Motorola MCP750 Specifications download pdf (Page 153)

Developing a Device Drive

10-

Each type of entry point is brieﬂy described as follows.

Initialization Routines 10

Typically, there are some initialization tasks that must be performed before the device is

ready to operate within the system. Typically, the initialization tasks include initializing

the device hardware, allocating control and data buffers, registering interrupt handlers, and

so on. These routines can perform these tasks: init(D2) and start(D2).

I/O Service Routines 10

Once the driver and associated devices have been initialized, the system is ready to inter-

face to the device via I/O service routines. The I/O service routines consist of several man-

datory and optional entry points. All character device driver entry points must have an

open() and close() entry point. Other entry points are optional—for example,

read(), write(), ioctl(), mmap() and chpoll(). These entry points are speci-

fied in the driver’s Master(4) configuration file. The driver’s I/O service routines are

called at program level.

Interrupt Service Routines 10

If the device for which you are developing a driver generates interrupts, the driver might

have one or more interrupt service routines. The driver’s interrupt service routines are

called at interrupt level. Typically, an interrupt service routine is invoked to handle the

completion of a data transfer or to signal an error condition or any other type of I/O event.

Developing the Driver Header File and Data Structures 10

Typically, a device drivers’s header file is used to declare device-dependent structures,

deﬁne symbolic constants and macros.

Most device drivers are written to control a piece of hardware plugged into the I/O back-

plane of the computer. Most boards are designed to have a set of control registers starting

at memory location zero of the board’s memory. This memory is mapped into the virtual

address space of the kernel at boot time. Because the registers on the board are accessed

frequently, it is helpful to declare a structure that represents those registers, declare a

pointer to this type of structure, and have the pointer point to the virtual address of the

board.

When declaring this structure, you must be aware of certain alignment considerations. The

simple data types and their alignment restrictions are described in Chapter 9 (“Under-

standing the Kernel Environment”).

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Motorola MCP750 Specifications Page 153