Supporting multiple outputs is something most of us take for granted in the world of X11 because the Xorg team has had years to implement and perfect support for multiple outputs. While we have all enjoyed the fruits of their labor, this has not been the case for Wayland. There are two primary types of multiple output configurations that are relevant: cloned and extended outputs.
Cloned output mode is the one that most people are familiar with. This means that the contents of the primary output will be duplicated on any additional outputs that are enabled. If you have ever hot plugged an external monitor into your Windows laptop, then you have seen this mode in action.
Extended output mode is somewhat less common, yet still very important. In this mode, the desktop is extended to span across multiple outputs, while the primary output retains sole ownership of any panels, shelves, or gadgets that reside there. This enables the secondary monitor to act as an extended desktop space where other applications can be run.
Multiple Output Support in Weston and EFL
Weston, the reference Wayland compositor, has had the ability to support multiple outputs in an extended configuration for quite some time now. This extended mode configuration allows each output to run with completely separate framebuffer and repaint loops. There are patches currently being worked on to enable cloned output support, but these have not been pushed upstream yet.
While EFL has had support for multiple outputs when running under X11 for quite some years now, the support for multiple outputs under Wayland has been missing. A major hurdle to implementing this support has been that Evas, the EFL rendering library, was not accounting for multiple outputs in it’s rendering update loop. With this commit, our very own Cedric Bail has removed that hurdle and enabled work to progress further on implementing this feature.
Complete Multiple Output Support is on the Way
While the current implementation of multiple output support in EFL Wayland only supports outputs in a cloned configuration, support for extended mode is in development and is expected to be available in upstream EFL soon. When these patches get published upstream, this will bring our EFL Wayland implementation much closer in parity to our X11 support. Thankfully there are little to no changes needed to add this support to our Enlightenment Wayland compositor.