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Adaptation/GUID-F75F975B-A490-4EB5-96CD-B2F8DFC63C7C.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-F75F975B-A490-4EB5-96CD-B2F8DFC63C7C" xml:lang="en"><title>Asynchronous
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Requests</title><shortdesc>This document describes how device drivers use asynchronous requests.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
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<section id="GUID-834B2C34-5C8B-4D6D-85F3-FADD357AD330">             <p>An
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asynchronous request is one which is used, typically, to communicate with
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the hardware itself. The time taken for such a request to complete depends
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on the hardware, and the client user-side thread may want to continue with
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some other processing.  The user-side thread is not blocked and can continue
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with other operations, including issuing other requests (synchronous or asynchronous).</p> <p>An
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asynchronous request is started by a call to <xref href="GUID-6FBFA078-8253-3E24-B1F8-5F75E86C3066.dita#GUID-6FBFA078-8253-3E24-B1F8-5F75E86C3066/GUID-FCC4A3A9-69F5-3947-8D0F-2AD12086F1E2"><apiname>RBusLogicalChannel::DoRequest()</apiname></xref>.
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Control normally returns as soon as the request has been issued, successfully
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or otherwise. Typically, the hardware indicates completion of an operation
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by generating an interrupt. This is handled by an Interrupt Service Routine
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(ISR) provided by the driver. This, in turn, schedules a DFC, which at some
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later time, runs in the context of a kernel-side thread, and signals the client-user
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side thread, marking the asynchronous request as complete. </p> <p>More than
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one asynchronous request can be outstanding at the same time, each one associated
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with its own <xref href="GUID-E0B34F3E-D4C4-3232-B8B1-7DB35B454646.dita"><apiname>TRequestStatus</apiname></xref> object and each identified
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by a specific request number. The device driver framework puts no explicit
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limit on the number of concurrent outstanding asynchronous requests; any limit
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must be enforced by the driver itself. However, the API to cancel a request
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implicitly prevents you from uniquely identifying more than 32 concurrent
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requests. </p> <p>An outstanding asynchronous request can be cancelled by
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a call to <xref href="GUID-6FBFA078-8253-3E24-B1F8-5F75E86C3066.dita#GUID-6FBFA078-8253-3E24-B1F8-5F75E86C3066/GUID-16E139F5-4473-33C0-BCDD-A92152D4E8E6"><apiname>RBusLogicalChannel::DoCancel()</apiname></xref>. </p>     </section>
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</conbody></concept>
MCL/sftools/depl/docscontent