output: fallback to other output modes if preferred mode fails

See sway@4cdc4ac6

Sometimes the preferred mode is not available due to hardware
constraints. In these cases it is better to fallback to lower modes than
to end up with a black screen.

diff --git a/src/output.c b/src/output.c
index a0c8379c06d80facef3edcc1b7db30ed80f82a6f..cf8d1507eb57ff12d767d56093cd32aa1bac7a00 100644 (file)
--- a/src/output.c
+++ b/src/output.c
@@ -828,21 +828,42 @@ output_destroy_notify(struct wl_listener *listener, void *data)
 static void
 new_output_notify(struct wl_listener *listener, void *data)
 {
-       /* This event is rasied by the backend when a new output (aka a display
-        * or monitor) becomes available. */
+       /*
+        * This event is rasied by the backend when a new output (aka display
+        * or monitor) becomes available.
+        */
        struct server *server = wl_container_of(listener, server, new_output);
        struct wlr_output *wlr_output = data;
 
+       /* The mode is a tuple of (width, height, refresh rate). */
+       wlr_log(WLR_DEBUG, "set preferred mode");
+       struct wlr_output_mode *preferred_mode =
+               wlr_output_preferred_mode(wlr_output);
+       wlr_output_set_mode(wlr_output, preferred_mode);
+
        /*
-        * The mode is a tuple of (width, height, refresh rate).
-        * TODO: support user configuration
+        * Sometimes the preferred mode is not available due to hardware
+        * constraints (e.g. GPU or cable bandwidth limitations). In these
+        * cases it's better to fallback to lower modes than to end up with
+        * a black screen. See sway@4cdc4ac6
         */
-       struct wlr_output_mode *mode = wlr_output_preferred_mode(wlr_output);
-       if (mode) {
-               wlr_output_set_mode(wlr_output, mode);
-               wlr_output_commit(wlr_output);
+       if (!wlr_output_test(wlr_output)) {
+               wlr_log(WLR_DEBUG, "preferred mode rejected, falling back to "
+                       "another mode");
+               struct wlr_output_mode *mode;
+               wl_list_for_each(mode, &wlr_output->modes, link) {
+                       if (mode == preferred_mode) {
+                               continue;
+                       }
+                       wlr_output_set_mode(wlr_output, mode);
+                       if (wlr_output_test(wlr_output)) {
+                               break;
+                       }
+               }
        }
 
+       wlr_output_commit(wlr_output);
+
        struct output *output = calloc(1, sizeof(struct output));
        output->wlr_output = wlr_output;
        output->server = server;