| DMA attributes |
| ============== |
| |
| This document describes the semantics of the DMA attributes that are |
| defined in linux/dma-attrs.h. |
| |
| DMA_ATTR_WRITE_BARRIER |
| ---------------------- |
| |
| DMA_ATTR_WRITE_BARRIER is a (write) barrier attribute for DMA. DMA |
| to a memory region with the DMA_ATTR_WRITE_BARRIER attribute forces |
| all pending DMA writes to complete, and thus provides a mechanism to |
| strictly order DMA from a device across all intervening busses and |
| bridges. This barrier is not specific to a particular type of |
| interconnect, it applies to the system as a whole, and so its |
| implementation must account for the idiosyncracies of the system all |
| the way from the DMA device to memory. |
| |
| As an example of a situation where DMA_ATTR_WRITE_BARRIER would be |
| useful, suppose that a device does a DMA write to indicate that data is |
| ready and available in memory. The DMA of the "completion indication" |
| could race with data DMA. Mapping the memory used for completion |
| indications with DMA_ATTR_WRITE_BARRIER would prevent the race. |
| |
| DMA_ATTR_WEAK_ORDERING |
| ---------------------- |
| |
| DMA_ATTR_WEAK_ORDERING specifies that reads and writes to the mapping |
| may be weakly ordered, that is that reads and writes may pass each other. |
| |
| Since it is optional for platforms to implement DMA_ATTR_WEAK_ORDERING, |
| those that do not will simply ignore the attribute and exhibit default |
| behavior. |
