Why did we replace wlc? May 27, 2018 on Drew DeVault's blog

For a little over a year, I’ve been working with a bunch of talented C developers to build a replacement for the wlc library. The result is wlroots, and we’re still working on completing it and updating our software to use it. The conventional wisdom suggests that rewriting your code from scratch is almost never the right idea. So why did we do it, and how is it working out? I have spoken a little about this in the past, but we’ll answer this question in detail today.

Sway will have been around for 3 years as of this August. When I started the project, I wanted to skip some of the hard parts and get directly to implementing i3 features. To this end, I was browsing around for libraries which provided some of the low-level plumbing for me - stuff like DRM (Display Resource Management) and KMS (Kernel Mode Setting), EGL and GLES wires, libinput support, and so on. I was more interested in whatever tool could get me up to speed and writing sway-specific code quickly. My options at this point came down to wlc and swc.

swc’s design is a little bit better in retrospect, but I ended up choosing wlc for the simple reason that it had an X11 backend I could use for easier debugging. If I had used swc, I would have been forced to work without a display server and test everything under the DRM backend - which would have been pretty annoying. So I chose wlc and go to work.

Designwise, wlc is basically a Wayland compositor with a plugin API, except you get to write main yourself and the plugin API communicates entirely in-process. wlc has its own renderer (which you cannot control) and its own desktop with its own view abstraction (which you cannot control). You have some events that it bubbles up for you and you can make some choices like where to arrange windows. However, if you just wire up some basics and run wlc_init, wlc will do all of the rest of the work and immediately start accepting clients, rendering windows, and dispatching input.

Over time we were able to make some small improvements to wlc, but sway 0.x still works with these basic principles today. Though this worked well at first, over time more and more of sway’s bugs and limitations were reflections of problems with wlc. A lengthy discussion on IRC and on GitHub ensued and we debated for several weeks on how we should proceed. I was originally planning on building a new compositor entirely in-house (similar to GNOME’s mutter and KDE’s kwin), and I wanted to abstract the i3-specific functionality of sway into some kind of plugin. Then, more “frontends” could be written on top of sway to add functionality like AwesomeWM, bspwm, Xmonad, etc.

After some discussion among the sway team and with other Wayland compositor projects facing similar problems with wlc, I decided that we would start developing a standalone library to replace wlc instead, and with it allow a more diverse Wayland ecosystem to flourish. Contrary to wlc’s design - a Wayland compositor with some knobs - wlroots is a set of modular tools with which you build the Wayland compositor yourself. This design allows it to be suited to a huge variety of projects, and as a result it’s now being used for many different Wayland compositors, each with their own needs and their own approach to leveraging wlroots.

When we started working on this, I wasn’t sure if it was going to be successful. Work began slowly and I knew we had a monumental task ahead of us. We spent a lot of time and a few large refactorings getting a feel for how we wanted the library to take shape. Different parts matured at different paces, sometimes with changes in one area causing us to rethink design decisions that affected the whole project. Eventually, we fell into our stride and found an approach that we’re very happy with today.

I think that the main difference with the approach that wlroots takes comes from experience. Each of the people working on sway, wlc, way cooler, and so on were writing Wayland compositors for the first time. I’d say the problems that arose as a result can also be seen throughout other projects, including Weston, KWin, and so on. The problem is that when we all set out, we didn’t fully understand the opportunities afforded by Wayland’s design, nor did we see how best to approach tying together the rather complicated Linux desktop stack into a cohesive project.

We could have continued to maintain wlc, fixed bugs, refactored parts of it, and maybe eventually arrived at a place where sway more or less worked. But we’d simply be carrying on the X11 tradition we’ve been trying to escape this whole time. wlc was a kludge and replacing it was well worth the effort - it simply could not have scaled to the places where wlroots is going. Today, wlroots is the driving force behind 6 Wayland compositors and is targeting desktops, tablets, and phones. Novel features never seen on any desktop - even beyond Linux - are possible with this work. Now we can think about not only replacing X11, but innovating in ways it never could have.

Our new approach is the way that Wayland compositors should be made. wlroots is the realization of Wayland’s potential. I am hopeful that our design decisions will have a lasting positive impact on the Wayland ecosystem.

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