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Writing Device Drivers
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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 guarantee stays in effect until a closing call to ddi_exit_critical(9F) is made. See the ddi_enter_critical(9F) man page 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 specification 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 the delays 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. The various kinds of internal sequencing logic include the following types:

  • The Intel 8251A and the Signetics 2651 alternate the same external register between two internal mode registers. Writing to the first internal register is accomplished by writing to the external register. This write, however, has the side effect of setting up the sequencing logic in the chip so that the next read/write operation refers to the second internal register.

  • The NEC PD7201 PCC has multiple internal data registers. To write a byte into a particular register, two steps must be performed. The first step is to write into register zero the number of the register into which the following byte of data will go. The data is then written to the specified data register. The sequencing logic automatically sets up the chip so that the next byte sent will go into data register zero.

  • The AMD 9513 timer has a data pointer register that points at the data register into which a data byte will go. When sending a byte to the data register, the pointer is incremented. The current value of the pointer register cannot be read.

Interrupt Issues

Note the following common interrupt-related issues:

  • A controller interrupt does not necessarily indicate that both the controller and one of its slave devices are ready. For some controllers, an interrupt can indicate that either the controller is ready or one of its devices is ready but not both.

  • Not all devices power up with interrupts disabled and can begin interrupting at any time.

  • Some devices do not provide a way to determine that the board has generated an interrupt.

  • Not all interrupting boards shut off interrupts when told to do so or after a bus reset.

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  Published under the terms fo the Public Documentation License Version 1.01. Design by Interspire