Asynchronous +Requests

An +asynchronous request is one which is used, typically, to communicate with +the hardware itself. The time taken for such a request to complete depends +on the hardware, and the client user-side thread may want to continue with +some other processing. The user-side thread is not blocked and can continue +with other operations, including issuing other requests (synchronous or asynchronous).

An +asynchronous request is started by a call to RBusLogicalChannel::DoRequest(). +Control normally returns as soon as the request has been issued, successfully +or otherwise. Typically, the hardware indicates completion of an operation +by generating an interrupt. This is handled by an Interrupt Service Routine +(ISR) provided by the driver. This, in turn, schedules a DFC, which at some +later time, runs in the context of a kernel-side thread, and signals the client-user +side thread, marking the asynchronous request as complete.

More than +one asynchronous request can be outstanding at the same time, each one associated +with its own TRequestStatus object and each identified +by a specific request number. The device driver framework puts no explicit +limit on the number of concurrent outstanding asynchronous requests; any limit +must be enforced by the driver itself. However, the API to cancel a request +implicitly prevents you from uniquely identifying more than 32 concurrent +requests.

An outstanding asynchronous request can be cancelled by +a call to RBusLogicalChannel::DoCancel().