DMABUF， DMA mapping，IOMMU的区别

1. DMABUF can be used as a wrapper to encapsulate other memory management frameworks. All these memory management framework(I mean mostly for graphics), buffer is the keypoint. DMABUF defines a standard buffer structure. So DMABUF can be used as a wrapper for  TTM/GEM/Android ION... and etc. Notice DMABUF can't replace these things, cause it doesn't cover everything. E.g: DMABUF has no   user space interfaces, right now only kernel interfaces(can be used in device driver).（DMABUF定义了一个标准的buffer结构，因此可用于封装其它的内存管理框架（ION），但不能够代替这些框架，因为DMABUF没有用户空间的接口，仅有内核接口）2. Kernel has DMA mapping API from origin. ARM defines IOMMU which can be used to connect scattered physical memory as a continuous region for devices which needs continue address towork(e.g: DMA). So IOMMU implementations & CMA should work behind kernel DMA mapping API. E.g: dma_alloc_from_contiguous can be implemented by CMA;dma_alloc_coherent can be implemented by IOMMU or by the normal case(just call__get_free_pages). So for device drivers need dma buffers, we should use dmamapping APIs, not call iommu api directly.（device drivers先使用dma mapping APIs（它调用IOMMU机制的函数））3. For tegra, GART & SMMU can be used to implement IOMMU apis.