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· DriverStudio Serialization Using Controller Objects
· DriverBundle
· Previews Windows NT offers several mechanisms for
· Compatibility synchronization and serialization. There are spin
[Downloads] · locks, which provide high priority mutual exclusion;
the "kernel dispatcher" objects (timers, events,
Wizards semaphores, mutexes), which interact with the
· Utilities scheduler at passive level; and device queues, which
· NT source provide a means for a device object to serialize
examples processing of I/O requests. A lesser known, but very
· VxD source useful, synchronization object is the controller
examples object, which has its own unique characteristics and
· WDM source usage.
examples
[Resources] · A controller is intended to model a resource shared
Technical papers by several devices. The classic example is a disk
· Useful links controller that provides a single data channel for a
· Technical tips set of disk drives. Although the individual drives
[Support] · operate independently, they must share the disk
controller for operations that involve data transfer.
Support Other operations, such as a head seek, do not require
· Knowledge base the disk controller's data channel. The device driver
· Problem uses a controller object to represent the shared
submission resource. When one of the disk drives requires the
· Product channel for a data transfer, it must first acquire
registration the controller object representing the shared
· Release notes resource. If an operation on another drive needs the
[Shop NuMega] · channel, it must wait until the operation on the
Buy it! first drive completes. The controller object provides
· Price list this serialization mechanism.
· How to buy
· Sales offices To create a controller object, a device driver calls
IoCreateController. The system returns a pointer to a
controller object, or NULL if the operation fails.
[Y2K Compliance] When the driver is unloads, or the controller is no
longer neeed, the device driver destroys the object
by calling IoDeleteController.
[More information]
Acquistion of a controller object requires calling
IoAllocateController. The semantics of this call are
"Please call my callback routine when the controller
object is free". If the controller object happens to
be not busy when the device driver calls
IoAllocateController, the system calls the callback
routine supplied by the caller immediately on the
same thread. If the controller object is busy,
IoAllocateController puts the address of the callback
routine on a queue, and returns to the caller
immediately. This is very similar to calling
IoStartPacket, which will cause a call to StartIo if
the device is not currently processing an IRP, or
will queue the IRP if the device is busy. Not
surprisingly, these internal implementation of these
two mechanisms share a common data structure, namely
KDEVICE_QUEUE.
When the device driver completes the operation that
required the shared resource represented by the
controller object, it calls IoFreeController. If
there is another callback request in the controller
object's queue, the system dequeues the request and
invokes the callback routine.
So, you can think of a controller object as a
specialized type of queue. Each item in the queue
consists of a callback routine and an associated
parameter. The controller object is responsible for
handling each item passed to it, either by
immediately calling the callback or by queuing the
request for later dispatching. The controller object
queues a new item only when the resource it
represents is busy. Initially not busy, the resource
becomes busy when a device driver calls
IoAllocateController. It becomes not busy when
IoFreeController is called and the controller's queue
is empty. The queuing mechanism implemented by a
controller object guarantees that IoFreeController
has been called between consecutive invocations of
the callback. The driver gets serialized access to
the resource without having to explicitly manage a
queue.
If you are writing your driver with DriverWorks, use
class KController to implement controller objects.
This class encapsulates and simplifies the system
services related to controllers, and enables you to
make the callback routine a member of your device
class, should you so choose.
As of this writing, Microsoft has removed the system
services for controllers from WDM.H, the primary
include file for Win32 Driver Model drivers. However,
if you are using DriverWorks, this is not a problem.
DriverWorks provides an implementation of KController
for WDM that does not rely on the system services.
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