DMA flags

DMA flags (attributes) are declared in the io/io_dma.h file.

• DMA_DIR_TO_DEVICE allows transactions from the main memory to the device memory.
• DMA_DIR_FROM_DEVICE allows transactions from the device memory to the main memory.
• DMA_DIR_BIDIR allows transactions from the main memory to the device memory, and vice versa.
• DMA_ZONE_DMA32 allows the use of only the first 4 GB of memory for the buffer.
• DMA_ATTR_WRITE_BACK, DMA_ATTR_WRITE_THROUGH, DMA_ATTR_CACHE_DISABLE, and DMA_ATTR_WRITE_COMBINE are for managing the cache of memory pages.