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<!DOCTYPE concept

  PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">

<concept id="GUID-6AC01B10-92C1-4E56-813B-6853DFCF3386-GENID-1-2-1-9-1-6-1-8-1-7-1-5-1" xml:lang="en"><title>DMA Requests</title><shortdesc>Describes how device drivers use DMA requests to initiate

a DMA transfer.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

<p>DMA transfer requests are the way in which a device driver sets

up and initiates a DMA transfer. A transfer request internally comprises

a linked list of DMA descriptor headers and is associated with a single

DMA channel. A transfer request also stores an optional client-provided

callback function, which can be invoked when the whole request completes,

whether successfully or not. A DMA request can be in any of the four

states: Not configured, Idle, Being transferred, and Pending. However,

these states are not used by the driver. </p>

<p>A device driver creates a DMA request by specifying a DMA channel

and an optional DMA callback function to be called after the request

completion. A DMA request must be fragmented. </p>

<p>The following shows the creation of a DMA request: </p>

<codeblock id="GUID-B2753DE7-800E-57CC-B2B6-CA2548AC25C4-GENID-1-2-1-9-1-6-1-8-1-7-1-5-1-3-4" xml:space="preserve">TInt DExDriverUartTxDma::Init()

    {

    ...

    // A DMA request has to be created for any DMA transfer. This 

    // specifies the channel on which the DMA request shall be 

    // made, the service callback function to be called after the 

    // request completion (TxDmaService), and an object pointer that will be passed 

    // as an argument to DMA request service function (this).

    // A DMA request is a list of fragments small enough to be 

    // transferred in one go by the DMA Controller.

    //

    iTxDmaRequest = new DDmaRequest(*iTxDmaChannel, TxDmaService,    this);

    if(iTxDmaRequest == NULL)

        {

        return KErrNoMemory;

        }

    ...

    }</codeblock>

</conbody></concept>