Running Weston

libweston uses the concept of a back-end to abstract the interface to the underlying environment where it runs on. Ultimately, the back-end is responsible for handling the input and generate an output. Weston, as a libweston user, can be run on different back-ends, including nested, by using the wayland backend, but also on X11 or on a stand-alone back-end like DRM/KMS and now deprecated fbdev.

In most cases, people should allow Weston to choose the backend automatically as it will produce the best results. That happens for instance when running Weston on a machine that already has another graphical environment running, being either another wayland compositor (e.g. Weston) or on a X11 server. You should only specify the backend manually if you know that what Weston picks is not the best, or the one you intended to use is different than the one loaded. In that case, the backend can be selected by using -B [backend.so] command line option. As each back-end uses a different way to get input and produce output, it means that the most suitable back-end depends on the environment being used.

Available back-ends:

  • drm – run stand-alone on DRM/KMS and evdev (recommend) (DRM kernel doc)

  • wayland – run as a Wayland application, nested in another Wayland compositor instance

  • x11 – run as a x11 application, nested in a X11 display server instance

  • rdp – run as an RDP server without local input or output

  • headless – run without input or output, useful for test suite

  • fbdev – run stand-alone on fbdev/evdev (deprecated)

The job of gathering all the surfaces (windows) being displayed on an output and stitching them together is performed by a renderer. By doing so, it is compositing all surfaces into a single image, which is being handed out to a back-end, and finally, displayed on the screen.

libweston has a CPU-based type of renderer by making use of the Pixman library, but also one that can make use of the GPU to do that, which uses OpenGL ES and it is simply called the GL-renderer.

Most of the back-ends provide a command line option to disable the GL-renderer, and use the CPU for doing that. That happens by appending to the command line --use-pixman when running Weston. One might use the CPU-based renderer to exclude any other potential issues with the GL-renderer.

Additional set-up steps

Depending on your distribution some additional set-up parts might be required, before actually launching Weston, although any fairly modern distribution should have it already set-up for you. Weston creates its unix socket file (for example, wayland-1) in the directory specified by the required environment variable $XDG_RUNTIME_DIR. Clients use the same variable to find that socket. Normally this should already be provided by systemd. If you are using a distribution that does not set-up $XDG_RUNTIME_DIR, you must set it using your shell profile capability. More info about how to set-up that up, which depends to some extent on your shell, can be found at Building/Running Weston

Running Weston in a graphical environment

As stated previously, if you are already in a graphical environment, Weston would infer and attempt to load up the correct back-end. Either running in a Wayland compositor instance, or a X11 server, you should be able to run Weston from a X terminal or a Wayland one.

Running Weston on a stand-alone back-end

Now that we are aware of the concept of a back-end and a renderer, it is time to introduce the concept of a seat, as stand-alone back-ends require one. A seat is a collection of input devices like a keyboard and a mouse, and output devices (monitors), forming the work or entertainment place for one person. It could also include sound cards or cameras. A single computer could be serving multiple seats.

Note

A graphics card is required to be a part of the seat, as among other things, it effectively drives the monitor.

By default Weston will use the default seat named seat0, but there’s an option to specify which seat Weston must use by passing --seat argument.

You can start Weston from a VT assuming that there’s a seat manager supported by libseat running, such as seatd or logind. The backend to be used by libseat can optionally be selected with $LIBSEAT_BACKEND. If libseat and seatd are both installed, but seatd is not already running, it can be started with sudo -- seatd -g video. If no seat manager supported by libseat is available, you can use the weston-launch application that can handle VT switching.

Another way of launching Weston is via ssh or a serial terminal. The simplest option here is to use the libseat launcher with seatd. The process for setting that up is identical to the one described above, where one just need to ensure that seatd is running with the appropriate arguments, after which one can just run weston. Alternatively and as a last resort, one can run Weston as root, specifying the tty to use on the command line: If TTY 2 is active, one would run weston --tty 2 as root.

Running Weston on a different seat on a stand-alone back-end

While Weston can be tested on top of an already running Wayland compositor or an X11 server, another option might be to have an unused GPU card which can be solely used by Weston. So, instead of having a dedicated machine to run Weston for trying out the DRM-backend, by just having an extra GPU, one can create a new seat that could access the unused GPU on the same machine (and potentialy other inputs) and assign it to that seat. All of the happening while you already have your graphical environment running.

In order to have that set-up, the requirements/steps would be:

  • have an extra GPU card – you could also use integrated GPUs, while your other GPU is in use by another graphical environment

  • create a udev file that assigns the card (and inputs) to another seat

  • start Weston on that seat

Start by creating a udev file, under /etc/udev/rules.d/ adding something similar to the following:

ACTION=="remove", GOTO="id_insecure_seat_end"

SUBSYSTEM=="drm", KERNEL=="card*", KERNELS=="0000:00:02.0", ENV{ID_SEAT}="seat-insecure"

SUBSYSTEM=="input", ATTRS{idVendor}=="222a", ATTRS{idProduct}=="004d", OWNER="your_user_id", ENV{ID_SEAT}="seat-insecure", ENV{WL_OUTPUT}="HDMI-A-1"
SUBSYSTEM=="input", ATTRS{idVendor}=="03f0", ATTRS{idProduct}=="1198", OWNER="your_user_id", ENV{ID_SEAT}="seat-insecure"

LABEL="id_insecure_seat_end"

By using the above udev file, devices assigned to that particular seat will be skipped by your normal display environment. Follow the naming scheme when creating the file (man 7 udev). For instance you could use 63-insecure-seat.rules as a filename, but take note that other udev rules might also be present and could potentially affect the way in which they get applied. Check that no other rules might take precedence before adding this new one.

Warning

This seat uses on purpose the name seat-insecure, to warn users that the input devices can be eavesdropped. Futher more, if you attempt doing this on a VT, without being already in a graphical environment (and although the udev rules do apply), there will be nothing stopping the events from input devices reaching the virtual terminal.

In the example above, there are two input devices, one of which is a touch panel that is being assigned to a specific output (HDMI-A-1) and another input which a mouse. Notice how ENV{ID_SEAT} and ENV{WL_OUTPUT} specify the name of the seat, respectively the input that should be assign to a specific output.

Resolving or extracting the udev key/value pair names, can be easily done with the help of udevadm command, for instance issuing udevadm info -a /dev/dri/cardX would give you the entire list of key values names for that particular card. Archaically, one would might also use lsusb and lspci commands to retrieve the PCI vendor and device codes associated with it.

If there are no input devices the DRM-backend can be started by appending --continue-without-input or by editing weston.ini and adding to the core section require-input=false.

Then, weston can be run by selecting the DRM-backend and the seat seat-insecure:

./weston -Bdrm-backend.so --seat=seat-insecure

If everything went well you should see weston be up-and-running on an output connected to that DRM device.