Deferred -Function Calls

- Deferred function calls

Asynchronous request processing -is normally done in a Deferred Function Call (DFC). The second stage of interrupt -handling is deferred to run as a DFC, which runs in a non-critical context.

Different -asynchronous requests can be handled using a single or multiple DFCs. The -number of DFCs to be created must be decided when the driver is designed. -For example, a driver that handles at a time an asynchronous receive request -and a transmit request would create two DFCs each for receive and transmit.

Creation

DFCs -are created using TDfc objects.

The DFC is initialised -with a DFC callback function, a pointer to an object to be passed to the callback -function, and a priority. It can also provide the DFC queue to be used by -this DFC at the time of construction.

TDfc iRxDfc; -iRxDfc(RxDataDfc,this,1); - or -iRxDfc(RxDataDfc,this,iDfcQ,1);

Queuing

To -initiate the process of DFC functionality, the DFC object must be queued to -the DFC queue associated with the Kernel thread.

Before adding the -DFC to a DFC queue of the thread, it must be associated with the queue (TDfcQue) -object. This can be either done at the time of construction, or can be set -later, by calling TDfc::SetDfcQ().

... -// Associate the Tx and Rx DFCs with the same queue set to receive -// the client messages for this driver. SetDfcQ() sets up the DFC -// queue for the DFCs constructed for Tx and Rx -// -iRxDfc.SetDfcQ(iDfcQ); -...

TDfc::Add() or TDfc::Enque() is -used to add the DFC object to the DFC queue. From an ISR, TDfc::Add() is -used. From a thread, TDfc::Enque() is used. If preemption -is disabled, TDfc::Add() could also be used.

// TDfc::Add() will queue an IDFC or a DFC from an ISR. This function -// is the only way to queue an IDFC and is the only way to queue a -// DFC from an ISR. To queue a DFC from an IDFC or a thread either -// TDfc::Enque() or TDfc::DoEnque() should be used. This function -// does nothing if the IDFC/DFC is already queued. -// -iRxDfc.Add(); -

Callback function

The -DFC callback function is a static function called when a DFC is executed. -A pointer to this function is provided at the time of DFC object creation. -This function implements the deferred functionality of an asynchronous request, -such as reading or writing data from or to an I/O peripheral. It would then -either complete the request or start another operation.

static void RxDataDfc(TAny* aPtr);

Cancellation

A -DFC function must be cancelled while cleaning up resources, for example, when -closing the channel. The function TDfc::Cancel() cancels -the DFC if it is already queued. It does nothing if a DFC is not queued. It -must be called to avoid orphaned DFCs.

... -// If it is a Transmit request, cancel the Transmit DFC. -// TDfc::Cancel() cancels the DFC if it is already queued. It -// does nothing if DFC is not queued. -// -iTxDfc.Cancel(); -...

+ + + + + +Deferred +Function CallsThis document describes how Deferred Function Calls are used to +implement asynchronous requests. +

+ Deferred function calls

Asynchronous request processing +is normally done in a Deferred Function Call (DFC). The second stage of interrupt +handling is deferred to run as a DFC, which runs in a non-critical context.

Different +asynchronous requests can be handled using a single or multiple DFCs. The +number of DFCs to be created must be decided when the driver is designed. +For example, a driver that handles at a time an asynchronous receive request +and a transmit request would create two DFCs each for receive and transmit.

Creation

DFCs +are created using TDfc objects.

The DFC is initialised +with a DFC callback function, a pointer to an object to be passed to the callback +function, and a priority. It can also provide the DFC queue to be used by +this DFC at the time of construction.

TDfc iRxDfc; +iRxDfc(RxDataDfc,this,1); + or +iRxDfc(RxDataDfc,this,iDfcQ,1);

Queuing

To +initiate the process of DFC functionality, the DFC object must be queued to +the DFC queue associated with the Kernel thread.

Before adding the +DFC to a DFC queue of the thread, it must be associated with the queue (TDfcQue) +object. This can be either done at the time of construction, or can be set +later, by calling TDfc::SetDfcQ().

... +// Associate the Tx and Rx DFCs with the same queue set to receive +// the client messages for this driver. SetDfcQ() sets up the DFC +// queue for the DFCs constructed for Tx and Rx +// +iRxDfc.SetDfcQ(iDfcQ); +...

TDfc::Add() or TDfc::Enque() is +used to add the DFC object to the DFC queue. From an ISR, TDfc::Add() is +used. From a thread, TDfc::Enque() is used. If preemption +is disabled, TDfc::Add() could also be used.

// TDfc::Add() will queue an IDFC or a DFC from an ISR. This function +// is the only way to queue an IDFC and is the only way to queue a +// DFC from an ISR. To queue a DFC from an IDFC or a thread either +// TDfc::Enque() or TDfc::DoEnque() should be used. This function +// does nothing if the IDFC/DFC is already queued. +// +iRxDfc.Add(); +

Callback function

The +DFC callback function is a static function called when a DFC is executed. +A pointer to this function is provided at the time of DFC object creation. +This function implements the deferred functionality of an asynchronous request, +such as reading or writing data from or to an I/O peripheral. It would then +either complete the request or start another operation.

static void RxDataDfc(TAny* aPtr);

Cancellation

A +DFC function must be cancelled while cleaning up resources, for example, when +closing the channel. The function TDfc::Cancel() cancels +the DFC if it is already queued. It does nothing if a DFC is not queued. It +must be called to avoid orphaned DFCs.

... +// If it is a Transmit request, cancel the Transmit DFC. +// TDfc::Cancel() cancels the DFC if it is already queued. It +// does nothing if DFC is not queued. +// +iTxDfc.Cancel(); +...

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