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<!DOCTYPE concept
  PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept id="GUID-6AC01B10-92C1-4E56-813B-6853DFCF3386" xml:lang="en"><title>DMA
Requests</title><shortdesc>This document 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 a client-provided call-back function, which
is 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 a
DMA callback function to be called after the request completion. A DMA request
can also be split up or fragmented. </p>
<p>The following shows the creation of a DMA request: </p>
<codeblock id="GUID-B2753DE7-800E-57CC-B2B6-CA2548AC25C4" 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>