DMA attributes
  			==============
  
  This document describes the semantics of the DMA attributes that are

  defined in linux/dma-mapping.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 idiosyncrasies 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 treat 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 synchronization operation on the CPU cache. CPU cache synchronization

  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

  specifying this attribute the allocated buffer is forced to be contiguous

  also in physical memory.

  
  DMA_ATTR_ALLOC_SINGLE_PAGES
  ---------------------------
  
  This is a hint to the DMA-mapping subsystem that it's probably not worth
  the time to try to allocate memory to in a way that gives better TLB
  efficiency (AKA it's not worth trying to build the mapping out of larger
  pages).  You might want to specify this if:
  - You know that the accesses to this memory won't thrash the TLB.
    You might know that the accesses are likely to be sequential or
    that they aren't sequential but it's unlikely you'll ping-pong
    between many addresses that are likely to be in different physical
    pages.
  - You know that the penalty of TLB misses while accessing the
    memory will be small enough to be inconsequential.  If you are
    doing a heavy operation like decryption or decompression this
    might be the case.
  - You know that the DMA mapping is fairly transitory.  If you expect
    the mapping to have a short lifetime then it may be worth it to
    optimize allocation (avoid coming up with large pages) instead of
    getting the slight performance win of larger pages.
  Setting this hint doesn't guarantee that you won't get huge pages, but it
  means that we won't try quite as hard to get them.
  
  NOTE: At the moment DMA_ATTR_ALLOC_SINGLE_PAGES is only implemented on ARM,
  though ARM64 patches will likely be posted soon.