dma_map_single
==> map DMA address for virtual memory,
==>use kmalloc if continuous memory is not enough, it returens fail
dma_unmap_single
==> free memory by allocating dma_map_single
dma_alloc_cohernet
==> allocate coherent memory for DMA
==> it allocates memory in conherent area,
==> refer to memory.h in architecture
==> it can be decided by CONSISTENT_DMA_SIZE
other APIs
==> dma_pool_create
==> dma_pool_alloc
==> dma_pool_free
==> dma_pool_destory
==> dma_alloc_writecombine
Also you can decide dma memory attributes by using kernel APIs
For more detail, refer to dma-api.txt in kernel doc
condiserations when you alloc DMA memory
1. allocate meory in coherent area ?
2. use memory as like pool?
3. how mush you are using coherent memory
4. swap area is used or not
But everytime you can not have enough memory......if you have, it's not real
