Overview

This chapter starts looking at the WdmIo and PHDIo drivers. Although the text only refers to the WdmIo driver, the techniques used in PHDIo are identical.

Any driver that talks to a real device will need to control access to the hardware. Chapter 9 mentioned that Critical section routines should be used to run routines that cannot be interrupted. Critical section routines are now used for real.

However, the main new requirement for a driver that accesses hardware is that it serializes all the IRPs that it processes. If more than one Win32 process has opened a device and is issuing IRPs, each IRP must not try to talk to the hardware at the same time ^[1]. If the IRPs can be processed very quickly, critical sections should be used. However, in most cases, the relevant IRPs need to be put into a queue and processed serially, one by one.

Processing IRPs serially will, of course, reduce performance. If jobs can be carried out in parallel, try to do it that way.

You may wish to impose further strictures on users of your device. You could insist that a certain IOCTL be used just before a read request. Although it is best to keep these restrictions to a minimum, do put in any relevant checks. You could use the FileObject pointer on the IRP stack to ensure that it is the same application that is issuing the IOCTL and read requests.

Processing IRPs...