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Data Transfer:-

The network function belonging to the sample CardBus device uses the following strategy for data transfer: The card expects the device driver to supply it with an array of two receive DMA descriptors and an array of two transmit DMA descriptors. Each DMA descriptor contains the address of an associated data buffer, its length, and a control word. You can use the control word to tell the device whether the descriptor contains valid data. For a transmit descriptor, you may also program it to request an interrupt after data transmission. The card looks for a valid descriptor and DMA's data to/from the associated data buffer. To suit this elementary scheme, the example driver uses only the coherent DMA interface. The driver coherently allocates a large buffer that holds the descriptors and their associated data buffers. The receive and transmit buffers are 1536 bytes long to match the maximum transmission unit (MTU) of Ethernet frames. The descriptors and buffers are pictorially shown in the Figure (given below). The top 24 bytes of each array in the figure hold two 12-byte DMA descriptors, and the rest of the memory is occupied by two 1536-byte DMA buffers. The 12-byte descriptor layout shown in the figure is assumed to match the format specified in the card's data sheet.



(DMA descriptors and buffers for the CardBus device.)