Chris Cant - Writing Windows WDM Device Drivers

The CallUSBDI routine in the UsbKbd driver is a candidate for this technique, as it is more than likely that the USB class drivers will take some time to process a USBDI request. However, it is usually the case that the USBDI call results are needed. Processing the results in a completion routine is just about possible. However, this will probably lead to code that is very complicated. In the end, it is probably simplest to leave the UsbKbd code as it is.

The DDK header files define several other HID IOCTLs. However, some of these are used by the HID class driver when it talks to a minidriver. It is not clear if any of these are available to HID clients.

This chapter concludes my look at the Human Input Device class driver. It is straightforward to write a user mode application to communicate with a HID device. A Win32 program looks for devices that use the HID device interface. They must then interrogate each device to see if their capabilities are of interest. The program can then read input reports and send output reports.

If need be, you can write a kernel mode HID client. It is best if you use the Plug and Play Notification technique to find any devices that support the HID device interface. Then, you can get the device capabilities and send and receive reports in a broadly similar way to user mode applications.

Both user mode and kernel mode HID clients can make use of the HID parsing routines. These make it much easier to find HID device's capabilities and to generate and understand reports.

Microsoft is the best source of information for most core Windows device driver development issues. Various books and newsgroups are also available to help driver writers. You may well need to check out other sources of information particular to your type of driver. You may need to seek help from vendors, standards bodies, and trade associations, as well as other driver writers.

A Microsoft Developer Network (MSDN) Professional Subscription provides most of the basic information you need to write device drivers. You get all the basic tools, Driver Development Kits (DDKs), and beta test versions of the Microsoft operating systems. However, it is worth keeping an eye on the Microsoft websites, as helpful articles and late-breaking news can often be posted. You will also need a C or C++ compiler, such as Visual Studio. The crucial Microsoft web sites are:

http://www.microsoft.com/hwdev/

http://www.microsoft.com/hwdev/driver/

http://msdn.microsoft.com/developer/

The DDKs include many example drivers that are very useful. Searching through these examples will often show you how to use a particular function or technique. Alternatively, you can base your entire driver on an existing example. Needless to say, do not use code blindly — make sure you understand what is going on. Finally, the sample driver directories often contain useful documentation in the form of Word or text files.

Table A.1 Information resources

The two primary Newsgroups for DDK information are comp.os.ms-windows.programmer.nt.kernel-mode and comp.os.ms-windows.programmer.vxd. The Microsoft news server at msnews.microsoft.com also has some useful newsgroups.

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