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Let s make this clearer with an example. Suppose we have a media driver that is designed to optimize data transfer to data addresses that are within a shared chunk. For example, a request might come via the le server from a multimedia application to save data to a le from a buffer in a shared chunk. The le server and media driver only support descriptorbased APIs, but if the driver uses the Kern::OpenSharedChunk() function, then we can still optimize the transfer using the shared chunk. Once the driver has opened the chunk, it next needs to obtain the address of the data within it. Remember that shared chunks may contain uncommitted regions (gaps) and the driver needs to detect these to avoid making an access attempt to a bad address, which would cause an exception. There are two functions provided for this the rst obtains just the linear address, the second also obtains the physical address. Taking the rst of these:
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TInt Kern::ChunkAddress(DChunk* aChunk, TInt aOffset, TInt aSize, TLinAddr& aKernelAddress)
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If chunk aChunk is a shared chunk, and if the region starting at offset aOffset from the start of the chunk and of size aSize (both in bytes) contains committed memory, then the function succeeds. In this case, the argument aKernelAddress returns the linear address in the kernel process corresponding to the start of the speci ed region. However, if the region isn t within the chunk, or the whole region doesn t contain committed memory, then an error is returned. Now the second function:
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TInt Kern::ChunkPhysicalAddress(DChunk* Chunk, TInt aOffset, TInt aSize, TLinAddr& aKernelAddress, TUint32& aMapAttr, TUint32& aPhysicalAddress, TUint32* aPageList)
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The rst four arguments are identical to those described for the previous function. If the function is successful, the argument aMapAttr will contain the mapping attributes that apply for the chunk, and the argument aPhysicalAddress will contain the physical address of the rst byte in the speci ed region. The argument aPageList returns the addresses of each of the physical pages that contain the speci ed region.
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An example driver using a shared chunk for DMA transfer
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To illustrate how shared chunks can be used, let us consider as an example a minimal device driver for an unspeci ed peripheral. Our driver supports only one type of request the asynchronous transmission
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of data out of the phone from a memory buffer residing within a shared chunk. The user-side interface to the driver is as follows:
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const TInt KMyDeviceBufSize=0x2000; // 8KB
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class RMyDevice : public RBusLogicalChannel { public: enum TRequest { EWriteBuf=0x0,EWriteBufCancel=0x1,}; enum TControl { EGetChunkHandle,EGetBufInfo,}; #ifndef __KERNEL_MODE__ public: inline TInt Open(); inline TInt GetChunkHandle(RChunk& aChunk); inline TInt GetBufInfo(TInt aBufNum,TInt& aBufOffset); inline void WriteBuffer(TRequestStatus& aStatus, TInt aBufNum,TUint aBufOffset,TInt aLen); #endif };
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The driver creates a shared chunk when it is opened. At the same time, it commits memory to two separate buffers within the chunk, each of size KMyDeviceBufSize, and each containing physically contiguous RAM pages. To gain access to these buffers, the user application must rst create a handle on the chunk, using the method GetChunkHandle(). This maps the chunk s memory into the address space of the process. The application obtains a pointer to the base of this region using the method RChunk::Base(). The application must then determine the offsets of the two buffers relative to this base address using the method GetBufInfo() the argument aBufNum specifying the buffer and aBufOffset returning its offset. Now the application can ll a buffer with the transmission data taking care not to access beyond the perimeter of the buffer, as this would normally cause it to panic. Finally, the application issues a request to transmit the contents of the buffer using the method WriteBuffer(). The argument aBufNum identi es which buffer should be used for the transfer and the argument aBufOffset provides the offset within this to start the transfer from. Argument aLen provides the number of bytes to transfer. When exchanging information on data locations within a shared chunk between driver and user code, you must always take care to specify this information as an offset rather than an address, since the chunk appears at different addresses in the address spaces of the different processes. Of course, the same applies when exchanging this information between user processes.
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