author | Graeme Price <GRAEME.PRICE@NOKIA.COM> |
Fri, 15 Oct 2010 14:32:18 +0100 | |
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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, |
307f4279f433
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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-2380FDDE-5489-5B1C-87BB-1FD882E385D2" xml:lang="en"><title>Migration |
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Tutorial: Direct Memory Addressing</title><shortdesc>To handle direct memory addressing, you must to make the following |
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changes to your code. </shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody> |
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<p>If a media driver uses a data transfer mechanism like <xref href="GUID-E5576D91-BC5C-588E-BF90-5A1E3C445133.dita">DMA</xref>, |
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data transfer can be faster if the media driver knows that an address passed |
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in an I/O request is a physical address. This is known as direct memory addressing. </p> |
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<ul> |
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<li id="GUID-E2C6E9D0-0150-529C-953E-32DFDEA68E77"><p> <xref href="GUID-2380FDDE-5489-5B1C-87BB-1FD882E385D2.dita#GUID-2380FDDE-5489-5B1C-87BB-1FD882E385D2/GUID-621EB44F-25BB-5B71-BF74-ED0304B7B591">Changes to registration</xref> </p> </li> |
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<li id="GUID-62701989-76A6-52D5-A713-226343BED4EE"><p> <xref href="GUID-2380FDDE-5489-5B1C-87BB-1FD882E385D2.dita#GUID-2380FDDE-5489-5B1C-87BB-1FD882E385D2/GUID-49992CF9-404A-585C-8770-E9034434A344">Changes to request handling</xref> </p> </li> |
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<li id="GUID-79FF128E-98B3-5613-B02A-08A81FC5B82D"><p> <xref href="GUID-2380FDDE-5489-5B1C-87BB-1FD882E385D2.dita#GUID-2380FDDE-5489-5B1C-87BB-1FD882E385D2/GUID-5F4A392F-0EAF-5E69-B902-D987B8FFD610">Issues about physical addresses</xref> </p> </li> |
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</ul> |
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<section id="GUID-621EB44F-25BB-5B71-BF74-ED0304B7B591"><title>Changes to |
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registration</title> <p>If the media driver code can handle physical addresses, |
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it must tell the local media subsystem. This process is called registration. |
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The media driver calls <xref href="GUID-9E60E8D9-619E-3A76-BAC8-93A60D62C7DF.dita#GUID-9E60E8D9-619E-3A76-BAC8-93A60D62C7DF/GUID-57B3380F-3CA6-3FF4-9D79-05B718E0743A"><apiname>LocDrv::RegisterDmaDevice()</apiname></xref> to register |
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as part of the <xref href="GUID-A70A01D2-467E-5BA8-A01D-6182558F3F52.dita">media |
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driver initialisation</xref> process. The media driver calls <codeph>RegisterDmaDevice()</codeph> after |
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the media driver has <xref href="GUID-A70A01D2-467E-5BA8-A01D-6182558F3F52.dita#GUID-A70A01D2-467E-5BA8-A01D-6182558F3F52/GUID-4A8DEEAB-32C4-5431-8226-5623E2BD9098">registered |
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with the local media subsystem</xref>. </p> <p>After the call to <xref href="GUID-9E60E8D9-619E-3A76-BAC8-93A60D62C7DF.dita#GUID-9E60E8D9-619E-3A76-BAC8-93A60D62C7DF/GUID-57B3380F-3CA6-3FF4-9D79-05B718E0743A"><apiname>LocDrv::RegisterDmaDevice()</apiname></xref>, |
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the local media subsystem will test if the address in an I/O request is a |
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physical address or a virtual address. The local media subsystem extracts |
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the information that the media driver requires. The media driver gets this |
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information through calls to the functions: </p> <ul> |
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<li id="GUID-93AEF5CB-CB70-5369-BC33-9F2B51903FFE"><p> <xref href="GUID-D84A9903-AE0F-3F54-8833-E8956A88E26C.dita#GUID-D84A9903-AE0F-3F54-8833-E8956A88E26C/GUID-DD6773B4-9EF5-322F-B53D-29174DF3B3BF"><apiname>TLocDrvRequest::IsPhysicalAddress()</apiname></xref>. </p> </li> |
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<li id="GUID-33328F10-BE3D-5ADA-8FFE-5E19764E43F1"><p> <xref href="GUID-D84A9903-AE0F-3F54-8833-E8956A88E26C.dita#GUID-D84A9903-AE0F-3F54-8833-E8956A88E26C/GUID-9879145B-D996-327E-87F6-3B8337A86A56"><apiname>TLocDrvRequest::GetNextPhysicalAddress()</apiname></xref>. </p> </li> |
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</ul> <p>A <xref href="GUID-D84A9903-AE0F-3F54-8833-E8956A88E26C.dita"><apiname>TLocDrvRequest</apiname></xref> object represents an I/O request |
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and is passed to the media driver. </p> <p>There are three pieces of information |
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that the local media subsystem needs from the media driver when the media |
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driver registers: </p> <ul> |
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<li id="GUID-7A889D6A-52E0-5FBB-BC2F-A1E10A0F0D4D"><p>The <i>minimum number |
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of bytes</i> that the media device can transfer. For example, the architecture |
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of some memory card types requires data transfer in blocks of 512 bytes. The |
