Linux Device Drivers, Second Edition

Automatic and Manual Configuration

Several parameters that a driver needs to know can change from system to system. For instance, the driver must know the hardware’s actual I/O addresses, or memory range (this is not a problem with well-designed bus interfaces and only applies to ISA devices). Sometimes you’ll need to pass parameters to a driver to help it in finding its own device or to enable/disable specific features.

Depending on the device, there may be other parameters in addition to the I/O address that affect the driver’s behavior, such as device brand and release number. It’s essential for the driver to know the value of these parameters in order to work correctly. Setting up the driver with the correct values (i.e., configuring it) is one of the tricky tasks that need to be performed during driver initialization.

Basically, there are two ways to obtain the correct values: either the user specifies them explicitly or the driver autodetects them. Although autodetection is undoubtedly the best approach to driver configuration, user configuration is much easier to implement. A suitable trade-off for a driver writer is to implement automatic configuration whenever possible, while allowing user configuration as an option to override autodetection. An additional advantage of this approach to configuration is that the initial development can be done without autodetection, by specifying the parameters at load time, and autodetection can be implemented later.

Many drivers ...