v4lcodecs: Validate src formats

This add src format validation, this avoid registering element for
drivers we don't support any of their src formats. This also special
case the AlphaDecodeBin wrapper, as we know that alphacombine element
only support I420 and NV12 for now.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/2272>

sys/v4l2codecs/gstv4l2codech264dec.c

@@ -1401,13 +1401,29 @@ gst_v4l2_codec_h264_dec_subclass_init (GstV4l2CodecH264DecClass * klass,
 }
 
 void
-gst_v4l2_codec_h264_dec_register (GstPlugin * plugin,
+gst_v4l2_codec_h264_dec_register (GstPlugin * plugin, GstV4l2Decoder * decoder,
     GstV4l2CodecDevice * device, guint rank)
 {
+  GstCaps *src_caps;
+
+  if (!gst_v4l2_decoder_set_sink_fmt (decoder, V4L2_PIX_FMT_H264_SLICE,
+          320, 240, 8))
+    return;
+  src_caps = gst_v4l2_decoder_enum_src_formats (decoder);
+
+  if (gst_caps_is_empty (src_caps)) {
+    GST_WARNING ("Not registering H264 decoder since it produces no "
+        "supported format");
+    goto done;
+  }
+
   gst_v4l2_decoder_register (plugin,
       GST_TYPE_V4L2_CODEC_H264_DEC,
       (GClassInitFunc) gst_v4l2_codec_h264_dec_subclass_init,
       gst_mini_object_ref (GST_MINI_OBJECT (device)),
       (GInstanceInitFunc) gst_v4l2_codec_h264_dec_subinit,
       "v4l2sl%sh264dec", device, rank, NULL);
+
+done:
+  gst_caps_unref (src_caps);
 }

sys/v4l2codecs/gstv4l2codech264dec.h

@@ -45,6 +45,7 @@ struct _GstV4l2CodecH264DecClass
 
 GType gst_v4l2_codec_h264_dec_get_type (void);
 void  gst_v4l2_codec_h264_dec_register (GstPlugin * plugin,
+                                        GstV4l2Decoder * decoder,
                                         GstV4l2CodecDevice * device,
                                         guint rank);
 

sys/v4l2codecs/gstv4l2codecvp8dec.c

@@ -872,10 +872,22 @@ static void gst_v4l2_codec_vp8_alpha_decode_bin_subclass_init
 }
 
 void
-gst_v4l2_codec_vp8_dec_register (GstPlugin * plugin,
+gst_v4l2_codec_vp8_dec_register (GstPlugin * plugin, GstV4l2Decoder * decoder,
     GstV4l2CodecDevice * device, guint rank)
 {
   gchar *element_name;
+  GstCaps *src_caps, *alpha_caps;
+
+  if (!gst_v4l2_decoder_set_sink_fmt (decoder, V4L2_PIX_FMT_VP8_FRAME,
+          320, 240, 8))
+    return;
+  src_caps = gst_v4l2_decoder_enum_src_formats (decoder);
+
+  if (gst_caps_is_empty (src_caps)) {
+    GST_WARNING ("Not registering VP8 decoder since it produces no "
+        "supported format");
+    goto done;
+  }
 
   gst_v4l2_decoder_register (plugin, GST_TYPE_V4L2_CODEC_VP8_DEC,
       (GClassInitFunc) gst_v4l2_codec_vp8_dec_subclass_init,
@@ -883,9 +895,18 @@ gst_v4l2_codec_vp8_dec_register (GstPlugin * plugin,
       (GInstanceInitFunc) gst_v4l2_codec_vp8_dec_subinit,
       "v4l2sl%svp8dec", device, rank, &element_name);
 
-  if (element_name) {
+  if (!element_name)
+    goto done;
+
+  alpha_caps = gst_caps_from_string ("video/x-raw,format={I420, NV12}");
+
+  if (gst_caps_can_intersect (src_caps, alpha_caps))
     gst_v4l2_codec_alpha_decode_bin_register (plugin,
         (GClassInitFunc) gst_v4l2_codec_vp8_alpha_decode_bin_subclass_init,
         element_name, "v4l2slvp8%salphadecodebin", device, rank);
-  }
+
+  gst_caps_unref (alpha_caps);
+
+done:
+  gst_caps_unref (src_caps);
 }

sys/v4l2codecs/gstv4l2codecvp8dec.h

@@ -45,6 +45,7 @@ struct _GstV4l2CodecVp8DecClass
 
 GType gst_v4l2_codec_vp8_dec_get_type (void);
 void  gst_v4l2_codec_vp8_dec_register (GstPlugin * plugin,
+                                       GstV4l2Decoder * decoder,
                                        GstV4l2CodecDevice * device,
                                        guint rank);
 

sys/v4l2codecs/plugin.c

@@ -50,12 +50,14 @@ register_video_decoder (GstPlugin * plugin, GstV4l2CodecDevice * device)
       case V4L2_PIX_FMT_H264_SLICE:
         GST_INFO_OBJECT (decoder, "Registering %s as H264 Decoder",
             device->name);
-        gst_v4l2_codec_h264_dec_register (plugin, device, GST_RANK_PRIMARY + 1);
+        gst_v4l2_codec_h264_dec_register (plugin, decoder, device,
+            GST_RANK_PRIMARY + 1);
         break;
       case V4L2_PIX_FMT_VP8_FRAME:
         GST_INFO_OBJECT (decoder, "Registering %s as VP8 Decoder",
             device->name);
-        gst_v4l2_codec_vp8_dec_register (plugin, device, GST_RANK_PRIMARY + 1);
+        gst_v4l2_codec_vp8_dec_register (plugin, decoder,
+            device, GST_RANK_PRIMARY + 1);
         break;
       default:
         GST_FIXME_OBJECT (decoder, "%" GST_FOURCC_FORMAT " is not supported.",