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local media subsystem can not support I/O requests for physical addresses |
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where the length of the data is smaller than this minimum number. This limit |
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prevents accidental access of the data outside the limits of the request. </p> </li> |
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<li id="GUID-C778CC22-699B-5B01-B217-C6567CBE01E3"><p>The <i>maximum number |
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of bytes</i> that the media driver can transfer in one burst. This value depends |
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on the hardware. For eaxample,<xref href="GUID-0D2F811C-81C3-526F-8EA4-98E50261BF4B.dita">DMA |
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Framework</xref> has limits that depend on the DMA controller. </p> </li> |
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<li id="GUID-49B448A8-B8ED-5EF6-9AB2-BA43FD6ED34F"><p>The <i>alignment of |
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memory</i> that the DMA controller requires. For example: a DMA controller |
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might require 2 byte (word or 16 bit) alignment or 4 byte (double word or |
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32 bit) alignment. For 2 byte alignment, specify 2; for 4 byte alignment specify |
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4 etc. The local media subsystem can not support I/O requests for physical |
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addresses that are not aligned according to this value. </p> </li> |
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</ul> <p>You get all of this information from the documentation for the platform. </p> <p>This |
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example code extends the code shown in the section <xref href="GUID-A70A01D2-467E-5BA8-A01D-6182558F3F52.dita">Media |
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driver initialisation before the system is initialised</xref>. It adds a call |
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to <xref href="GUID-9E60E8D9-619E-3A76-BAC8-93A60D62C7DF.dita#GUID-9E60E8D9-619E-3A76-BAC8-93A60D62C7DF/GUID-57B3380F-3CA6-3FF4-9D79-05B718E0743A"><apiname>LocDrv::RegisterDmaDevice()</apiname></xref>. This call follows registration |
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of the media driver with the local media subsystem. </p> <codeblock id="GUID-A56AFE4C-729A-57DA-B232-60A29CE7F735" xml:space="preserve">DECLARE_STANDARD_EXTENSION() |
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{ |
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TInt r=KErrNoMemory; |
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DPrimaryMediaBase* pM=new DPrimaryMediaBase; |
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if (pM) |
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{//…Required here for Asynchronous creation (if supported) |
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pM->iDfcQ = &MyDfcQ; |
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|
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// Perform registration here |
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r = LocDrv::RegisterMediaDevice(MEDIA_DEVICE_TYPE, |
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MEDIA_DRIVECOUNT, |
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&IMediaDriveNumbers[0], |
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pM,MEDIA_NUMMEDIA,KMediaDriveName |
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); |
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if ® != KErrNone) |
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{ |
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return r; |
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} |
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|
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// Register support for physical addressing. |
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// |
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// Note : in practice the values passed to RegisterDmaDevice() would be |
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// either symbolic constants or functions that return values. If the |
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// media driver is split into platform independent and platform dependent |
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// layers, and this code is in the independent layer, then you will need |
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// functions in the dependent layer to provide these values. |
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r = LocDrv::RegisterDmaDevice(pM, |
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512, // Block Addressing 512 Bytes |
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1024, // 1024 Byte address range |
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2 ); // 2 Byte alignment |
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if ® != KErrNone) |
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92 |
{ |
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93 |
return r; |
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94 |
} |
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95 |
... |
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96 |
} |
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97 |
return®); |
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98 |
} |
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99 |
</codeblock> </section> |
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100 |
<section id="GUID-49992CF9-404A-585C-8770-E9034434A344"><title> Changes to |
