,6 @@ gst_v4l2_video_dec_loop (GstVideoDecoder * decoder)
     ret = gst_buffer_pool_acquire_buffer (pool, &buffer, NULL);
     g_object_unref (pool);
 
-    if (ret == GST_V4L2_FLOW_RESOLUTION_CHANGE) {
-      GST_INFO_OBJECT (decoder, "Received resolution change");
-      goto resolution_changed;
-    }
-
     if (ret != GST_FLOW_OK)
       goto beach;
 
@@ -788,11 +791,6 @@ gst_v4l2_video_dec_loop (GstVideoDecoder * decoder)
       if (cpool)
         gst_object_unref (cpool);
     }
-
-    if (ret == GST_V4L2_FLOW_RESOLUTION_CHANGE) {
-      GST_INFO_OBJECT (decoder, "Received resolution change");
-      goto resolution_changed;
-    }
   } while (ret == GST_V4L2_FLOW_CORRUPTED_BUFFER);
 
   if (ret != GST_FLOW_OK)
@@ -865,15 +863,26 @@ gst_v4l2_video_dec_loop (GstVideoDecoder * decoder)
 
   return;
 
-resolution_changed:
-  GST_VIDEO_DECODER_STREAM_LOCK (decoder);
-  /* FIXME, should be draining here */
-  gst_v4l2_object_stop (self->v4l2capture);
-  gst_v4l2_object_unlock_stop (self->v4l2capture);
-  GST_VIDEO_DECODER_STREAM_UNLOCK (decoder);
-  return;
-
 beach:
+  if (ret == GST_V4L2_FLOW_RESOLUTION_CHANGE) {
+    GST_VIDEO_DECODER_STREAM_LOCK (decoder);
+    self->draining = TRUE;
+    GST_VIDEO_DECODER_STREAM_UNLOCK (decoder);
+    GST_INFO_OBJECT (decoder, "Received resolution change");
+    return;
+  }
+
+  if (ret == GST_FLOW_EOS) {
+    GST_VIDEO_DECODER_STREAM_LOCK (decoder);
+    if (self->draining) {
+      self->draining = FALSE;
+      gst_v4l2_object_stop (self->v4l2capture);
+      GST_VIDEO_DECODER_STREAM_UNLOCK (decoder);
+      return;
+    }
+    GST_VIDEO_DECODER_STREAM_UNLOCK (decoder);
+  }
+
   GST_DEBUG_OBJECT (decoder, "Leaving output thread: %s",
       gst_flow_get_name (ret));
 
@@ -990,6 +999,7 @@ gst_v4l2_video_dec_handle_frame (GstVideoDecoder * decoder,
     /* Start the processing task, when it quits, the task will disable input
      * processing to unlock input if draining, or prevent potential block */
     self->output_flow = GST_FLOW_FLUSHING;
+    self->draining = FALSE;
     if (!gst_pad_start_task (decoder->srcpad,
             (GstTaskFunction) gst_v4l2_video_dec_loop, self, NULL))
       goto start_task_failed;

subprojects/gst-plugins-good/sys/v4l2/gstv4l2videodec.h

@@ -65,6 +65,7 @@ struct _GstV4l2VideoDec
 
   /* Source Change Events */
   gboolean wait_for_source_change;
+  gboolean draining;
 
   /* Capabilities */
   gboolean supports_source_change;