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Adaptation/GUID-132349A6-9A5F-4866-A54D-F01B6F58ABDD.dita
author Graeme Price <GRAEME.PRICE@NOKIA.COM>
Fri, 15 Oct 2010 14:32:18 +0100
changeset 15 307f4279f433
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<?xml version="1.0" encoding="utf-8"?>
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<!-- Copyright (c) 2007-2010 Nokia Corporation and/or its subsidiary(-ies) All rights reserved. -->
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<!-- This component and the accompanying materials are made available under the terms of the License
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"Eclipse Public License v1.0" which accompanies this distribution,
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and is available at the URL "http://www.eclipse.org/legal/epl-v10.html". -->
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<!-- Initial Contributors:
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    Nokia Corporation - initial contribution.
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Contributors:
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-->
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<!DOCTYPE concept
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  PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
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<concept id="GUID-132349A6-9A5F-4866-A54D-F01B6F58ABDD" xml:lang="en"><title>Shared
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Chunks</title><shortdesc>This document describes the use of shared chunks to share data.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
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<section id="GUID-DBFFE006-4E6C-4B4C-9E50-0A9FC9CB51FF">       <title> Shared
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chunks</title>       <p>Device drivers often need to share data between user
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space and kernel space threads. Though there are APIs for doing this, such
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as <xref href="GUID-C6946ECB-775F-3EC2-A56F-78F25B9FBE3D.dita#GUID-C6946ECB-775F-3EC2-A56F-78F25B9FBE3D/GUID-904A42A8-8077-3FC6-BEF2-29619F079842"><apiname>Kern::ThreadRawRead()</apiname></xref> and <xref href="GUID-C6946ECB-775F-3EC2-A56F-78F25B9FBE3D.dita#GUID-C6946ECB-775F-3EC2-A56F-78F25B9FBE3D/GUID-182C88F4-326C-376E-9FBE-889E3CB9B68A"><apiname>Kern::ThreadRawWrite()</apiname></xref>,
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these involve an overhead of a memory copy, which can be a problem when there
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is a large amount of data. </p> <p>To avoid unnecessary data transfer, Symbian
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platform provides shared chunks, which are similar to shared memory in other
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operating systems. Shared chunks can be created and used by both a driver
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and user-side code directly. An example application for shared chunks is for
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a camera driver. Without shared chunks, the image data would have to be copied
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from the camera driver to the user process, which would then copy it to the
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display driver. This would have a high memory copy overhead. Using shared
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chunks instead would improve the performance of the camera view finder. </p> <p>A
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shared chunk is created and controlled by the kernel side code, for example,
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the device driver, rather than the user code. This memory is safe to be used
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by ISRs and DMA. A shared chunk can be mapped into multiple user processes
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in sequence or at the same time. The memory object can be transferred between
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another process and another device driver. Multiple drivers and multiple user
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processes can have access to the same shared chunk. </p>     </section>
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<section id="GUID-8C27DA12-5396-4229-8723-480D5846604C"><title>Comparison
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between shared memory and shared I/O buffers</title><p>The EKA1 versions of
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Symbian platform had a mechanism called shared I/O buffers. These are now
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deprecated, and shared chunks should be used instead. The disadvantage of
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a shared I/O buffer is that only one user process can access the chunk at
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a time, and that it does not have a user handle: the user process is only
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supplied with the address and size of the buffer. Shared chunks solve these
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issues, and give more flexibility for users to do safe operations. </p></section>
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</conbody></concept>
MCL/sftools/depl/docscontent