The Windows Driver Model has two separate but equally important aspects. First, the core model describes the standard structure for device drivers. Second, Microsoft provides a series of bus and class drivers for common types of devices.
The core WDM model describes how device drivers are installed and started, and how they should service user requests and interact with hardware. A WDM device driver must fit into the Plug and Play (PnP) system that lets users plug in devices that can be configured in software.
Microsoft provides a series of system drivers that have all the basic functionality needed to service many standard types of device. The first type of system driver supports different types of bus, such as the Universal Serial Bus (USB), IEEE 1394 (FireWire) and Audio port devices. Other class drivers implement standard Windows facilities such as Human Input Devices (HID) and kernel streaming. Finally, the Still Image Architecture (STI) provides a framework for handling still images, scanners, etc.
These system class drivers can make it significantly easier to write some types of device driver. For example, the USB system drivers handle all the low-level communications across this bus. A well defined interface is made available to other drivers. This makes it fairly straightforward to issue requests to the USB bus.
Conclusion
Conclusion
This chapter has shown how to queue IRPs for serial processing in a driver's StartIo routine. It has then shown how to cope with the necessary evil of cancelling IRPs. Make sure you clean up any IRPs that are still not completed when the device handle is being closed. Use techniques similar to that used in the example WdmIoCancel application to test that cancelling and cleanup happen correctly.
The next chapter inspects the next part of the WdmIo and PHDIo drivers, how to handle interrupts and do interrupt-driven programmed I/O. It also looks at timers, both for IRP time-outs in the order of seconds, and custom timers for finer grain intervals.