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request handling</title> <p>To use physical addreses, you need to make changes |
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102 |
to the code that deals with <codeph>ERead</codeph> and <codeph>EWrite</codeph> requests |
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103 |
in your implementation of the <xref href="GUID-EBF025DB-1552-5E99-8C07-09932DB60552.dita#GUID-EBF025DB-1552-5E99-8C07-09932DB60552/GUID-EC193360-31C2-5012-8ED2-19F1C48C8FC5">DMediaDriver::Request()</xref> function. This section discusses the issues with <codeph>ERead</codeph> requests, |
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but the principles apply to <codeph>EWrite</codeph> requests. </p> <p>There |
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105 |
are a number of points to consider: </p> <p><b>Check if the address is physical </b> </p> <p>Call <xref href="GUID-D84A9903-AE0F-3F54-8833-E8956A88E26C.dita#GUID-D84A9903-AE0F-3F54-8833-E8956A88E26C/GUID-DD6773B4-9EF5-322F-B53D-29174DF3B3BF"><apiname>TLocDrvRequest::IsPhysicalAddress()</apiname></xref> to |
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106 |
test if the address passed is a physical address. For example, for a <codeph>ERead</codeph> request: </p> <codeblock id="GUID-90F1EE30-DAE5-5DE5-B42E-9EF48EB12509" xml:space="preserve">... |
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107 |
// iReadReq points to a TLocDrvRequest object |
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108 |
... |
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109 |
iMediaStartPos = iReadReq->Pos(); |
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iTotalLength = I64LOW(iReadReq->Length()); |
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iDoPhysicalAddress = iReadReq->IsPhysicalAddress(); |
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if(iDoPhysicalAddress) |
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113 |
{ |
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..< Physical address memory code >.. |
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115 |
} |
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116 |
else |
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117 |
{ |
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...< Virtual address memory code >...</codeblock> <p><b>Physical address code </b> </p> <p>If you want to use the physical address, |
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119 |
you need to get the physical address and the length of contiguous memory at |
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120 |
this location. Call <xref href="GUID-D84A9903-AE0F-3F54-8833-E8956A88E26C.dita#GUID-D84A9903-AE0F-3F54-8833-E8956A88E26C/GUID-9879145B-D996-327E-87F6-3B8337A86A56"><apiname>TLocDrvRequest::GetNextPhysicalAddress()</apiname></xref> to |
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121 |
get the physical address and the length of physically contiguous memory. The |
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122 |
length of physically contiguous memory can be smaller than the length supplied |
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123 |
in the read request, because physical memory can be fragmented into a number |
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of small blocks. If the length is smaller than the length supplied in the |
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125 |
read or write request, you call <xref href="GUID-D84A9903-AE0F-3F54-8833-E8956A88E26C.dita#GUID-D84A9903-AE0F-3F54-8833-E8956A88E26C/GUID-9879145B-D996-327E-87F6-3B8337A86A56"><apiname>TLocDrvRequest::GetNextPhysicalAddress()</apiname></xref> again |
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126 |
to get the address of the next physically contiguous block of memory. You |
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repeat this operation until the read request is complete. For example, for |
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a <codeph>ERead</codeph> request: </p> <codeblock id="GUID-2311A626-38BF-582F-B310-8E30D73AA1A7" xml:space="preserve">... |
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// iReadReq points to a TLocDrvRequest object |
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130 |
... |
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TPhysAddr physAddr; |
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132 |
TInt physLength; |
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TInt err = KErrNone; |
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err = iReadReq->GetNextPhysicalAddress(physAddr, physLength); |
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135 |
if(err == KErrNone) |
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136 |
{ |
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137 |
if (physLength < iTotalLength) |
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138 |
{ |
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139 |
// Memory is fragmented, note remainder. You will need |
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140 |
// to use this code again using the remainder value. |
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141 |
iRemaining = iTotalLength – physLength; |
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142 |
iTotalLength -= physLength; |
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143 |
} |
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144 |
|
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145 |
// Start data transfer into the current physically |
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146 |
// contiguous block of physical memory. |
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147 |
