DEVICE DRIVERS AND EXTENSIONS in VS .NET

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DEVICE DRIVERS AND EXTENSIONS
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This example demonstrates some of the following fundamental device driver concepts: Loading of a logical and physical device User::LoadLogicalDevice User::LoadPhysicalDevice Opening a channel to the device driver RSimpleSerialChannel::Open Performing a synchronous operation RSimpleSerialChannel::Config RSimpleSerialChannel::SetConfig Performing an asynchronous operation RSimpleSerialChannel::Read RSimpleSerialChannel::Write Closing the channel to the device driver RSimpleSerialChannel::Close In the following sections I ll be discussing the underlying principles behind each of these concepts, both from a user-side perspective and how these operations are translated and implemented by the kernel and device driver.
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12.4.4.1 Loading the driver from user-side code
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As I have previously mentioned, device drivers are kernel DLLs that are initialized by the loader in the same manner as any other DLL; this contrasts with the extensions I mentioned previously, which are XIP modules initialized explicitly by the kernel without the use of the loader. Before a client can use a device driver, its DLLs must be loaded using a combination of the following APIs:
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TInt User::LoadLogicalDevice(const TDesC &aFileName) TInt User::LoadPhysicalDevice(const TDesC &aFileName)
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These functions ask the loader to search the system path for the required LDD or PDD. If you don t supply a lename extension, then the required extension (.LDD or .PDD) will be added to the lename. If the le is found, its UID values are veri ed to make sure the DLL is a valid LDD or PDD before the image is loaded. Once loaded, the kernel proceeds to call the DLL entry point as described in Section 12.1.6, and this constructs any global objects in the DLL. After loading the DLL, the loader calls its rst export immediately. LDDs and PDDs are polymorphic DLLs, and the rst export is de ned as the factory function required to create an object of a class derived
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DEVICE DRIVERS
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from either DLogicalDevice for an LDD, or DPhysicalDevice for a PDD. (I ll describe these classes in detail in the next section.) As I described in Section 12.2.2, the kernel de nes two macros, DECLARE_STANDARD_LDD and DECLARE_STANDARD_PDD, which are used by the device driver to de ne both the kernel module entry point and the exported factory function, as shown in the following example:
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DECLARE_STANDARD_LDD() { return new DSimpleSerialLDD; }
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If the device driver needs to perform some one-off initialization at system boot time, you should ensure that it is also an extension. In this case, you would use the DECLARE_STANDARD_EXTENSION macro discussed in Section 12.2.2 to de ne the custom kernel module entry point, and use the alternative DECLARE_EXTENSION_LDD and DECLARE_EXTENSION_PDD macros to export the factory function. Note: If you are using this feature to allocate resources early in the boot process, consider carefully whether such initialization would be better off being deferred to some later point in the process (such as when the driver is actually loaded or a channel is created). Any resources allocated at boot time will remain allocated until the system is rebooted, which may not be the behavior that you are looking for. Once the factory object is created, the kernel calls its second phase constructor, Install(). You must register an appropriate name for the newly created LDD or PDD factory object with this function (see the following example), as well as performing any other driver-speci c initialization as required. If this function is successful, the kernel will add the named object to its kernel object container. The kernel reserves two object containers speci cally to maintain the list of currently loaded LDD and PDD factory objects: Object DLogicalDevice DPhysicalDevice Container ELogicalDevice EphysicalDevice Name <ldd> <ldd>.<pdd>
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All future references to the name of the device driver should now refer to the object name, rather than the LDD/PDD lename. The object name is also used to associate a PDD with a speci c class of LDD I ll talk about this more later in the chapter. Note: Some device drivers deviate from the standard <ldd>.<pdd> naming convention and de ne a different pre x for PDD names than the
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