Linux Device Drivers, Second Edition

Interrupt Sharing

The notion of an IRQ conflict is almost synonymous with the PC architecture. In general, IRQ lines on the PC have not been able to serve more than one device, and there have never been enough of them. As a result, frustrated users have often spent much time with their computer case open, trying to find a way to make all of their hardware play well together.

But, in fact, there is nothing in the design of the hardware itself that says that interrupt lines cannot be shared. The problems are on the software side. With the arrival of the PCI bus, the writers of system software have had to work a little harder, since all PCI interrupts can explicitly be shared. So Linux supports shared interrupts—and on all buses where it makes any sense, not just the PCI. Thus, suitably aware drivers for ISA devices can also share an IRQ line.

The question of interrupt sharing under the ISA bus brings in the issue of level-triggered versus edge-triggered interrupt lines. Although the former kind of interrupt reporting is safe with regard to sharing, it may lead to software lockup if not handled correctly. Edge-triggered interrupts, on the other hand, are not safe with regard to sharing; ISA is edge triggered, because this signaling is easier to implement at hardware level and therefore was the common choice in the 1980s. This issue is unrelated to electrical signal levels; in order to support sharing, the line must be able to be driven active by multiple sources whether it ...