1. 根目录:
2. include
3. linux
4. dma-direction.h

#ifndef _LINUX_DMA_DIRECTION_H
#define _LINUX_DMA_DIRECTION_H
/*
 * These definitions mirror those in pci.h, so they can be used
 * interchangeably with their PCI_ counterparts.
 */
enum dma_data_direction {
	DMA_BIDIRECTIONAL = 0,
	DMA_TO_DEVICE = 1,
	DMA_FROM_DEVICE = 2,
	DMA_NONE = 3,
};
#endif