Linux Device Drivers, Second Edition

SBus

While most computers nowadays are equipped with a PCI or ISA interface bus, most not-so-recent SPARC-based workstations use SBus to connect their peripherals.

SBus is quite an advanced design, although it has been around for a long time. It is meant to be processor independent (even though only SPARC computers use it) and is optimized for I/O peripheral boards. In other words, you can’t plug additional RAM into SBus slots (RAM expansion boards have long been forgotten even in the ISA world, and PCI does not support them either). This optimization is meant to simplify the design of both hardware devices and system software, at the expense of some additional complexity in the motherboard.

This I/O bias of the bus results in peripherals using virtual addresses to transfer data, thus bypassing the need to allocate a contiguous DMA buffer. The motherboard is responsible for decoding the virtual addresses and mapping them to physical addresses. This requires attaching an MMU (memory management unit) to the bus; the chipset in charge of the task is called IOMMU. Although somehow more complex than using physical addresses on the interface bus, this design is greatly simplified by the fact that SPARC processors have always been designed by keeping the MMU core separate from the CPU core (either physically or at least conceptually). Actually, this design choice is shared by other smart processor designs and is beneficial overall. Another feature of this bus is that device boards exploit massive geographical addressing, so there’s no need to implement an address decoder in every peripheral or to deal with address conflicts.

SBus peripherals use the Forth language in their PROMs to initialize themselves. Forth was chosen because the interpreter is lightweight and therefore can be easily implemented in the firmware of any computer system. In addition, the SBus specification outlines the boot process, so that compliant I/O devices fit easily into the system and are recognized at system boot. This was a great step to support multi-platform devices; it’s a completely different world from the PC-centric ISA stuff we were used to. However, it didn’t succeed for a variety of commercial reasons.

Although current kernel versions offer quite full-featured support for SBus devices, the bus is so little used nowadays that it’s not worth covering in detail here. Interested readers can look at source files in arch/sparc/kernel and arch/sparc/mm.