[media] media: Print information on failed link validation

The Media controller doesn't tell much to the user in cases such as pipeline
startup failure. The link validation is the most common media graph (or in
V4L2's case, format) related reason for the failure. In more complex
pipelines the reason may not always be obvious to the user, so point them to
look at the right direction.

Signed-off-by: Sakari Ailus <[e-mail]>
Acked-by: Laurent Pinchart <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

drivers/media/media-entity.c

···
 				continue;
 
 			ret = entity->ops->link_validate(link);
-			if (ret < 0 && ret != -ENOIOCTLCMD)
+			if (ret < 0 && ret != -ENOIOCTLCMD) {
+				dev_dbg(entity->parent->dev,
+					"link validation failed for \"%s\":%u -> \"%s\":%u, error %d\n",
+					entity->name, link->source->index,
+					link->sink->entity->name,
+					link->sink->index, ret);
 				goto error;
+			}
 		}
 
 		/* Either no links or validated links are fine. */
···
 
 		if (!bitmap_full(active, entity->num_pads)) {
 			ret = -EPIPE;
+			dev_dbg(entity->parent->dev,
+				"\"%s\":%u must be connected by an enabled link\n",
+				entity->name,
+				find_first_zero_bit(active, entity->num_pads));
 			goto error;
 		}
 	}