v4l2codecs: Detect missing M2M_HOLD_CAPTURE_BUF

Produce an error if we try to use the feature of holding capture buffer
but it is not supported by the driver. Ingoring this can lead to stalls
as the driver will run-out of capture buffer to decode into. This
affects slice decoders but also split-field interlaced decoding.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2009>

diff --git a/subprojects/gst-plugins-bad/sys/v4l2codecs/gstv4l2decoder.c b/subprojects/gst-plugins-bad/sys/v4l2codecs/gstv4l2decoder.c
index a024c96..7f7b3f9 100644 (file)
--- a/subprojects/gst-plugins-bad/sys/v4l2codecs/gstv4l2decoder.c
+++ b/subprojects/gst-plugins-bad/sys/v4l2codecs/gstv4l2decoder.c
@@ -87,6 +87,9 @@ struct _GstV4l2Decoder
   gchar *media_device;
   gchar *video_device;
   guint render_delay;
+
+  /* detected features */
+  gboolean supports_holding_capture;
 };
 
 G_DEFINE_TYPE_WITH_CODE (GstV4l2Decoder, gst_v4l2_decoder, GST_TYPE_OBJECT,
@@ -488,6 +491,13 @@ gst_v4l2_decoder_request_buffers (GstV4l2Decoder * self,
     return ret;
   }
 
+  if (direction == GST_PAD_SINK) {
+    if (reqbufs.capabilities & V4L2_BUF_CAP_SUPPORTS_M2M_HOLD_CAPTURE_BUF)
+      self->supports_holding_capture = TRUE;
+    else
+      self->supports_holding_capture = FALSE;
+  }
+
   return reqbufs.count;
 }
 
@@ -1086,6 +1096,15 @@ gst_v4l2_request_queue (GstV4l2Request * request, guint flags)
 
   GST_TRACE_OBJECT (decoder, "Queuing request %i.", request->fd);
 
+  /* this would lead to stalls if we tried to use this feature and it wasn't
+   * supported. */
+  if ((flags & V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF)
+      && !decoder->supports_holding_capture) {
+    GST_ERROR_OBJECT (decoder,
+        "Driver does not support holding capture buffer.");
+    return FALSE;
+  }
+
   if (!gst_v4l2_decoder_queue_sink_mem (decoder, request,
           request->bitstream, request->frame_num, flags)) {
     GST_ERROR_OBJECT (decoder, "Driver did not accept the bitstream data.");