Linux-GGI Project

In this article, we will explain the intentions and goals of the Linux-GGI Project along with the basic concepts used by the GGI programmers to allow fast, easy to use access to graphical services, hide hardware level issues from applications and introduce extensible support for multiple displays under Linux. The Linux-GGI project wants to set up a General Graphical Interface for Linux that will allow easy use of graphical hardware and input facilities under the Linux OS. Already existing solutions and standards like X or OpenGL do deal with graphic's issues, but these current implementations under Linux have several (sometimes serious) drawbacks:

GGI addresses all these points and several more in a clean and extensible way. (GGI does not wish to be a substitute for these existing standards nor does it want to implement its graphical services completely inside the kernel.) Now, let's have a look at the concepts of GGI—some of which have already been implemented and have shown their usability.

The GGI hardware driver consists of a kernel space module called Kernel Graphical Interface (KGI) and a user space library called libGGI. The KGI part of GGI will consist of a display manager that takes care of accessing multiple video cards and does MMU-supported page flipping on older hardware. This method allows for incredibly fast access to the frame buffer from user space whenever possible. (This technique has already been proven—the GO32 graphics library for DJPGG, the GNU-C-compiler for DOS, uses this method and has astonishingly fast graphical support on older hardware.) If this memory-mapped access method can be used in GGI, there will be no loss in performance as the application reads or writes the pixel buffer directly.

Each type of video card in the system has its own driver, a simple loadable module that registers as many displays as the card can address. (Video cards exist that support two monitors or a monitor and a TV screen.) The driver module gives the system the information needed to access the frame buffer and to access special accelerated features, the setup of a certain video mode and the limits of the hardware (e.g., the graphic card, the monitor, and any other part of the display system). The module can either be obtained from a single source file or be linked using precompiled subdrivers for each graphical hardware subsystem (ramdac, monitor, clock chip, chipset, accelerator engine). This last option is the favourite approach, since it allows support for new cards to be added quite easily, as only the subdrivers for hardware not already supported need to be implemented and tested. (The others are already in use or bug fixes there will improve all drivers using them.) This scheme has been used to derive support for many of the S3 accelerator-based cards, and has proved to be very efficient and easy to use. It also allows for efficient simultaneous development for several graphic cards. The subdrivers to be linked together are now selected at configuration time, but they can also be selected after automatic detection or according to a database (not yet built). Note that the subdrivers do not need to be in source form; as a result, precompiled subdriver object files can be linked together during installation.

As each subdriver knows the hardware exactly, it can prevent the display hardware from being damaged due to a bad configuration and make suggestions about the optimal use of the hardware. For example, the current implementation has drivers for fixed- and multisync monitors that allow optimal timings for any resolution to be calculated on the fly without any further configuration. Of course, completely user- configurable drivers are also possible. In short, in addition to the hardware level code, the subsystem drivers provide as much information about the hardware as possible. This way the kernel will have sufficient methods to initialize the card, to reset consoles and video modes when an application gets terminated, and to make optimal use of the hardware. The KGI manager will allow a single kernel image to support GGI on all hardware, as any hardware-specific code is in the loadable module and only common services (such as memory mapping code) are provided from the kernel. The KGI manager will also provide data structures and support to almost any imaginable kind of input devices.

The user space library, called libGGI, will implement an abstract programming interface to applications. It interfaces to the kernel part using special device files and standard file operations. Applications should use this interface (or APIs provided by applications based on it) to gain maximum performance; however, other APIs can be built accessing the special files directly. Understand that in this case the X server will just be a normal application in terms of graphic access. Since X is considered to be the main customer for graphical services, the API will be designed according to the X protocol definition and will implement a set of low level drawing routines required by X servers. The library will use accelerated functions whenever possible and emulate features not efficiently supported by the hardware found. An important feature of future generation graphical hardware is 3D acceleration which easily fits into the GGI point of view. We plan to provide support for 3D features based on MESA, which is close to OpenGL and ensures compatibility with other platforms than Linux.

Another issue when dealing with graphics is game programming as games need the highest possible performance. They also need special support by the video hardware to produce flicker-free animation or realistic images. The current approaches can't support this need in a reasonable way, since they cannot get help from the kernel (e.g., to use retrace interrupts). GGI can provide this support easily and give maximum hardware support to all applications.