IEEE 1394 Unit Filter Driver Architecture (Windows CE 5.0)
The unit filter driver is not in a device stack underneath the virtual subunit driver. The virtual subunit driver cannot send commands to the unit filter driver by sending them down the device stack as it does for AVC_STREAM. As a result, a virtual subunit driver must communicate both with the drivers below it and communicate through a specific reference to the unit filter driver.
To facilitate the process, the unit filter driver exports functions that the virtual subunit driver can call to locate, open, and reference the appropriate device object on the unit filter driver, and to close the same device object when it is no longer required. These functions are UNIT_OpenUnitDeviceObject and UNIT_CloseUnitDeviceObject.
When you open the virtual subunit driver, it obtains a device object pointer. You can use this device object as the destination of virtual subunit I/O request packets (IRPs). The virtual subunit can request functionality of the unit filter driver by generating IOCTL requests and sending them in IRPs to the device object.
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