Bus Systems

Whereas expansion devices are addressed by device drivers that communicate with the remaining code only via a fixed set of interfaces and therefore have no effect on the core kernel sources, the kernel is also responsible for a more basic issue — how devices are attached to the rest of the system by means of buses.

Bus drivers are much more closely linked with the central kernel code than drivers for specific devices can ever be. Also, there is no standardized interface via which a bus driver makes its functions and options available to associated drivers. This is because the hardware techniques used differ greatly between the various bus systems. However, this does not mean that the code responsible for managing the different buses has no commonalities. Similar buses adopt similar concepts, and the generic driver model has been introduced to manage all system buses in a collection of central data structures, reducing them as far as possible to the smallest common denominator.

The kernel supports a large number of buses, sometimes on several hardware platforms, sometimes on just a single platform. It is therefore impossible to discuss all versions in detail. I shall therefore limit us to a close examination of the PCI bus since its design is relatively modern, it features all the common and key elements of a powerful system bus, and it is used on most architectures supported by Linux. I shall also discuss the widely used and system-independent USB for external peripherals.18

18Whether this is a classical bus is a matter of controversy because USB does not offer the functionality of a system bus but is reliant on an additional distribution mechanism ''within the computer.'' I take a pragmatic approach and am little concerned with this controversial issue.

Continue reading here: The Generic Driver Model

Was this article helpful?

0 0