drm/vc4: A present but empty dmas disables audio

Overlays are unable to remove properties in the base DTB, but they
can overwrite them. Allow a present but empty 'dmas' property
to also disable the HDMI audio interface.

See: #2489

Signed-off-by: Phil Elwell <[email protected]>

Author: Phil Elwell

drivers/gpu/drm/vc4/vc4_hdmi.c

@@ -2037,10 +2037,12 @@ static int vc4_hdmi_audio_init(struct vc4_hdmi *vc4_hdmi)
 	const __be32 *addr;
 	int index;
 	int ret;
+	int len;
 
-	if (!of_find_property(dev->of_node, "dmas", NULL)) {
+	if (!of_find_property(dev->of_node, "dmas", &len) ||
+	    len == 0) {
 		dev_warn(dev,
-			 "'dmas' DT property is missing, no HDMI audio\n");
+			 "'dmas' DT property is missing or empty, no HDMI audio\n");
 		return 0;
 	}