linux/Documentation/DMA-attributes.txt
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   1                        DMA attributes
   2                        ==============
   3
   4This document describes the semantics of the DMA attributes that are
   5defined in linux/dma-attrs.h.
   6
   7DMA_ATTR_WRITE_BARRIER
   8----------------------
   9
  10DMA_ATTR_WRITE_BARRIER is a (write) barrier attribute for DMA.  DMA
  11to a memory region with the DMA_ATTR_WRITE_BARRIER attribute forces
  12all pending DMA writes to complete, and thus provides a mechanism to
  13strictly order DMA from a device across all intervening busses and
  14bridges.  This barrier is not specific to a particular type of
  15interconnect, it applies to the system as a whole, and so its
  16implementation must account for the idiosyncracies of the system all
  17the way from the DMA device to memory.
  18
  19As an example of a situation where DMA_ATTR_WRITE_BARRIER would be
  20useful, suppose that a device does a DMA write to indicate that data is
  21ready and available in memory.  The DMA of the "completion indication"
  22could race with data DMA.  Mapping the memory used for completion
  23indications with DMA_ATTR_WRITE_BARRIER would prevent the race.
  24
  25DMA_ATTR_WEAK_ORDERING
  26----------------------
  27
  28DMA_ATTR_WEAK_ORDERING specifies that reads and writes to the mapping
  29may be weakly ordered, that is that reads and writes may pass each other.
  30
  31Since it is optional for platforms to implement DMA_ATTR_WEAK_ORDERING,
  32those that do not will simply ignore the attribute and exhibit default
  33behavior.
  34
  35DMA_ATTR_WRITE_COMBINE
  36----------------------
  37
  38DMA_ATTR_WRITE_COMBINE specifies that writes to the mapping may be
  39buffered to improve performance.
  40
  41Since it is optional for platforms to implement DMA_ATTR_WRITE_COMBINE,
  42those that do not will simply ignore the attribute and exhibit default
  43behavior.
  44
  45DMA_ATTR_NON_CONSISTENT
  46-----------------------
  47
  48DMA_ATTR_NON_CONSISTENT lets the platform to choose to return either
  49consistent or non-consistent memory as it sees fit.  By using this API,
  50you are guaranteeing to the platform that you have all the correct and
  51necessary sync points for this memory in the driver.
  52
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