Linux Device Drivers, Second Edition

External Buses

One of the most recent entries in the field of interface buses is the whole class of external buses. This includes USB, FireWire, and IEEE1284 (parallel-port-based external bus). These interfaces are somewhat similar to older and not-so-external technology such as PCMCIA/CardBUS and even SCSI.

Conceptually, these buses are neither full-featured interface buses (like PCI is) nor dumb communication channels (like the serial ports are). It’s hard to classify the software that is needed to exploit their features, as it’s usually split into two levels: the driver for the hardware controller (like drivers for PCI SCSI adaptors or PCI controllers introduced earlier in Section 15.1) and the driver for the specific “client” device (like sd.c handles generic SCSI disks and so-called PCI drivers deal with cards plugged in the bus).

But there’s another problem with these new buses. With the exception of USB, their support is either not mature or is somehow in need of a revision (the latter condition applies especially to the SCSI kernel subsystem, which is reported to be far from optimal by several of the best kernel hackers).

USB

USB, the Universal Serial Bus, is the only external bus that is currently mature enough to deserve some discussion. Topologically, a USB subsystem is not laid out as a bus; it is rather a tree built out of several point-to-point links. The links are four-wire cables (ground, power, and two signal wires) that connect a device and a hub (just like ...