Replacing X11

In Linux and other Unix-like systems, the X stack has grown to encompass functionality arguably belonging in client libraries, helper libraries, or the host operating system kernel. Support for things like PCI resource management, display configuration management, direct rendering, and memory management has been integrated into the X stack, imposing limitations like limited support for standalone applications, duplication in other projects (e.g. the Linux fb layer or the DirectFB project), and high levels of complexity for systems combining multiple elements (for example radeon memory map handling between the fb driver and X driver, or VT switching).

Moreover, X has grown to incorporate modern features like offscreen rendering and scene composition, but subject to the limitations of the X architecture. For example, the X implementation of composition adds additional context switches and makes things like input redirection difficult.

Figure: X with a compositing manager.

The diagram above illustrates the central role of the X server and compositor in operations, and the steps required to get contents on to the screen.

Over time, X developers came to understand the shortcomings of this approach and worked to split things up. Over the past several years, a lot of functionality has moved out of the X server and into client-side libraries or kernel drivers. One of the first components to move out was font rendering, with freetype and fontconfig providing an alternative to the core X fonts. Direct rendering OpenGL as a graphics driver in a client side library went through some iterations, ending up as DRI2, which abstracted most of the direct rendering buffer management from client code. Then cairo came along and provided a modern 2D rendering library independent of X, and compositing managers took over control of the rendering of the desktop as toolkits like GTK+ and Qt moved away from using X APIs for rendering. Recently, memory and display management have moved to the Linux kernel, further reducing the scope of X and its driver stack. The end result is a highly modular graphics stack.