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.

  
  DMA_ATTR_WRITE_COMBINE
  ----------------------
  
  DMA_ATTR_WRITE_COMBINE specifies that writes to the mapping may be
  buffered to improve performance.
  
  Since it is optional for platforms to implement DMA_ATTR_WRITE_COMBINE,
  those that do not will simply ignore the attribute and exhibit default
  behavior.

  
  DMA_ATTR_NON_CONSISTENT
  -----------------------
  
  DMA_ATTR_NON_CONSISTENT lets the platform to choose to return either
  consistent or non-consistent memory as it sees fit.  By using this API,
  you are guaranteeing to the platform that you have all the correct and
  necessary sync points for this memory in the driver.

  
  DMA_ATTR_NO_KERNEL_MAPPING
  --------------------------
  
  DMA_ATTR_NO_KERNEL_MAPPING lets the platform to avoid creating a kernel
  virtual mapping for the allocated buffer. On some architectures creating
  such mapping is non-trivial task and consumes very limited resources
  (like kernel virtual address space or dma consistent address space).
  Buffers allocated with this attribute can be only passed to user space
  by calling dma_mmap_attrs(). By using this API, you are guaranteeing
  that you won't dereference the pointer returned by dma_alloc_attr(). You
  can threat it as a cookie that must be passed to dma_mmap_attrs() and
  dma_free_attrs(). Make sure that both of these also get this attribute
  set on each call.
  
  Since it is optional for platforms to implement
  DMA_ATTR_NO_KERNEL_MAPPING, those that do not will simply ignore the
  attribute and exhibit default behavior.

  
  DMA_ATTR_SKIP_CPU_SYNC
  ----------------------
  
  By default dma_map_{single,page,sg} functions family transfer a given
  buffer from CPU domain to device domain. Some advanced use cases might
  require sharing a buffer between more than one device. This requires
  having a mapping created separately for each device and is usually
  performed by calling dma_map_{single,page,sg} function more than once
  for the given buffer with device pointer to each device taking part in
  the buffer sharing. The first call transfers a buffer from 'CPU' domain
  to 'device' domain, what synchronizes CPU caches for the given region
  (usually it means that the cache has been flushed or invalidated
  depending on the dma direction). However, next calls to
  dma_map_{single,page,sg}() for other devices will perform exactly the
  same sychronization operation on the CPU cache. CPU cache sychronization
  might be a time consuming operation, especially if the buffers are
  large, so it is highly recommended to avoid it if possible.
  DMA_ATTR_SKIP_CPU_SYNC allows platform code to skip synchronization of
  the CPU cache for the given buffer assuming that it has been already
  transferred to 'device' domain. This attribute can be also used for
  dma_unmap_{single,page,sg} functions family to force buffer to stay in
  device domain after releasing a mapping for it. Use this attribute with
  care!

  
  DMA_ATTR_FORCE_CONTIGUOUS
  -------------------------
  
  By default DMA-mapping subsystem is allowed to assemble the buffer
  allocated by dma_alloc_attrs() function from individual pages if it can
  be mapped as contiguous chunk into device dma address space. By
  specifing this attribute the allocated buffer is forced to be contiguous
  also in physical memory.