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    10 <!DOCTYPE concept

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

    12 <concept id="GUID-204B4155-AE35-5C53-867B-299AF7E5D16A" xml:lang="en"><title>Device

    13 driver services</title><shortdesc>Describes asynchronous services provided by device drivers.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

    14 <p>Some asynchronous services are provided directly by device drivers.</p>

    15 <p>As with services provided by the client-server framework, device driver

    16 provided services are presented to clients through a client API known as the

    17 user-side device driver. This is typically an <codeph>R</codeph> class, derived

    18 from <codeph>RLogicalChannel</codeph>. </p>

    19 <p>When the <codeph>RLogicalChannel</codeph> is opened, the device driver

    20 is initialised and the client’s thread id is noted. Request functions are

    21 converted into a message which encapsulates all the parameters, and sent to

    22 the Kernel-side driver. Completion of a request function usually corresponds

    23 closely with completion of a device operation. On completion, the driver posts

    24 the request status and signals the client thread’s request semaphore.</p>

    25 <p>Device drivers often have limited capability. For instance, an inability

    26 to be opened by multiple client threads simultaneously. Therefore, devices

    27 are often handled by specially tailored server threads. These servers present

    28 a client API with more sophistication, sharing and checking. </p>

    29 <p>Usually, therefore, clients use the server-provided classes rather than

    30 the user-side device driver directly.</p>

    31 </conbody></concept>