1 <?xml version="1.0" encoding="utf-8"?>

     2 <!-- Copyright (c) 2007-2010 Nokia Corporation and/or its subsidiary(-ies) All rights reserved. -->

     3 <!-- This component and the accompanying materials are made available under the terms of the License 

     4 "Eclipse Public License v1.0" which accompanies this distribution, 

     5 and is available at the URL "http://www.eclipse.org/legal/epl-v10.html". -->

     6 <!-- Initial Contributors:

     7     Nokia Corporation - initial contribution.

     8 Contributors: 

     9 -->

    10 <!DOCTYPE concept

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

    12 <concept id="GUID-132349A6-9A5F-4866-A54D-F01B6F58ABDD" xml:lang="en"><title>Shared

    13 Chunks</title><shortdesc>This document describes the use of shared chunks to share data.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

    14 <section id="GUID-DBFFE006-4E6C-4B4C-9E50-0A9FC9CB51FF">       <title> Shared

    15 chunks</title>       <p>Device drivers often need to share data between user

    16 space and kernel space threads. Though there are APIs for doing this, such

    17 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>,

    18 these involve an overhead of a memory copy, which can be a problem when there

    19 is a large amount of data. </p> <p>To avoid unnecessary data transfer, Symbian

    20 platform provides shared chunks, which are similar to shared memory in other

    21 operating systems. Shared chunks can be created and used by both a driver

    22 and user-side code directly. An example application for shared chunks is for

    23 a camera driver. Without shared chunks, the image data would have to be copied

    24 from the camera driver to the user process, which would then copy it to the

    25 display driver. This would have a high memory copy overhead. Using shared

    26 chunks instead would improve the performance of the camera view finder. </p> <p>A

    27 shared chunk is created and controlled by the kernel side code, for example,

    28 the device driver, rather than the user code. This memory is safe to be used

    29 by ISRs and DMA. A shared chunk can be mapped into multiple user processes

    30 in sequence or at the same time. The memory object can be transferred between

    31 another process and another device driver. Multiple drivers and multiple user

    32 processes can have access to the same shared chunk. </p>     </section>

    33 <section id="GUID-8C27DA12-5396-4229-8723-480D5846604C"><title>Comparison

    34 between shared memory and shared I/O buffers</title><p>The EKA1 versions of

    35 Symbian platform had a mechanism called shared I/O buffers. These are now

    36 deprecated, and shared chunks should be used instead. The disadvantage of

    37 a shared I/O buffer is that only one user process can access the chunk at

    38 a time, and that it does not have a user handle: the user process is only

    39 supplied with the address and size of the buffer. Shared chunks solve these

    40 issues, and give more flexibility for users to do safe operations. </p></section>

    41 </conbody></concept>