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<concept id="GUID-4E3C086B-25BE-4DAC-9E21-CFC4F8B792A5" xml:lang="en"><title>DMA Technology Guide</title><shortdesc>Describes the Direct Memory Access framework.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

<section id="GUID-E987EF55-27CE-4EC2-B250-2C3D7C36CAF9"><title>Purpose</title><p>The DMA framework provides a generic simple interface for the

clients to access DMA resources in a device. The primary clients are

device drivers that will use the DMA framework to set up DMA transfers.

A device can have more than one DMA controller (DMAC). </p></section>

<section id="GUID-7607CC80-0FA0-4FC2-897A-C4AB6B6C74FC"><title>Architectural

concepts</title><p>The key concepts related to DMA framework are:</p><fig id="GUID-ECCF1F57-DBD0-4653-A631-F8972E24DECF">

<title>DMA Framework</title>

<image href="GUID-10FE825A-4383-4A10-A507-58577BB230FB_d0e89974_href.png" placement="inline"/>

</fig><dl>

<dlentry>

<dt>DMA Client</dt>

<dd><p>A device driver or a kernel object that needs to use the resources

of the DMA framework. On the Symbian platform, the Physical Device

Drivers are the primary clients of the DMA framework. </p></dd>

</dlentry>

<dlentry>

<dt>DMA Platform Service API</dt>

<dd><p>The generic interface to provide DMA services to the clients.</p></dd>

</dlentry>

<dlentry>

<dt>DMA Import Library</dt>

<dd><p>The DMA clients link against the <filepath>dma.lib</filepath> file.</p></dd>

</dlentry>

<dlentry>

<dt>DMA Platform-Independent Layer (PIL)</dt>

<dd><p>The platform-independent layer contains the generic framework

independent from the hardware.</p></dd>

</dlentry>

<dlentry>

<dt>DMA Platform-Specific Layer (PSL)</dt>

<dd><p>The platform-specific layer is specific to the baseport and

the hardware used.</p></dd>

</dlentry>

<dlentry>

<dt>DMA Controller (DMAC)</dt>

<dd><p>The DMAC is the DMA hardware implementation. The number of

DMAC in the implementation of the DMA Framework depends on the hardware.

The DMAC is a peripheral to the CPU and is programmed to control data

transfers without using the CPU. </p></dd>

</dlentry>

</dl></section>

<section id="GUID-CAC0FE8F-A338-4B0E-92ED-7A1D1FD52C24"><title>DMA

functionality</title><p>The key concepts related to functionality

provided by the DMA framework are:</p><dl>

<dlentry>

<dt>Scatter/Gather Mode</dt>

<dd><p>Some DMA controllers transfer data using scatter/gather mode.

In this mode, the operating system creates a DMA descriptor with source

and destination memory addresses and the configuration. The source

and destination data can be a list of scattered memory blocks. The

DMA controller uses this configuration to perform the data transfer.</p></dd>

</dlentry>

<dlentry>

<dt>DMA Descriptors</dt>

<dd><p>The data structure used by the DMA framework to store the configuration

of the data transfer. These will store details such as source address,

destination address, number of bytes and pointer to the next descriptor,

when the DMA is used to transfer disjoint blocks of data. This structure

is defined in the platform specific layer. The descriptors are always

used to store the configuration even if the DMA controller does not

support scatter/gather mode. When the scatter/gather mode is not supported

by the DMAC, the descriptors used to store the configuration are called

as pseudo-descriptors.</p></dd>

</dlentry>

<dlentry>

<dt>Descriptor Headers</dt>

<dd><p>The descriptor headers are used to store additional information

about the descriptors. The descriptors are accessed using the descriptor

headers which contain the pointer to the descriptor.</p></dd>

</dlentry>

<dlentry>

<dt>Transfer Request</dt>

<dd><p>The device drivers configure and initialize a data transfer

using the class <xref href="GUID-780F4D53-5546-3B69-B328-0226C70EBDE2.dita"><apiname>DDmaRequest</apiname></xref>. The data can be transferred

between:<ul>

<li><p>memory to memory</p></li>

<li><p>memory to peripheral</p></li>

<li><p>peripheral to memory</p></li>

</ul>The transfer request stores the callback function of the clients.

The callback function is used to notify the success or failure of

a data transfer.</p></dd>

</dlentry>

<dlentry>

<dt>DMA Channel</dt>

<dd><p>The object which represents a single hardware, logical or a

virtual channel. In the DMA platform service API, each DMA channel

is referred to by a 32-bit value called a cookie. Each DMA transfer

is in the form of a description header and its associated DMA descriptor.

The queue of transfer requests (both being transferred and pending)

consists of a linked list of the DMA headers. </p></dd>

</dlentry>

<dlentry>

<dt>DMA Channel Allocator</dt>

<dd><p>The channel manager controls the opening and closing of DMA

channels. The channel manager functions are defined in the platform

independent layer and implemented in the platform specific layer. </p></dd>

</dlentry>

</dl></section>

<section id="GUID-F64155FC-5245-447F-88E7-C642EDEB2CEE"><title>Typical

Uses</title><p>The typical use cases of a DMA framework are:</p><ul>

<li><p>programmable data transfer between hardware peripherals and

memory buffers</p></li>

<li><p>programmable data transfer of block of data between different

regions of memory</p></li>

</ul></section>

</conbody><related-links>

<link href="http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0424b/index.html.dita">

<linktext>ARM DMA Controller Reference Manual</linktext></link>

</related-links></concept>