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<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>