backend/drm: add support for the subconnector property

The subconnector property indicates the connector sub-type. This is
useful because that usually indicates what kind of connector the user
has plugged in to their monitor, e.g. a DisplayPort-to-DVI cable will
indicate a DVI subconnector. Also some laptops have non-DP connectors
that are internally linked to a DP port on the GPU.

Set the output description accordingly.

See https://drmdb.emersion.fr/properties/3233857728/subconnector

backend/drm/drm.c

@@ -1349,12 +1349,25 @@ void scan_drm_connectors(struct wlr_drm_backend *drm) {
 			parse_edid(&wlr_conn->output, edid_len, edid);
 			free(edid);
 
+			char *subconnector = NULL;
+			if (wlr_conn->props.subconnector) {
+				subconnector = get_drm_prop_enum(drm->fd,
+					wlr_conn->id, wlr_conn->props.subconnector);
+			}
+			if (subconnector && strcmp(subconnector, "Native") == 0) {
+				free(subconnector);
+				subconnector = NULL;
+			}
+
 			struct wlr_output *output = &wlr_conn->output;
 			char description[128];
-			snprintf(description, sizeof(description), "%s %s %s (%s)",
+			snprintf(description, sizeof(description), "%s %s %s (%s%s%s)",
-				output->make, output->model, output->serial, output->name);
+				output->make, output->model, output->serial, output->name,
+				subconnector ? " via " : "", subconnector ? subconnector : "");
 			wlr_output_set_description(output, description);
 
+			free(subconnector);
+
 			wlr_log(WLR_INFO, "Detected modes:");
 
 			for (int i = 0; i < drm_conn->count_modes; ++i) {

backend/drm/properties.c

@@ -1,3 +1,4 @@
+#define _POSIX_C_SOURCE 200809L
 #include <stdbool.h>
 #include <stddef.h>
 #include <stdint.h>
@@ -24,6 +25,7 @@ static const struct prop_info connector_info[] = {
 	{ "EDID", INDEX(edid) },
 	{ "PATH", INDEX(path) },
 	{ "link-status", INDEX(link_status) },
+	{ "subconnector", INDEX(subconnector) },
 	{ "vrr_capable", INDEX(vrr_capable) },
 #undef INDEX
 };
@@ -151,3 +153,27 @@ void *get_drm_prop_blob(int fd, uint32_t obj, uint32_t prop, size_t *ret_len) {
 	drmModeFreePropertyBlob(blob);
 	return ptr;
 }
+
+char *get_drm_prop_enum(int fd, uint32_t obj, uint32_t prop_id) {
+	uint64_t value;
+	if (!get_drm_prop(fd, obj, prop_id, &value)) {
+		return NULL;
+	}
+
+	drmModePropertyRes *prop = drmModeGetProperty(fd, prop_id);
+	if (!prop) {
+		return NULL;
+	}
+
+	char *str = NULL;
+	for (int i = 0; i < prop->count_enums; i++) {
+		if (prop->enums[i].value == value) {
+			str = strdup(prop->enums[i].name);
+			break;
+		}
+	}
+
+	drmModeFreeProperty(prop);
+
+	return str;
+}

include/backend/drm/properties.h

@@ -17,6 +17,7 @@ union wlr_drm_connector_props {
 		uint32_t link_status; // not guaranteed to exist
 		uint32_t path;
 		uint32_t vrr_capable; // not guaranteed to exist
+		uint32_t subconnector; // not guaranteed to exist
 
 		// atomic-modesetting only
 
@@ -69,5 +70,6 @@ bool get_drm_plane_props(int fd, uint32_t id, union wlr_drm_plane_props *out);
 
 bool get_drm_prop(int fd, uint32_t obj, uint32_t prop, uint64_t *ret);
 void *get_drm_prop_blob(int fd, uint32_t obj, uint32_t prop, size_t *ret_len);
+char *get_drm_prop_enum(int fd, uint32_t obj, uint32_t prop);
 
 #endif