how to launch wayland app on second display?

Question

I have a yocto/poky build of linux using Weston/Wayland. I have 2 displays. The desktop environment is showing on both displays, but I can only launch apps on display-1 from the command line (ssh shell). I'm just trying to run weston-simple-egl.

This command works and I see the app running on display-1:

WAYLAND_DISPLAY=wayland-1 XDG_RUNTIME_DIR=/run/user/1000 /usr/bin/weston-simple-egl

But this fails:

WAYLAND_DISPLAY=wayland-2 XDG_RUNTIME_DIR=/run/user/1000 /usr/bin/weston-simple-egl

weston-simple-egl: ../weston-11.0.1/clients/simple-egl.c:1157: int main(int, char **): Assertion `display.display' failed.
Aborted

I also tried other display-ids: 0, 2, 3, 4...

Is there a way to check which display-ids are valid? Any logs I should be looking at? or other ways to debug this?

Edit: The display is currently 'extended' I can move an app from one to the other. In the XDG_RUNTIME_DIR, I see only wayland-1:

# ls /run/user/1000          
wayland-1       wayland-1.lock

Answer 1

It seems this is not possible via any weston environment variable or configuration.

The answer was to modify the app itself, using this wayland api:

xdg_toplevel::set_fullscreen(output: object<wl_output>)

Calling set_fullscreen(NULL) will allow the compositor to fullscreen the window on a display of its choosing. But you can iterate over wl_outputs and choose one to use. I added a command line option to set the output index.

Example for glmark2:

- output = display_->outputs.at(0);
+ output = display_->outputs.at(NativeStateWayland::outputIndex);

Example for weston-simple-egl:

create_surface(struct window *window)
 {
    struct display *display = window->display;
+   struct wl_output *wl_output = NULL;
+   if (window->outputIndex != -1) {
+       int index = 0;
+       struct output *itr = NULL;
+       wl_list_for_each(itr, &display->output_list, link) {
+           if (index == window->outputIndex) {
+               wl_output = itr->wl_output;
+               break;
+           }
+           index++;
+       }
+   }
    ...
    ...
    if (window->fullscreen) {
-       xdg_toplevel_set_fullscreen(window->xdg_toplevel, NULL);
+       xdg_toplevel_set_fullscreen(window->xdg_toplevel, wl_output);
    }

It was not possible to select a display without going fullscreen.

resources:

• https://wayland.app/protocols/xdg-shell#xdg_toplevel:request:set_fullscreen
• https://wayland.freedesktop.org/ (IRC chat helped me find the answer)