Writing Device Drivers

Device Issues

This section describes issues with special devices.

Timing-Critical Sections

While most driver operations can be performed without mechanisms for synchronization and protection beyond those provided by the locking primitives, some devices require that a sequence of events occur in order without interruption. In conjunction with the locking primitives, the function ddi_enter_critical(9F) asks the system to guarantee, to the best of its ability, that the current thread will neither be preempted nor interrupted. This stays in effect until a closing call to ddi_exit_critical(9F) is made. See the ddi_enter_critical(9F) man pages for details.


Many chips specify that they can be accessed only at specified intervals. For example, the Zilog Z8530 SCC has a “write recovery time” of 1.6 microseconds. This means that a delay must be enforced with drv_usecwait(9F) when writing characters with an 8530. In some instances, the specifications do not make explicit what delays are needed, so they must be determined empirically.

Be careful not to compound delays for parts of devices that might exist in large numbers, for example thousands of SCSI disk drives.

Internal Sequencing Logic

Devices with internal sequencing logic map multiple internal registers to the same external address. There are various kinds of internal sequencing logic:

Interrupt Issues

The following are some common interrupt-related issues: