There are many ways to interact with computers—a keyboard, a mouse, even a cybersuit. All of these methods have special protocols to report user actions and even need special hardware to be accessed. GGI will allow any kind of input to be supported without recompiling the kernel for each new device, thus allowing for flexibility and easy configuration. This support is achieved by having a loadable module for each device or device class. Just like the video card drivers, any input device driver will register abstract input devices that convert user actions to events.
For example, an application might query for the registered devices and select the events it wants to receive, so that a game program could default to use joysticks or keyboard input depending on the environment. Installing a game or an X server will not require any further configuration other than copying the binary to its destination directory and starting it. Please note that this methodology will also considerably reduce the effort required to maintain several differently-equipped machines as the application binaries will be the same for all machines and can be shared via network file systems. Only the GGI modules to be loaded will differ from machine to machine.
GGI defines a console as a pair—a display and a (mandatory) character input device. Optionally, other input facilities like a pointing device or controllers attached to a console may be present. The display is capable of providing alphanumeric data or graphics while the character device provides character input (just as the name implies). We use these diffuse terms as the display actually might be something other than a monitor, e.g., braille lines or other devices that help disabled or handicapped people to work with computers. Similarly, the input might be a keyboard or a voice recognition program or hardware—just about anything you can imagine. However, the character input device is mandatory, because it focuses on one and only one virtual console that is shown on one of the displays registered by the loaded modules. Any other devices are associated with one of the keyboards, and any user activity is reported to applications running on the focused console. Thus, it is not only possible to have multiple virtual consoles but also (in conjunction with multiple displays) to have several real consoles.
If the user wants to switch between two virtual consoles, the keyboard driver will tell the KGI manager to bring the specified virtual console on the display assigned to it and then report any keyboard, pointer and controller events to the application. One problem arising from the virtualization is that an application accessing accelerated features might first have to terminate the current command or that the frame buffer needs to be preserved even if the application goes into background mode. GGI will effectively hide this operation from the application. Applications can be placed into one of the following categories with examples given:
The application can redraw its screen without noticeable overhead at any given time, e.g., X server.
The application can be programmed considerably more easily when a back-up buffer is provided in case the frame buffer needs to be accessible at any time, e.g., a ray tracer or any other program that needs to do a lot of calculations to draw an image. This back-up would also allow running the application in background mode while continuing to draw to its frame buffer.
The application can skip output or simply sleep, if not in foreground mode, thereby reducing system load significantly, e.g., games or software video decoders. SVGALIB works in this manner.
Class one and three are easy to virtualize—they just have to redraw their buffers when switched to foreground mode, and therefore, when switching to background mode, the screen contents are discarded and drawing requests are ignored. The only difficult class is class two. However, since the kernel knows the exact state of the hardware, it can tell a user space daemon to allocate sufficient memory, save the frame buffer there, redirect the memory mapping of the application and tell the library to use optimized drawing methods for memory- mapped buffers instead of accelerated drawing functions.
GGI plans to add powerful graphical hardware support to the Linux operating system. As with any hardware driver, it needs to have a kernel segment that is kept to a minimum (currently the modules are about 30K in size, and should not become greater than 80K). If accepted by the Linux community, GGI can provide a clean method of dealing with multiple display and input hardware as well as an architecture-independent programming interface that will give good performance on any platform. Also, it will allow hardware manufacturers to provide optimized drivers for their hardware if they wish. During development much care has been and will continue to be taken to isolate machine or hardware-dependent code, whenever possible, in order to provide good portability.
As GGI is still under development, several features are not yet implemented, but there is a first implementation that demonstrates that our concepts are capable of providing easy access to video hardware and solving all of the points addressed in this article. Currently being worked on is support for multiple displays and libGGI. Of course, introducing a new concept to the kernel to access video hardware will cause several (non-X) applications to be incompatible, but on the other hand, adding this concept will ease the configuration of Linux, and open up new vistas to game programmers with an operating system and graphical support that will allow maximum performance on any system.
Andreas Beck (firstname.lastname@example.org) studies physics at the University of Duesseldorf, Germany and started the GGI project. He developed the memory mapping code for GGI, worked on the library implementation and made major contributions to the concepts used.
Steffen Seeger (email@example.com) also studies physics at the University of Technology at Chemnitz-Zwickau. He wrote most of the S3 driver code and made major contributions to the console concepts and the kernel drivers.
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