| Arthur Kepner | a75b0a2 | 2008-04-29 01:00:31 -0700 | [diff] [blame] | 1 | DMA attributes |
| 2 | ============== |
| 3 | |
| 4 | This document describes the semantics of the DMA attributes that are |
| 5 | defined in linux/dma-attrs.h. |
| 6 | |
| 7 | DMA_ATTR_WRITE_BARRIER |
| 8 | ---------------------- |
| 9 | |
| 10 | DMA_ATTR_WRITE_BARRIER is a (write) barrier attribute for DMA. DMA |
| 11 | to a memory region with the DMA_ATTR_WRITE_BARRIER attribute forces |
| 12 | all pending DMA writes to complete, and thus provides a mechanism to |
| 13 | strictly order DMA from a device across all intervening busses and |
| 14 | bridges. This barrier is not specific to a particular type of |
| 15 | interconnect, it applies to the system as a whole, and so its |
| 16 | implementation must account for the idiosyncracies of the system all |
| 17 | the way from the DMA device to memory. |
| 18 | |
| 19 | As an example of a situation where DMA_ATTR_WRITE_BARRIER would be |
| 20 | useful, suppose that a device does a DMA write to indicate that data is |
| 21 | ready and available in memory. The DMA of the "completion indication" |
| 22 | could race with data DMA. Mapping the memory used for completion |
| 23 | indications with DMA_ATTR_WRITE_BARRIER would prevent the race. |
| 24 | |