Linux Device Drivers, Second Edition

Task Queues

One feature many drivers need is the ability to schedule execution of some tasks at a later time without resorting to interrupts. Linux offers three different interfaces for this purpose: task queues, tasklets (as of kernel 2.3.43), and kernel timers. Task queues and tasklets provide a flexible utility for scheduling execution at a later time, with various meanings for “later”; they are most useful when writing interrupt handlers, and we’ll see them again in Section 9.5, in Chapter 9. Kernel timers are used to schedule a task to run at a specific time in the future and are dealt with in Section 6.5, later in this chapter.

A typical situation in which you might use task queues or tasklets is to manage hardware that cannot generate interrupts but still allows blocking read. You need to poll the device, while taking care not to burden the CPU with unnecessary operations. Waking the reading process at fixed time intervals (for example, using current->timeout) isn’t a suitable approach, because each poll would require two context switches (one to run the polling code in the reading process, and one to return to a process that has real work to do), and often a suitable polling mechanism can be implemented only outside of a process’s context.

A similar problem is giving timely input to a simple hardware device. For example, you might need to feed steps to a stepper motor that is directly connected to the parallel port—the motor needs to be moved by single steps on a timely ...