In Qt we have the Platform Abstraction (QPA) which allows to better interact with the used windowing system through a plugin. In case of KWin we use the “xcb” plugin on X11 and on Wayland we used to use the “wayland” plugin provided by QtWayland. For quite some time I had been thinking about migrating away from those and use an own KWin-specific plugin at least for Wayland.
What’s wrong with QtWayland?
QtWayland provides a plugin for Wayland clients, but KWin is not a Wayland client: it is the Wayland server. This created some interesting “problems” which we had to workaround in KWin. KWin just doesn’t fit the usecase of QtWayland QPA plugin and it would not be a good idea to change QtWayland in a way that it supports KWin as a first-class citizen – it’s just a too special use case. So let’s look at some of the issues which motivated me to write an own QPA plugin.
Connecting to the Wayland server
The wayland plugin is meant for Wayland clients and tries to connect to the Wayland server during the creation of the QGuiApplication. As KWin is the Wayland server the plugin wants to connect to, we needed to make sure that the Wayland server and the event loop are up and running before constructing the QGuiApplication. This added a restriction that we have to monitor the server socket from the main thread.
The wayland plugin performs a blocking roundtrip to the Wayland server during startup to wait for all outputs to be announced. But this happens from the main gui thread, in which also the server lives. You can guess what happens: dead-lock. We fixed this one in QtWayland, but the problem was still there: any blocking call from the main gui thread to the Wayland server would freeze KWin. We experienced this with e.g. eglInitialize, startup of dbus services (e.g. kded, kamd), rendering of Qt internal windows in case the compositor didn’t send the frame rendered yet. Overall I found this aspect way to fragile and consider especially the fact that the server is in the main thread as a problem.
KWin uses the Qt::BypassWindowManagerHint on all it’s windows because this is needed on X11. The hint is X11 specific and doesn’t make much sense on other platforms, so the Wayland plugin doesn’t show such windows at all. That is a valid approach – there is nothing I could say against it. So in KWin we needed to remove the hint for all windows on Wayland.
OpenGL Context for QtQuick
QtQuick requires an OpenGL context and if it cannot create one it will abort. That is normally not a problem but can easily become one in the case of KWin: KWin creates a “low-level” OpenGL context using the specific platform (e.g. DRM/GBM, hwcomposer, whatever). Qt(Wayland) doesn’t know which one KWin uses and might not even have code for it. QtWayland tries to create an OpenGL context using the EGL platform Wayland. In the case of Mesa everything works fine, but we noticed that on libhybris the creation of a Wayland context fails if there is already one created for hwcomposer. One could consider this as a libhybris specific issue, but I wouldn’t say so – interacting with multiple EGL platforms from one process is kind of out of the specification.
This is a small issue: QtWayland creates window decorations around all Qt windows. But in the context of KWin we do not want any decorations. So we had to disable them by setting the QT_WAYLAND_DISABLE_WINDOWDECORATION environment variable.
And here is the plugin
So after Akademy I decided to work on it and see whether we can get a plugin working. Now it’s in a state where it works good enough to blog about it. The plugin does not support everything that might be used by Qt, but rather reduces to the feature set used by KWin. For example we don’t need to forward any input events, because KWin (mostly) handles them internally anyway and sends them directly to the respective internal Qt window.
The plugin brings the advantage that it no longer depends on the Wayland server being started before creating the QApplication. In fact it does not even try to connect to it, but can use KWin’s internal connection. Similar we can use this to shortcut e.g. creation of low level windows, etc.
On the OpenGL side the plugin approach shows it’s true strength. As it can access KWin internals we can use the EGLDisplay used by the compositing scene and create a sharing OpenGL context. So we can bypass the windowing system completely to render QtQuick windows and this will also allow us to make our window textures available to QtQuick. So a nice improvement.
In case our compositor doesn’t use OpenGL the plugin creates an OpenGL context for the platform Wayland. This is done in a way that it doesn’t hit the blocking issue I mentioned above.
So is the plugin something you should use in other applications?
The plugin interacts with KWin internals and thus is not in any way useable for non-KWin applications. In fact it even checks whether the running binary is KWin and does nothing if that’s not the case. It will also never be turned into a generic plugin as there is no need for that (just use QtWayland).
When will this plugin get merged?
The plugin can be found in branch “qpa” on my personal kwin clone on git.kde.org. It’s not yet in a state that I could push it to master. I still need to test more and make sure that everything works as intended.
Now to something else!
KDE is currently running a fundraiser for the Randa Meetings with the topic “Bring Touch to KDE!”. I personally will not participate in the Randa Meetings, but many other KDE developers will go there and we need your support for it. Please help and make the Randa Meetings possible.
6 Replies to “A Qt Platform Abstraction plugin for KWin”
Not about this Post, but because of it i came back into my mind.
what will happen to “wayland” and KWin in case of Khronos Vulkan? Would is be possible to use Vulkan in KWin “on” Wayland?
You need OpenGL ES for Wayland, right? But Since Vulkan will be much faster, for reasons, that would be a better choice. But would it be possible to use Vulkan and still load a Wayland compatible compositor?
(i know.. nothing to care about for the moment, but would like to know)
Not answering all of it, because there will be a note on Vulkan here, soon.
No! KWin supports OpenGL, OpenGL ES or no GL at all through the QPainter backend.
Can you explain why Qt::BypassWindowManagerHint is specific to X11? This is not clear for me.
Funny fact “qpa” is readed in Polish as “kupa” thats means poop 😛
It’s documented like that. Also it’s Qt::X11BypassWindowManagerHint
Could it be that Qt is lacking in its abstraction capabilities? I would think that if Qt effectively abstracted away the differences between X and Wayland, glx and egl, OpenGL, ES and DirectX – and possibly even Mir and whatever Android uses, KDE including kwin would be able run on any platform without modification.
If your new plug-in is a step to achieving more platform independence then that sounds great!
No, Qt’s abstraction is awesome!
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