Multiscreen in Plasma 5.7 and beyond

Here’s a quick status update about where we currently stand with respect to multiscreen support in Plasma Desktop.

While for many people, multiscreen support in Plasma works nicely, for some of our users, it doesn’t. There are problems with restoring previously set up configurations, and around the primary display mechanism. We’re really unhappy about that, and we’re working on fixing it for all of our users. These kinds of bugs are the stuff nightmares are made of, so there’s not a silver bullet to fix everything of it, once and for all right away. Multiscreen support requires many different components to play in tune with each other, and they’re usually divided into separate processes communicating via different channels with each other. There’s X11 involved, XCB, Qt, libkscreen and of course the Plasma shell. I can easily at least three different protocols in this game, Wayland being a fourth (but likely not used at the same time as X11). There’s quite some complexity involved, and the individual components involved are actually doing their jobs quite well and have their specific purposes. Let me give an overview.

Multiscreen components

Plasma Shell renders the desktop, places panels, etc., When a new screen is connected, it checks whether it has an existing configuration (wallpaper, widgets, panels etc.) and extends the desktop. Plasma shell gets its information from QScreen now (more on that later on!)

KWin is the compositor and window manager. KWin/X11 interacts with X11 and is responsible for window management, movement, etc.. Under Wayland, it will also take the job of the graphical and display server work that X11 currently does, though mostly through Wayland and *GL APIs.

KScreen kded is a little daemon (actually a plugin) that keeps track of connected monitors and applies existing configs when they change

KScreen is a module in systemsettings that allows to set up the display hardware, positioning, resolution, etc.

Libkscreen is the library that backs the KScreen configuration. It offers an API abstraction over XRandR and Wayland. libkscreen sits pretty much at the heart of proper multiscreen support when it comes to configuring manually and loading the configuration.

Primary Desktop

The primary display mechanism is a bit of API (rooted in X11) to mark a display as primary. This is used to place the Panel in Plasma, and for example to show the login manager window on the correct monitor.

Libkscreen and Qt’s native QScreen are two different mechanism to reflect screen information. QScreen is mainly used for querying info (and is of course used throughout QtGui to place windows, get information about resolution and DPI, etc.). Libkscreen has all this information as well, but also some more, such as write support. Libkscreen’s backends get this information directly from Xorg, not going through Qt’s QScreen API. For plasmashell, we ended up needing both, since it was not possible to find the primary display using Qt’s API. This causes quite some problems since X11 is async by its nature, so essentially we ended up having “unfixable” race conditions, also in plasmashell. These are likely the root cause of the bug you’re seeing here.

This API has been added in Qt 5.6 (among a few other fixes) by Aleix Pol, one of our devs in the screen management team. We have removed libkscreen from plasmashell today and replaced it with “pure QScreen” code, since all the API we need for plasmashell is now available in the Qt we depend on.

These changes should fix much of the panel placement grief that bug 356225 causes. It does need some good testing, now it’s merged. Therefore, we’d like to see as many people, especially those reporting problem with multiscreen, to test against latest Plasma git master (or the upcoming Plasma 5.7 Beta, which is slated for release on June, 16th).

Remember the config

Another rough area that is under observation right now is remembering and picking the right configuration from a previous setup, for example when you return to your docking station which has another display connected. Bug 358011 is an example for that. Here, we get “spurious” events about hardware changes from X11, and I’m unsure where they come from. The problem is that it’s not easy to reproduce, it only happens for certain setups. This bug was likely introduced with the move to Qt 5 and Frameworks, it’s a regression compared to Plasma 4.
I’ve re-reviewed the existing code, added more autotests and made the code more robust in some places that seemed relevant, but I can’t say that we’ve found a sure solution to these problems. The code is now also better instrumented for debugging the areas at play here. Now we need some more testing of the upcoming beta. This is certainly not unfixable, but needs feedback from testing so we can apply further fixes if needed.

Code quality musings

From a software engineering point of view, we’re facing some annoying problems. It took us a long time to get upstream QScreen code to be robust and featureful enough to draw the lines between the components involved, especially QScreen and libkscreen more clearly, which certainly helps to reduce hard-to-debug race conditions involving hardware events. The fact that it’s almost impossible to properly unit test large parts of the stack (X11 and hardware events are especially difficult in that regard) means that it’s hard to control the quality. On the other hand, we’re lacking testers, especially those that face said problems and are able to test the latest versions of Qt and Plasma.
QA processes is something we spent some serious work on, on the one hand, our review processes for new code and changes to current code are a lot stricter, so we catch more problems and potential side-effects before code gets merged. For new code, especially the Wayland support, our QA story also looks a lot better. We’re aiming for near-100% autotest coverage, and in many cases, the autotests are a lot more demanding than the real world use cases. Still, it’s a lot of new code that needs some real world exposure, which we hope to get more of when users test Plasma 5.7 using Wayland.