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 ==============
 DMA attributes
 ==============
 
 This document describes the semantics of the DMA attributes that are
 defined in linux/dma-mapping.h.
 
 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_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.
 
 DMA_ATTR_NO_WARN
 ----------------
 
 This tells the DMA-mapping subsystem to suppress allocation failure reports
 (similarly to __GFP_NOWARN).
 
 On some architectures allocation failures are reported with error messages
 to the system logs.  Although this can help to identify and debug problems,
 drivers which handle failures (eg, retry later) have no problems with them,
 and can actually flood the system logs with error messages that aren't any
 problem at all, depending on the implementation of the retry mechanism.
 
 So, this provides a way for drivers to avoid those error messages on calls
 where allocation failures are not a problem, and shouldn't bother the logs.
 
 .. note:: At the moment DMA_ATTR_NO_WARN is only implemented on PowerPC.
 
 DMA_ATTR_PRIVILEGED
 -------------------
 
 Some advanced peripherals such as remote processors and GPUs perform
 accesses to DMA buffers in both privileged "supervisor" and unprivileged
 "user" modes.  This attribute is used to indicate to the DMA-mapping
 subsystem that the buffer is fully accessible at the elevated privilege
 level (and ideally inaccessible or at least read-only at the
 lesser-privileged levels).

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