DoDataTransfer(iMediaStartPos, physLength, physAddr); |
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148 |
... |
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149 |
} |
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150 |
</codeblock> <p>If you do not want to deal with fragmented physical memory, |
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|
151 |
you can use your original code. </p> <p><b>Virtual to physical address translation </b> </p> <p>Your code must not |
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152 |
perform a virtual to physical address translation when it deals with physical |
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153 |
memory. For example: </p> <codeblock id="GUID-4377A0D5-8E41-59AF-B26E-C468ABAFCF47" xml:space="preserve">void DMMCRxDmaHelp::ChannelTransfer(const SDmaPseudoDes& aDes) |
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154 |
{ |
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155 |
… |
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156 |
TPhysAddr dest; |
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157 |
|
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158 |
if (iCurrentReq->IsPhysicalAddress()) |
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159 |
dest = (TPhysAddr) aDes.iDest; |
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160 |
else |
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|
161 |
dest = Epoc::LinearToPhysical(aDes.iDest); |
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|
162 |
TPlatDma::SetDMARegister(KHoDMA_CDSA(iChno), dest); |
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163 |
… |
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|
164 |
}</codeblock> <p><b>Eliminate inter-process copy </b> </p> <p>You must change your code to |
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|
165 |
remove calls to <xref href="GUID-D84A9903-AE0F-3F54-8833-E8956A88E26C.dita#GUID-D84A9903-AE0F-3F54-8833-E8956A88E26C/GUID-842F91FF-C780-3E2A-8860-8F34815452FC"><apiname>TLocDrvRequest::WriteRemote()</apiname></xref>. For <codeph>ERead</codeph> requests, |
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|
166 |
data for transfer is already at the physical address. For example: </p> <codeblock id="GUID-1B14C66A-476C-5E3F-B0D7-502B54324D74" xml:space="preserve">if (!iCurrentReq->IsPhysicalAddress()) |
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Initial contribution of the Adaptation Documentation.
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|
167 |
{ |
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|
168 |
if( (id == DMediaPagingDevice::ERomPageInRequest)|| |
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169 |
(id == DMediaPagingDevice::ECodePageInRequest) ) |
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170 |
{ |
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|
171 |
r = iCurrentReq->WriteToPageHandler((TUint8 *)(& iBufPtr [0]), len, usrOfst); |
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172 |
} |
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173 |
else if(id==DLocalDrive::ERead) |
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Initial contribution of the Adaptation Documentation.
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|
174 |
{ |
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Initial contribution of the Adaptation Documentation.
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|
175 |
r = iCurrentReq->WriteRemote(&iBufPtr,usrOfst); |
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Initial contribution of the Adaptation Documentation.
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176 |
} |
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|
177 |
}</codeblock> <p>The same logic applies to <codeph>EWrite</codeph> requests. |
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178 |
You need to remove calls to <xref href="GUID-D84A9903-AE0F-3F54-8833-E8956A88E26C.dita#GUID-D84A9903-AE0F-3F54-8833-E8956A88E26C/GUID-96B80631-AD1C-35E8-8613-0A630BD7D9E9"><apiname>TLocDrvRequest::ReadRemote()</apiname></xref>. </p> <p><b>Test your changes </b> </p> <p>You are recommended to run regression tests |
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179 |
on your changed code to makes sure that the media driver operates correctly. </p> </section> |
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180 |
<section id="GUID-5F4A392F-0EAF-5E69-B902-D987B8FFD610"><title>Issues about |
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181 |
physical addresses</title> <p>If the media driver can use physical addresses, |
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|
182 |
you need to be aware of a number of issues. </p> <p><b>The address scheme used by the hardware </b> </p> <p>All media devices |
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183 |
have a minimum number of bytes that they can transfer. For example, the architecture |
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|
184 |
of some memory card types requires data transfer in blocks of 512 bytes. To |
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|
185 |
read one byte from this type of media device, the media driver must read a |
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186 |
block of 512 bytes and extract the byte from the block. To write one byte |
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187 |
to a media device, the media driver must read a block of 512 bytes, change |
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|
188 |
the content of the byte, and write the block to the media device. </p> <p><b>Data transfer smaller than the minimum size </b> </p> <p>If the local |
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Initial contribution of the Adaptation Documentation.
