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Thoughts on Vulkan in KWin

Lately I have been asked a lot about using Vulkan in KWin: in fact almost every blog post in the last few months has questions about it and that seems to me there is something to write about it.

So the quick tldr: I don’t have any plans on adding support for Vulkan. Over the last years I ported to so many new technologies, including at least 3 incompatible OpenGL versions. I am not looking forward to another incompatible OpenGL version (whether it’s called Vulkan or OpenGL doesn’t matter to me on that).

Now let’s look at the strength of Vulkan: going closer to the hardware and doing multi-threaded rendering. KWin still performs the rendering in the main GUI thread. Qt tried to do rendering in an off-thread for QtQuick’s scene-graph, in case of KWin we also do that in the main-gui thread. Reworking our compositor to use threading is a lot of work and would also probably improve the performance with OpenGL. Anyway as long as KWin doesn’t support threaded rendering this improvement by Vulkan is rather moot.

Now let’s look on the closer to hardware: yesterday I did a blog post on how I want to use e.g. KMS Planes to get closer to hardware and bypass the OpenGL compositing. Vulkan will allow us to perform rendering closer to hardware, but if we don’t render at all, we don’t benefit from it. Vulkan is probably more power saving, but not using the rendering will save even more.

So Vulkan can only improve when the scene needs to be rendered: e.g. when you wobble a window or spin the desktop cube. I don’t think that those effects are what we should optimize for and spend our time on and even if: there are low-hanging fruits to optimize using OpenGL, we do not yet use OpenGL 3 or 4 features to improve these effects.

Overall Vulkan looks to me like a lot of work as once again we would have to add a new compositing backend, write a new low level interaction, rewrite all shaders, etc. etc. Going to Vulkan early would mean introducing more complexity to KWin, more different code paths our users might use. It sounds like a mood exercise to do so. But I also doubt that there will be useable Vulkan drivers any time soon.

Vulkan is a great technology which will make graphics much better. But I doubt that an application like KWin is the use-case it was developed for.

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.

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Layered compositing

On X11 the (OpenGL) compositor renders into a single buffer through the overlay window. This is needed to get features like translucency, shadows, wobbled windows or a desktop cube as Xorg itself doesn’t have any support for such features. The disadvantage of this approach is that we basically always have to perform a “copy” of what needs to be rendered. Consider VLC is playing a fullscreen video the compositor needs to take VLC’s video pixmap and render it onto the overlay window. The compositor needs to run, evaluate the scene and then render the one window.

Of course the compositors optimize for this case by only repainting what is needed and repainting from top to bottom in the window stack so that only the one fully opaque window gets rendered. Even more they have features like unredirecting fullscreen windows or (in the case of KWin) blocking compositing while specific windows are active. Unredirecting is a nice idea but it doesn’t really cover the use case. If one has just a small window (a tool tip, menu, on-screen volume change display) on top of the VLC video window, we need to start compositing again. Although we still would like to just forward the one VLC window.

On Wayland the situation looks better. Instead of rendering to an overlay window the compositor can directly interact with the hardware through e.g. DRM. This allows to for example take the buffer provided by VLC and pass it directly to DRM and bypass the compositing all together. But modern hardware allows us to do more than Xorg ever allowed us through “layers”. Be it the raspberry pi, Android’s hwcomposer or Linux’s KMS Planes, the idea is always similar: let the hardware compose the final image together.

If we think of Plasma phone we realize that in most cases we just need three layers: top panel, bottom panel and a maximized window in between. There is no need to do complex composition of the scene at all as all that is needed is putting the three visible (and opaque) windows in the three layers.

On a desktop system it’s a little bit more complicated. Our windows are not all maximized, they have translucent window decorations, shadows, there is a panel with blurred background. Especially the latter one needs a composition through OpenGL. Nevertheless there should be situations where we can make use of layers. Especially in the case of a fullscreen or maximized window this should help making the compositor faster.

Using layers will mean a large change on how KWin’s compositor works. The infrastructure for unredirecting of fullscreen windows cannot be reused in a sensible way as it is evaluated only for fullscreen windows and before the compositor runs. That is before the effects are executed. The decision whether to put a window into a layer needs to come at a later point: once we have evaluated whether we need to compose the complete screen (you might be wobbling a window) or can short pass through layers.

What I have in mind is to give this architecture a try with the VC4 hardware of the raspberry pi which supports layers in a decent way. And to first adjust the QPainter based compositor (as it’s the simpler one) to make use of this architecture. The idea is to experiment with it to get a useful generic KWin-internal backend API, so that we can also use it for OpenGL compositor and even more for our DRM and hwcomposer backend.

For the DRM backend I want to use the new atomic modesetting feature which will be introduced in Linux 4.2. Given that we cannot depend on it anytime soon, this will still need some time till it will be implemented. Overall all of this is not going to happen tomorrow – there are still more important things to work on. But if you, dear reader, are interested on working on this, please contact me. I’m happy to walk you through the code and share my ideas.

As this blog post is mostly about optimizing the compositor by making use of new hardware features which we didn’t have on X11, I can already point out that I will do a follow up blog post on Vulkan.

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.

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A Qt Platform Abstraction plugin for KWin

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.

Blocking roundtrips

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.

BypassWindowManagerHint

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.

Window decorations

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.

Fundraiser

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Should we target EGL as the default?

When we started the compositing work in KWin the only way to initialize an OpenGL context was by using GLX. In fact GLX is even part of the OpenGL library on Linux. Being an X11 window manager and an X11 compositor it was not a big problem.

Years later we started the effort to get KWin using optionally OpenGL ES in addition to normal OpenGL. One of the differences is that OpenGL ES doesn’t use GLX, but rather EGL. So we gained code to setup compositing using EGL. But this was still a compile time switch and only supported together with using OpenGL ES. If I remember correctly the option to create on OpenGL context on top of EGL was not yet available in our drivers back then.

But there was a time when the FLOSS drivers started to support it and since the 4.10 release in beginning of 2013 we can provide a normal OpenGL context over EGL. This option was not really exposed and only bound to an environment variable. Very few users knew about it and this was also wanted. Using EGL was a good way to shoot yourself in the foot given that not all drivers support it and that we had much more testing on the GLX backend.

Now the situation has changed. With 5.x we expose EGL also through a config interface, but more importantly EGL is the only way to get OpenGL on Wayland. The assumption that every user is using GLX doesn’t hold any more. The assumption that we don’t need to spend so much time (as it’s only relevant in the reduced feature set of mobile devices) on the EGL backend is wrong. It must reach the same quality as the GLX backend, otherwise our Wayland experience suffers.

So this raises an important question: should we try targeting EGL as the default? In the past one of the main reasons to not even think about EGL as default is the reason that the NVIDIA driver didn’t support it. If we would have switched to EGL by default it would have meant dealing with the situation that EGL doesn’t work and we need to fall back to GLX. It would have complicated the startup significantly and would have resulted in our user base using different rendering paths.

Now this one road blocker seems to resolve itself. While I haven’t tested it yet, the latest NVIDIA beta driver supports OpenGL over EGL, so we should be able to have the majority of our user base on the same backend after all. Given that it is only a beta driver and the feature is marked as “experimental” it’s probably still some time till we can default to it.

Nevertheless it’s time to start thinking about it. I encourage NVIDIA users to give EGL now a try and to report issues. Also I encourage all other users to report issues they see with the EGL backend. If we get the EGL backend into a good state maybe 5.6 might be a good point in time to default to it and then start to slowly deprecate the GLX backend with the aim to get it removed in the distant future.