One of the most often repeated misconceptions about Wayland is that it requires hardware acceleration. I would have thought that this issues would have been resolved once the reference compositor, Weston, supported rendering through Pixman. The reason for this misconception is most likely that the earlier versions of Weston required hardware acceleration.
Now KWin in 4.11 introduces the same mistake. Our experimental Wayland support is only available in the OpenGL compositor – even OpenGL ES is not supported (something is broken on my system I cannot start it). I think this is a bad situation. One of the huge advantages of KWin is the exchangeable compositor allowing to switch to software based XRender in case there are no proper drivers available. In fact KWin switches automatically to XRender if it detects a driver which recommends the XRender backend (e.g. software rasterizer).
So last week I started to work on some new Wayland related features and extended the XRender compositor in a way that it renders to a Wayland surface. It’s a pretty simple setup. Our XRender compositor uses an off-screen pixmap for rendering. The adjusted backend uses a pixmap created through XShm, so we get the frame rendered into a shared memory segment. From there we just copy it into a shared memory Wayland buffer to share it with the Wayland compositor. It would be better if we could create the shared Wayland buffer directly from the XShm segment, but I did not see a solution on how to achieve this.
In the end it’s also not really relevant. After all we want to use Wayland and the last thing we want to do on Wayland is using an X11 rendering API for compositing. So we can consider this more as a proof-of-concept and as a task so that I can get more familiar with Wayland It helped me to improve the code, so it was worth the effort. Even if it is a more academic solution it serves a concrete use-case: KWin uses XRender as a fallback and we can expect that the backend works. Given that we have to enforce compositing in future, it’s good to know that all backends work. The current state of this work can be found in branch kwin/wayland-backend of my clone (note: the link might change in case I force push).
If XRender is an academic solution, we are still facing the problem that we lost our software compositor. So I thought I need to also do something about this problem and started to work on a new compositing backend based on QPainter. It’s a quite simple compositor which uses a QImage as the rendering target. The QImage is created directly from the Wayland buffer, so in opposite to the XRender backend we don’t have the needless memory copy from one to another buffer. This is a very interesting compositor as it’s the first part of KWin which has been written from scratch only with Wayland in mind. It doesn’t support X11 (at the moment). This is the first new compositor written in years, so I found some things to improve and which are already under review. And as it was much easier to write a new compositing backend than I expected, I decided to document it in our wiki. It would be totally awesome if someone could have a look at it and write a dedicated compositor for the RaspberryPi The new compositor can be found in branch kwin/qpainter-scene.
The switch to Wayland does not only affect our software compositors, but compositing in general. So far KWin has always had the possibility to not require compositing. This doesn’t make any sense in a Wayland world. So I needed to teach KWin that we might need to enforce compositing. Of course on X11 we still want to have the option to disable compositing, so the changes only affect the Wayland backend. When starting KWin with the Wayland display environment variable set, KWin will require compositing. Even if it is disabled in the config options it will be enabled. Even more it ignores the shortcut to suspend compositing and options like unredirection of fullscreen windows. But still the backend might fail and in this case KWin is just exiting. There is no need to continue execution if one cannot see the output. This work can be found in branch kwin/enforce-compositing (which also contains all the other mentioned branches).