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|
189 |
media subsystem receives a request to transfer data with a length smaller |
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190 |
than the minimum transfer size, the local media subsystem does not make a |
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191 |
physical address available to the media driver. A call to <xref href="GUID-D84A9903-AE0F-3F54-8833-E8956A88E26C.dita#GUID-D84A9903-AE0F-3F54-8833-E8956A88E26C/GUID-DD6773B4-9EF5-322F-B53D-29174DF3B3BF"><apiname>TLocDrvRequest::IsPhysicalAddress()</apiname></xref> returns |
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Initial contribution of the Adaptation Documentation.
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false. It is considered unsafe to give access to the physical data surrounding |
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the requested memory location. </p> <p><b>Data transfer not aligned to the media device block boundary </b> </p> <p>If |
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the local media subsystem receives a request to transfer data, and the address |
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on the media device is <i>not aligned</i> to the media device block boundary, |
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you need to adopt the technique suggested below. The local media subsystem |
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will make the physical address available to the media driver. A call to <xref href="GUID-D84A9903-AE0F-3F54-8833-E8956A88E26C.dita#GUID-D84A9903-AE0F-3F54-8833-E8956A88E26C/GUID-DD6773B4-9EF5-322F-B53D-29174DF3B3BF"><apiname>TLocDrvRequest::IsPhysicalAddress()</apiname></xref> returns |
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true. </p> <p>Consider the following case. A request has been made to read |
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1024 bytes from a media device that has a block size of 512 bytes. The 1024 |
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bytes start at offset +256 on the media device. </p> <fig id="GUID-2689C022-180B-51EF-A02D-E63ACA832EB2"> |
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<image href="GUID-647ADEDA-AB5A-548F-93C3-D099EAE6A030_d0e17089_href.png" placement="inline"/> |
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</fig> <p>To get the first 256 bytes, you must read the first block of 512 |
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bytes from the media device. This can corrupt the physical memory passed in |
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the I/O request. The solution is to read the first block from the media device |
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into an intermediate buffer. Copy the 256 bytes from that buffer into the |
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physical memory passed in the I/O request. </p> <p>To get the last 256 bytes, |
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you must read the third block of 512 bytes from the media device into the |
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intermediate buffer. Copy the 256 bytes from that buffer into the correct |
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position in the physical memory passed in the I/O request. </p> <p>The middle |
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512 bytes are aligned on the media device block boundary. The media driver |
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can read this data into the correct position in the physical memory passed |
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in the I/O request. </p> <p><b>Scatter/Gather DMA controllers </b> </p> <p>DMA controllers can support |
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the Scatter/Gather mode of operation. Each request in this mode of operation |
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consists of a set of smaller requests chained together. This chain of requests |
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is called the Scatter/Gather list. Each item in the list consists of a physical |
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address and a length. </p> <p>Use <xref href="GUID-D84A9903-AE0F-3F54-8833-E8956A88E26C.dita#GUID-D84A9903-AE0F-3F54-8833-E8956A88E26C/GUID-9879145B-D996-327E-87F6-3B8337A86A56"><apiname>TLocDrvRequest::GetNextPhysicalAddress()</apiname></xref> to |
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help you populate the Scatter/Gather list. </p> <p>The following code fragment |
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shows how you do this. The example assumes that the DMA controller supports |
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a Scatter/Gather list with an unlimited number of entries. In practice, the |
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number of entries in the list is finite. </p> <codeblock id="GUID-EC3A441C-080F-5EB8-B0E8-E658F80687D4" xml:space="preserve">TPhysAddr physAddr; |
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TInt physLength; |
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TInt err = KErrNone; |
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|
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while (iRemaining > 0) |
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{ |
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err = iCurrentReq->GetNextPhysicalAddress(physAddr, physLength); |
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if(err != KErrNone) |
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return err; |
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|
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iRemaining -= physLength; |
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PopulateScatterGatherList(physAddr, physLength); |
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} |
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|
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return DoDataTransfer(pos, length);</codeblock> </section> |
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</conbody><related-links> |
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<link href="GUID-86082C0C-B0EE-5E7C-85B4-4A509066012F.dita"><linktext>MMC Porting |
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Implementation Tutorial</linktext></link> |
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</related-links></concept> |