Change Details

In order to work properly with an IOMMU the LinuxKPI DMA related interfaces should use bus_dma(9) to obtain DMA addresses rather than use physical addresses.

Currently the LinuxKPI uses physical addresses to create mappings for the DMA related interfaces. These physical addresses are not true DMA addresses and hence can't be used to program a device's DMA engine in conjunction with an IOMMU. To properly supply DMA addresses for use with an IOMMU, the LinuxKPI should use the bus_dma(9) interfaces to setup and teardown the mappings. The linux API provides an implementation of allocate/free plus map/unmap large memory regions, map/unmap a single memory region, map/unmap a scatterlist, and a dma_pool interface for a cache of smaller memory regions. For the most part the linux allocation/mapping setup translates nicely into bus_dmamap_create() and _bus_dmamap_load_phys(). The invalidation/teardown presents more of a challenge as both bus_dmamap_unload() and bus_dmamap_destroy() need to be called for a given DMA address. To find the bus_dmamap_t associated with a given DMA address a path-compressed radix trie is used with the DMA address functioning as a key. The tree is populated with entries (struct linux_dma_obj) containing the dmamap, the DMA address and in the case of a dma_pool entry the virtual address too. A UMA cache is used for the tree's entries as well as the tree’s internal nodes. The dma_pool supplies both allocated and mapped memory regions with characteristics specified when the pool is created. For efficiency a cache-only UMA zone is used. The items in the cache contain a DMA-able memory region and a DMA pctrie entry. The zones import and release routines allocate and free the DMA-memory with bus_dmamem_alloc() and bus_dmamem_free() as well the associated DMA tree entries (struct linux_dma_obj).

In order to work properly with an IOMMU the LinuxKPI DMA related interfaces should use bus_dma(9) to obtain DMA addresses rather than use physical addressesCurrently the LinuxKPI uses physical addresses to create mappings for the DMA related interfaces. These physical addresses are not true DMA addresses and hence can't be used to program a device's DMA engine in conjunction with an IOMMU. To properly supply DMA addresses for use with an IOMMU, the LinuxKPI should use the bus_dma(9) interfaces to setup and teardown the mappings. The linux API provides an implementation of allocate/free plus map/unmap large memory regions, map/unmap a single memory region, map/unmap a scatterlist, and a dma_pool interface for a cache of smaller memory regions. For the most part the linux allocation/mapping setup translates nicely into bus_dmamap_create() and _bus_dmamap_load_phys(). The invalidation/teardown presents more of a challenge as both bus_dmamap_unload() and bus_dmamap_destroy() need to be called for a given DMA address. To find the bus_dmamap_t associated with a given DMA address a path-compressed radix trie is used with the DMA address functioning as a key. The tree is populated with entries (struct linux_dma_obj) containing the dmamap, the DMA address and in the case of a dma_pool entry the virtual address too. A UMA cache is used for the tree's entries as well as the tree’s internal nodes. The dma_pool supplies both allocated and mapped memory regions with characteristics specified when the pool is created. For efficiency a cache-only UMA zone is used. The items in the cache contain a DMA-able memory region and a DMA pctrie entry. The zones import and release routines allocate and free the DMA-memory with bus_dmamem_alloc() and bus_dmamem_free() as well the associated DMA tree entries (struct linux_dma_obj).