Device drivers in .NET

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Device drivers
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In this section of the chapter I ll be discussing device drivers in a little more depth. I ll talk about the execution model, and about how to create a device driver. I ll also discuss how user code interacts with a device driver. To make this concrete, I ll be walking through the creation of my own device driver, a simple comms driver. But rst, let s look at the device driver counterpart to the extension s entry point macros.
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12.4.1 Device driver entry point macros
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DECLARE STANDARD LDD and DECLARE STANDARD PDD The following macros are provided to support device driver LDDs and PDDs:
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#define DECLARE_STANDARD_LDD() TInt KernelModuleEntry(TInt) { return KErrNone; } EXPORT_C DLogicalDevice* CreateLogicalDevice() \ \ \
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DEVICE DRIVERS
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#define DECLARE_STANDARD_PDD() TInt KernelModuleEntry(TInt) { return KErrNone; } EXPORT_C DPhysicalDevice* CreatePhysicalDevice()
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\ \ \
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This would be implemented in an LDD as follows:
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DECLARE_STANDARD_LDD() { return new DSimpleSerialLDD; }
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Notice that KernelModuleEntry does not provide any initialization hooks. As we shall see later, LDDs and PDDs are polymorphic DLLs which are dynamically loaded after the kernel has booted. Instead, this macro de nes the rst export to represent the DLL factory function.
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DECLARE EXTENSION LDD and DECLARE EXTENSION PDD Although device drivers are dynamically loadable DLLs, there may be some instances where a device driver must perform some one-off initialization at boot time. For example, media drivers register themselves with the local media sub-system at boot time to provide information about the number of supported drives, partitions and drive numbers prior to the driver being loaded (this is described in detail in 13, Peripheral Support ). To support this, you should use the DECLARE_STANDARD_EXTENSION macro previously described, in conjunction with the following macros to export the required factory function:
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#define DECLARE_EXTENSION_LDD() \ EXPORT_C DLogicalDevice* CreateLogicalDevice() #define DECLARE_EXTENSION_PDD() \ EXPORT_C DPhysicalDevice* CreatePhysicalDevice()
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12.4.2 Device driver classes
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Throughout this chapter I shall be referring to the various classes that make up the EKA2 device driver framework. Figure 12.3 gives an overview of these classes which you can refer back to while you are reading this chapter. In Figure 12.3, the white boxes represent classes provided by the EKA2 device driver framework. The shaded boxes indicate classes that must be implemented by the device driver. Two components make up the logical device driver (LDD) the LDD factory (derived from DLogicalDevice) and the logical channel
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DEVICE DRIVERS AND EXTENSIONS
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The PDD factory The logical channel
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DPhysicalDevice
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The LDD factory
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DLogicalDevice
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DLogicalChannelBase iDevice iPhysicalDevice iPdd DDevice1PddFactory
DDriver1Factory
DBase
DDriver1
DLogicalChannel TDfcQue iDfcQ iMsgQ TMessageQue DDriver1Device
The physical channel
DDriver1Channel
Figure 12.3 The EKA2 device driver classes
(derived from DLogicalChannelBase). The LDD factory is responsible for creating an instance of the logical channel, which, as I described in the overview in Section 12.1.2.1, contains functionality that is common to a speci c class of devices (such as communications devices). A user-side application communicates with the logical channel via a handle to the logical channel (RBusLogicalChannel). Similarly, two components make up the physical device driver (PDD) the PDD factory (derived from DPhysicalDevice) and the physical channel (derived from DBase). As I also described in Section 12.1.2.2, the physical channel is responsible for communicating with the underlying hardware on behalf of the more generic logical channel. The physical channel exists purely to provide functionality to the logical channel, so is not directly accessible from the user side. Note: In Figure 12.3, two shaded boxes appear in the physical channel (DDriver1 and DDriver1Device). These represent a further abstraction known as the platform-independent and platform-speci c layers (PIL/PSL). The PIL (DDriver1) contains functionality that, although not generic enough to live in the logical channel, is applicable to all hardware that your driver may be implemented on. The PSL (DDriver1Device) contains functionality that is too speci c to belong in the LDD or PSL, such as the reading and writing of hardware-speci c registers. Such
DEVICE DRIVERS
layering is often bene cial when porting your device driver to a new platform, but is optional so it is up to you when designing your device driver to determine if such layering is appropriate.