DEVICE DRIVERS AND EXTENSIONS in .NET

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variant HAL handler within the Asic::VariantHal() method of your variant class. This provides a simple low level interface through which the caller can obtain the processor speed and machine ID, select the active debug port, and control the debug LEDs and switches. EHalGroupDisplay The LCD extension provides the HAL functions de ned by TDisplayHalFunction, which include functions to retrieve the current operating mode of the display, set the contrast, modify the palette and switch the display on or off. This is usually one of the rst set of HAL functions that you would implement during a base port. EHalGroupPower If a power model is registered with the kernel, it will handle the HAL functions de ned by TPowerHalFunction. This provides an interface to retrieve information on the state of the power supply, battery capacity and case open/close switches. Several other HAL groups also have handlers that are implemented and registered by various modules in the OS, depending on the hardware supported by the mobile device.
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12.3.1 Registering HAL entries
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The Kern class exports the following methods to allow modules to register and deregister a HAL group handler:
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TInt AddHalEntry(TInt aId, THalFunc aFunc, TAny* aPtr); TInt AddHalEntry(TInt aId, THalFunc aFunc, TAny* aPtr, TInt aDeviceNumber); TInt RemoveHalEntry(TInt aId); TInt RemoveHalEntry(TInt aId, TInt aDeviceNumber);
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Note: An extension is unlikely to remove a HAL entry as extensions are never unloaded from the system. However, device drivers are dynamically loaded so must remove their handlers as part of their shutdown process. The arguments to the AddHalEntry APIs are the ID of a HAL group, a pointer to the handler function and a pointer to a data structure that will be passed to the handler function. A handler may also take a device number as an argument, so that it can be made device-speci c. For example, a second video driver could make itself a handler for display attributes by calling:
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Kern::AddHalEntry(EHalGroupDisplay, &handler, this, 1).
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The device number for a HAL function is determined by the top 16 bits of the associated HAL group number passed to the function. If a handler already exists for the HAL group, this handler will not be registered.
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THE HARDWARE ABSTRACTION LAYER
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The HAL handler function prototype is de ned by THalFunc.
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typedef TInt (*THalFunc)(TAny*,TInt,TAny*,TAny*);
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The arguments to this are the pointer registered with the HAL handler, the HAL function number and two optional arguments, the de nition of which are dependent on the HAL function. They are usually used to read or write data passed from the client to the handler. Let s take a look at how the LCD extension registers its HAL handler with the system. This is done when the extension is initialized:
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DECLARE_STANDARD_EXTENSION() { // create LCD power handler TInt r = KErrNoMemory; DLcdPowerHandler* pH = new DLcdPowerHandler; // LCD specific initialisation omitted for clarity if(pH != NULL) { r = Kern::AddHalEntry(EHalGroupDisplay, halFunction, pH); } return r; }
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This creates the LCD driver and registers a HAL handler (halFunction) for the EHalGroupDisplay group, passing a pointer to the LCD driver for context when the handler is invoked. When a client makes a HAL request, halFunction is invoked, which is implemented as follows (most HAL functions omitted for clarity):
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LOCAL_C TInt halFunction(TAny* aPtr, TInt aFunction, TAny* a1, TAny* a2) { DLcdPowerHandler* pH=(DLcdPowerHandler*)aPtr; return pH->HalFunction(aFunction,a1,a2); } TInt DLcdPowerHandler::HalFunction(TInt aFunction, TAny* a1, TAny* a2) { TInt r=KErrNone; switch(aFunction) { case EDisplayHalScreenInfo: { TPckgBuf<TScreenInfoV01> vPckg; ScreenInfo(vPckg()); Kern::InfoCopy(*(TDes8*)a1,vPckg); break; }
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DEVICE DRIVERS AND EXTENSIONS
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case EDisplayHalSecure: kumemput32(a1, &iSecureDisplay, sizeof(TBool)); break; case EDisplayHalSetState: { if(!Kern::CurrentThreadHasCapability(ECapabilityPowerMgmt, NULL)) return KErrPermissionDenied; if ((TBool)a1) WsSwitchOnScreen(); else WsSwitchOffScreen(); } default: r=KErrNotSupported; break; } return r; }
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Note in particular how this example performs API policing and safely accesses user-side memory using the APIs described in Sections 12.1.7 and 12.1.8. These are absolutely essential when implementing HAL handlers, device drivers or any other service that responds to requests from user-side code.
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