![](https://csdnimg.cn/release/download_crawler_static/9617374/bg12.jpg)
This is the Title of the Book, eMatter Edition
Copyright © 2005 O’Reilly & Associates, Inc. All rights reserved.
The Role of the Device Driver
|
3
array of data blocks. Higher levels of the system provide policies, such as who may
access the floppy drive, whether the drive is accessed directly or via a filesystem, and
whether users may mount filesystems on the drive. Since different environments usu-
ally need to use hardware in different ways, it’s important to be as policy free as
possible.
When writing drivers, a programmer should pay particular attention to this funda-
mental concept: write kernel code to access the hardware, but don’t force particular
policies on the user, since different users have different needs. The driver should deal
with making the hardware available, leaving all the issues about how to use the hard-
ware to the applications. A driver, then, is flexible if it offers access to the hardware
capabilities without adding constraints. Sometimes, however, some policy decisions
must be made. For example, a digital I/O driver may only offer byte-wide access to
the hardware in order to avoid the extra code needed to handle individual bits.
You can also look at your driver from a different perspective: it is a software layer
that lies between the applications and the actual device. This privileged role of the
driver allows the driver programmer to choose exactly how the device should appear:
different drivers can offer different capabilities, even for the same device. The actual
driver design should be a balance between many different considerations. For
instance, a single device may be used concurrently by different programs, and the
driver programmer has complete freedom to determine how to handle concurrency.
You could implement memory mapping on the device independently of its hardware
capabilities, or you could provide a user library to help application programmers
implement new policies on top of the available primitives, and so forth. One major
consideration is the trade-off between the desire to present the user with as many
options as possible and the time you have to write the driver, as well as the need to
keep things simple so that errors don’t creep in.
Policy-free drivers have a number of typical characteristics. These include support for
both synchronous and asynchronous operation, the ability to be opened multiple
times, the ability to exploit the full capabilities of the hardware, and the lack of soft-
ware layers to “simplify things” or provide policy-related operations. Drivers of this
sort not only work better for their end users, but also turn out to be easier to write
and maintain as well. Being policy-free is actually a common target for software
designers.
Many device drivers, indeed, are released together with user programs to help with
configuration and access to the target device. Those programs can range from simple
utilities to complete graphical applications. Examples include the tunelp program,
which adjusts how the parallel port printer driver operates, and the graphical cardctl
utility that is part of the PCMCIA driver package. Often a client library is provided as
well, which provides capabilities that do not need to be implemented as part of the
driver itself.
,ch01.2168 Page 3 Thursday, January 20, 2005 9:21 AM