KDE's Nate Graham On X11 Being A Bad Platform & The Wayland Future

Over on Nate's other blog entry he indicates this:

The fundamental X11 development model was to have a heavyweight window server–called Xorg–which would handle everything, and everyone would use it. Well, in theory there could be others, and at various points in time there were, but in practice writing a new one that isn’t a fork of an old one is nearly impossible

And I think this is something people tend to forget. X11 as a protocol is complex and writing an implementation of it is difficult to say the least. Because of this, we've all kind of relied on Xorg's implementation of it and things like KDE and GNOME piggyback on top of that. However, nothing (outside of the pure complexity) prevented KWin (just as an example) implementing it's own X server. KWin having it's own X server would give it specific things that would better handle the things KWin specifically needed.

Good parallel is how crazy insane the HTML5 spec has become and how now pretty much only Google can write a browser for that spec (with thankfully Firefox also keeping up) and everyone is just cloning that browser and putting their specific spin to it. But if a deep enough core change happens, that's likely to find its way into all of the spins. And that was some of the issue with X. Good example here, because of the specific way X works an "OK" button (as an example) is actually implemented by your toolkit as a child window. Menus those are windows too. In fact pretty much no toolkit uses primitives anymore. It's all windows with lots and lots of text attributes. And your toolkit Qt, Gtk, WINGs, EFL, etc handle all those attributes so that events like "clicking a mouse button" work like had you clicked a button and not a window that's drawn to look like a button.

That's all because these toolkits want to do things that X won't explicitly allow them to do. Now the various DEs can just write an X server that has their concept of what a button should do, how it should look, etc... And that would work except that, say you fire up GIMP that uses Gtk and Gtk has it's idea of how that widget should look and work and boom things break with the KDE X server. That's because of the way X11 is defined. There's this middle man that always sits there dictating how things work. Clients draw to you, not to the screen in X. And that's fundamentally how X and Wayland are different.

I think people think of Wayland in the same way of X11. That there's this Xorg that exists and we'll all be using it and configuring it. And that's not wholly true. In X we have the X server and in that department we had Xorg/XFree86 (and some other minor bit players). The analog for that in Wayland (roughly, because Wayland ≠ X) is the Compositor. Of which we have Mutter, Clayland, KWin, Weston, Enlightenment, and so on. Which that's more than just one that we're used to. That's because the Wayland protocol is simple enough for these multiple implementations.

The skinny is that a Compositor needs to at the very least provide these:

• wl_display - This is the protocol itself.
• wl_registry - A place to register objects that come into the compositor.
• wl_surface - A place for things to draw.
• wl_buffer - When those things draw there should be one of these for them to pack the data into.
• wl_output - Where rubber hits the road pretty much, wl_surface should display wl_buffer onto this thing.
• wl_keyboard/wl_touch/etc - The things that will interact with the other things.
• wl_seat - The bringing together of the above into something a human being is interacting with.

And that's about it. The specifics of how to interface with hardware and what not is mostly left to the kernel. In fact, pretty much compositors are just doing everything in EGL, that is KWin's wl_buffer (just random example here) is a eglCreatePbufferSurface with other stuff specific to what KWin needs and that's it. I would assume Mutter is pretty much the same case here. This gets a ton of the formality stuff that X11 required out of the way and allows Compositors more direct access to the underlying hardware. Which was pretty much the case for all of the Window Managers since 2010ish. All of them basically Window Manage in OpenGL because OpenGL allowed them to skip a lot of X, but of course there is GLX (that one bit where X and OpenGL cross) but that's so much better than dealing with Xlib and everything it requires that would routinely require "creative" workarounds.

This is what's great about Wayland, it allows KWin to focus on what KWin needs, mutter to focus on what mutter needs, but provides enough generic interface that Qt applications will show up on mutter just fine. Wayland goes out of its way to get out of the way. BUT that means things we've enjoyed previously aren't there, like clipboards, screen recording, etc. Because X dictated those things and for Wayland, that's outside of scope.