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<concept id="GUID-58AA7257-0951-42AB-9B6E-AAF63343FEFA" xml:lang="en"><title>DMA Channel Operations</title><shortdesc>This document describes how a device driver should open and close

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

<section id="GUID-C4241738-D4C3-4171-87B2-5EDFFACE4002"><title>Open</title><p>To

use a DMA channel it has to be opened. <xref href="GUID-83882548-FAC5-3EFF-92ED-14D1D9A85D37.dita#GUID-83882548-FAC5-3EFF-92ED-14D1D9A85D37/GUID-514BA285-3563-3C24-A067-F360098A6ADF"><apiname>TDmaChannel::Open()</apiname></xref> must

be called to open the channel, and the required information provided in a <xref href="GUID-83882548-FAC5-3EFF-92ED-14D1D9A85D37.dita#GUID-83882548-FAC5-3EFF-92ED-14D1D9A85D37/GUID-6B973278-8BE7-3CE5-97D8-60E8D3F4D473"><apiname>TDmaChannel::SCreateInfo</apiname></xref> structure.

The actual DMA channel selection is done by the hardware specific layer. The

cookie passed to the <codeph>SCreateInfo</codeph> structure holds an object

from the hardware specific layer, which provides the DMA channel information. </p> <codeblock id="GUID-A1CA8DBA-1378-502B-8CC8-6D83F13DD151" xml:space="preserve">/**

 Initializes the DMA for transmit. This function does all the 

 required generic initialization to use the DMA framework. This 

 function does not have any variant specific initializations. 

    The implementation of the function can be the same on all 

    variants, except if the variant has a different DFCQ buffer size.

 @return     KErrNone on success, else standard error code on failure

 */

TInt DExDriverUartTxDma::Init()

    {

    ...

    // Create the helper object for DMA transmit. This object 

    // pointer is provided to System DMA controller driver during 

    // channel operations such as opening, closing and starting

    //

    iHelper = new DExDriverTxDmaHelper(iUartComm);

    if (iHelper == NULL)

        {

        return KErrNoMemory;

        }

    // Initialize the channel information to select and configure the DMA     

    // channel

    TDmaChannel::SCreateInfo info;

    info.iCookie = (TUint)iHelper;

    info.iDesCount = 1; // For single buffered transfers

    info.iDfcPriority = 4; // Set the priority of this DFC for DMA

    info.iDfcQ = iUartComm->DfcQ(); // DFCQ onto which this request

 // has to be queued    

    // DMA channel has to be opened before doing any operations on 

    // the channel. TDmaChannel::Open() opens the DMA channel as 

    // set by info passed and selected by the hardware-specific layer. 

    // Using the cookie, the helper class's ChannelOpen() is called 

    // for DMA channel initialization.

    // 

    r = TDmaChannel::Open(info, iTxDmaChannel);

    if (r!=KErrNone)

        {

        return r;

        }

    ...

    }</codeblock> </section>

<section id="GUID-5E09A55C-A2B9-4E02-9BCF-F23558357B70"><title>Close</title> <p>A DMA channel has to be closed after completing

all the DMA operations. This releases the DMA channel for other resources

and peripherals. To close a previously opened channel, the channel should

be idle, with no pending requests. So, before closing, call <xref href="GUID-83882548-FAC5-3EFF-92ED-14D1D9A85D37.dita#GUID-83882548-FAC5-3EFF-92ED-14D1D9A85D37/GUID-0A98D2DC-2A61-3FBD-AB7A-623FE926A183"><apiname>TDmaChannel::CancelAll()</apiname></xref>,

which cancels the current request and any pending DMA requests. This should

then be followed by closing the channel using <xref href="GUID-83882548-FAC5-3EFF-92ED-14D1D9A85D37.dita#GUID-83882548-FAC5-3EFF-92ED-14D1D9A85D37/GUID-8204AFBD-2A60-372E-B626-35BD19851FF7"><apiname>TDmaChannel::Close()</apiname></xref>. </p> <codeblock id="GUID-4BA5CDD2-6BD3-5179-89B8-7337ED5809AA" xml:space="preserve">/**

 Destructor. Deinitializes the member variables, cancels any 

 requests, closes the channel, deletes the DMA requests, frees the 

 hardware chunks allocated.

 */    

DExDriverUartTxDma::~DExDriverUarttxDma()

    {

    ...

    // if DMA channel is existing, cancel all requests on it.    

    //

    if (iTxDmaChannel)

        {

        // TDmaChannel::CancelAll() cancels the current request and 

        // any pending requests on the DMA channel

        //     

        iTxDmaChannel->CancelAll();

        }

    ...

    if(iTxDmaChannel)

        {

        // Close the DMA channel. TDmaChannel::Close() closes a 

        // previously opened channel. All the requests on this 

        // channel should have been cancelled prior to this, i.e. 

        // TDmaChannel::CancelAll() should have been called before 

        // this call.

        //

        iTxDmaChannel->Close();

        }

    ...

    }</codeblock> <p>To stop DMA without closing the channel, simply cancel

the request. </p></section>