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+++ b/Adaptation/GUID-6CE68645-D8C5-52E1-88B3-76AEFB80DE51.dita	Fri Oct 15 14:32:18 2010 +0100
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+<?xml version="1.0" encoding="utf-8"?>
+<!-- Copyright (c) 2007-2010 Nokia Corporation and/or its subsidiary(-ies) All rights reserved. -->
+<!-- This component and the accompanying materials are made available under the terms of the License 
+"Eclipse Public License v1.0" which accompanies this distribution, 
+and is available at the URL "http://www.eclipse.org/legal/epl-v10.html". -->
+<!-- Initial Contributors:
+    Nokia Corporation - initial contribution.
+Contributors: 
+-->
+<!DOCTYPE concept
+  PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
+<concept id="GUID-6CE68645-D8C5-52E1-88B3-76AEFB80DE51" xml:lang="en"><title>Peripherals
+As Power Resources</title><shortdesc>Peripherals that provide services to other peripherals can be considered
+as power resources from the point of view of the client driver.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
+<p>As they can offer a service to more than one other peripheral, we suggest
+that the driver that manages them presents a <xref href="GUID-10105403-AF16-3908-AE9E-96CF2A03AD3A.dita"><apiname>MPowerInput</apiname></xref> derived
+interface to client peripheral drivers, providing <codeph>Use()</codeph> and <codeph>Release()</codeph> functions
+that can be used by dependent peripheral drivers to issue power requests.
+The driver for the peripheral providing that interface may then use the built-in
+usage counting feature to control the peripheral power state. </p>
+<p>A typical example of a shared Peripheral is an Inter-Component serial Bus
+(I2C, SPI, etc): </p>
+<codeblock id="GUID-2E454D22-496D-5D9C-9F67-281D6F340459" xml:space="preserve">class MySharedPeripheral : public MPowerInput , (other parent classes)
+    {
+public:    
+    // from MPowerInput
+    void Use();
+    void Release();
+    TUint GetCount();
+    ...
+    (other methods and data members required to model the interface)
+    };
+      </codeblock>
+<p>Special care must be taken to not create circular dependencies, for example,
+Peripheral A depends on Peripheral B, which depends on Peripheral C which
+depends on Peripheral A. Such circular dependencies would cause the <xref href="GUID-10105403-AF16-3908-AE9E-96CF2A03AD3A.dita#GUID-10105403-AF16-3908-AE9E-96CF2A03AD3A/GUID-606ECD77-A5F7-3408-9B63-C68C0A7B73C6"><apiname>MPowerInput::Release()</apiname></xref> mechanism
+not to work properly, and preventing the chain of peripherals from powering
+down when due. </p>
+<p>The shared peripheral driver’s power handler asynchronous <codeph>PowerDown()</codeph> must
+examine the usage count of that peripheral if the power manager issues a <codeph>PowerDown()</codeph> request
+to the shared peripheral driver’s power handler while the peripheral is still
+in use by another driver. In this situation, the shared peripheral driver
+must defer powering down until all the dependent peripherals have released
+their request on it. </p>
+<p>Some peripherals may also be represented as power resources to prevent
+the base port power saving operations inhibiting their usage. For example,
+a base port may consider DRAM to be a shared power resource with the DMA controller,
+or the DSP controller as clients. This will allow them to still be able to
+access DRAM whilst the CPU is idling, thus stopping the DRAM going into self-refresh
+if either one of its clients has pending operations on it. </p>
+<p>As another example, to save power some base ports might decide to reduce
+the refresh rate of the LCD whilst the CPU is idling or even power it down
+for short periods and rely on the persistence of the LCD to prevent degredation
+of the user experience. However, on a multiple core solution the LCD might
+be used by the second core (e.g. DSP), for example, for playing back video
+whilst the main CPU is idling. In this situation the LCD should be represented
+as a shared power resource and the DSP must be able to request usage of this
+resource. The DSP controller software could assert a request on the LCD preventing
+the main CPU from initiating the power saving measures. </p>
+</conbody></concept>
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