Device Classes

摘要

In the last Driving Me Nuts column [see LJ, June 2003], we introduced the kernel driver model framework with an explanation of how the generic bus and driver and device code works. The i2c core was used as an example to show how these different subsystems work. This month, we cover how the driver class code works, again using the i2c code to provide some working examples. As discussed in the last column, device classes do not meet the general object-oriented definition of a class; rather they are something that provides a single type of function to the user. For example, kernel classes are used for tty devices, block devices, network devices, SCSI hosts and, in the near future, filesystems. In the 2.5.69 kernel, the driver class support was rewritten radically. In previous kernel versions, class support was tied tightly to the driver and device support. A class would be bound to the device at the same time it was registered to a driver. This did work for a number of devices and classes, but some real-world devices did not fit very well into this model. Now, class support is tied only loosely to devices and drivers; in fact, a device or driver is not even needed to use the class code now, as the tty class code shows. The class code is now split into three different types of structures: classes, class devices and class interfaces.