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Kernel extensions
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Since the ASSP and variant modules are initialized before the kernel is initialized, and the kernel loads all extensions before the le server (which itself is loaded by an extension), then there must be some special mechanism in place to load these modules. And indeed there is extensions are simply execute-in-place DLLs that are speci ed at ROM build time, allowing the build tools to place the address of the extension in the ROM header. This allows the kernel to initialize extensions without using the loader.
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12.2.1 Installing an extension
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To install a kernel extension into a Symbian OS ROM image, you need to specify one of the following keywords in the kernel s OBY le: Variant: Extension: The variant extension A standard kernel or ASSP extension
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The ROMBUILD tool uses these to build the ROM header (represented by the TRomHeader class), which contains two extension lists iVariantFile contains the list of variants, and iExtensionFile contains the list of all other extensions. As a consequence of this, extensions are always initialized in the order at which they appear in the kernel s IBY le. This is an extremely desirable feature, as many extensions and device drivers depend on other extensions being present to initialize. The MMC controller is a good example of this, as it depends on the power management extension and possibly the DMA framework too. Note that although you can specify more than one variant keyword to include several variant extensions in the ROM, the kernel will only initialize one of them. Each variant DLL contains an identi er which speci es which CPU, ASIC and variant it was built for. The build tools place this information in the ROM header, and the bootstrap may later modify it when it copies it to the kernel superpage, thus providing the ability to create multi-platform ROMs. The same principle applies to standard extensions. However, this feature is rarely used in a production device due to ROM budget constraints.
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12.2.2 Extension entry point macros
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You should de ne the extension entry point, KernelModuleEntry, when writing a kernel extension, and interpret the supplied reason codes
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KERNEL EXTENSIONS
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according to the rules I have described. For example, a standard kernel extension entry point would look something like this:
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TInt KernelModuleEntry(Tint aReason) { if (aReason==KModuleEntryReasonExtensionInit0) return KErrNone; if (aReason!=KModuleEntryReasonExtensionInit1) return KErrArgument; //... do extension specific initialisation here }
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Since all extensions follow the same pattern (and it s easy to make a mistake and dif cult to debug what has gone wrong), the kernel provides you with a set of standard macros (de ned in kernel.h) that do the hard work for you:
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DECLARE STANDARD EXTENSION This is de ned by the kernel as follows:
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#define DECLARE_STANDARD_EXTENSION() GLREF_C TInt InitExtension(); TInt KernelModuleEntry(TInt aReason) { if (aReason==KModuleEntryReasonExtensionInit0) return KErrNone; if (aReason!=KModuleEntryReasonExtensionInit1) return KErrArgument; return InitExtension(); } GLDEF_C TInt InitExtension()
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Thus reducing the entry point for a standard extension to:
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DECLARE_STANDARD_EXTENSION() { // Initialisation code here }
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DECLARE STANDARD ASSP The ASSP extension entry point simply re-invokes the DLL entry point to initialize the extension s constructors:
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#defineDECLARE_STANDARD_ASSP() \ extern "C" { GLREF_C TInt _E32Dll(TInt); } \ GLDEF_C TInt KernelModuleEntry(TInt aReason) \ { return (aReason==KModuleEntryReasonExtensionInit1)\ KErrNone:KErrGeneral; } \
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EXPORT_C void AsicInitialise() \ { E32Dll(KModuleEntryReasonExtensionInit1); }
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Thus reducing the entry point for the ASSP to:
DECLARE_STANDARD_ASSP()
You only need to declare this you don t need to write any extra code.
12.2.2.1 Extensions on the emulator
Since the emulator is based on the Windows DLL model and there is no ROMBUILD stage involved, we are not able to specify the order in which extensions are loaded at build time. To solve this problem, the Symbian OS emulator has its own mechanism for loading extensions, which ensures that the behavior of the emulated platform is identical to that of target hardware. The kernel loads the emulator s variant extension explicitly by name (ECUST.DLL) and invokes the exported VariantInitialise() function. This registers the extensions to be loaded by publishing a Publish/Subscribe key named Extension :
if (iProperties.Append("Extension", "winsgui;elocd.ldd;medint.pdd;medlfs.pdd; epbusv.dll;mednand.pdd") == NULL) return KErrNoMemory;
The emulator-speci c parts of the kernel that would be responsible for loading extensions from ROM on target hardware are then able to read this string and explicitly load each DLL in the order it appears in the list. You should modify this list if you are developing extensions that need to be loaded into the emulator. The code can be found in the Wins::InitProperties() function in the source le \wins \specific\property.cpp.