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  1                         DMA attributes
  2                         ==============
  4 This document describes the semantics of the DMA attributes that are
  5 defined in linux/dma-mapping.h.
  8 ----------------------
 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 idiosyncrasies of the system all
 17 the way from the DMA device to memory.
 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.
 26 ----------------------
 28 DMA_ATTR_WEAK_ORDERING specifies that reads and writes to the mapping
 29 may be weakly ordered, that is that reads and writes may pass each other.
 31 Since it is optional for platforms to implement DMA_ATTR_WEAK_ORDERING,
 32 those that do not will simply ignore the attribute and exhibit default
 33 behavior.
 36 ----------------------
 38 DMA_ATTR_WRITE_COMBINE specifies that writes to the mapping may be
 39 buffered to improve performance.
 41 Since it is optional for platforms to implement DMA_ATTR_WRITE_COMBINE,
 42 those that do not will simply ignore the attribute and exhibit default
 43 behavior.
 46 -----------------------
 48 DMA_ATTR_NON_CONSISTENT lets the platform to choose to return either
 49 consistent or non-consistent memory as it sees fit.  By using this API,
 50 you are guaranteeing to the platform that you have all the correct and
 51 necessary sync points for this memory in the driver.
 54 --------------------------
 56 DMA_ATTR_NO_KERNEL_MAPPING lets the platform to avoid creating a kernel
 57 virtual mapping for the allocated buffer. On some architectures creating
 58 such mapping is non-trivial task and consumes very limited resources
 59 (like kernel virtual address space or dma consistent address space).
 60 Buffers allocated with this attribute can be only passed to user space
 61 by calling dma_mmap_attrs(). By using this API, you are guaranteeing
 62 that you won't dereference the pointer returned by dma_alloc_attr(). You
 63 can treat it as a cookie that must be passed to dma_mmap_attrs() and
 64 dma_free_attrs(). Make sure that both of these also get this attribute
 65 set on each call.
 67 Since it is optional for platforms to implement
 68 DMA_ATTR_NO_KERNEL_MAPPING, those that do not will simply ignore the
 69 attribute and exhibit default behavior.
 72 ----------------------
 74 By default dma_map_{single,page,sg} functions family transfer a given
 75 buffer from CPU domain to device domain. Some advanced use cases might
 76 require sharing a buffer between more than one device. This requires
 77 having a mapping created separately for each device and is usually
 78 performed by calling dma_map_{single,page,sg} function more than once
 79 for the given buffer with device pointer to each device taking part in
 80 the buffer sharing. The first call transfers a buffer from 'CPU' domain
 81 to 'device' domain, what synchronizes CPU caches for the given region
 82 (usually it means that the cache has been flushed or invalidated
 83 depending on the dma direction). However, next calls to
 84 dma_map_{single,page,sg}() for other devices will perform exactly the
 85 same synchronization operation on the CPU cache. CPU cache synchronization
 86 might be a time consuming operation, especially if the buffers are
 87 large, so it is highly recommended to avoid it if possible.
 88 DMA_ATTR_SKIP_CPU_SYNC allows platform code to skip synchronization of
 89 the CPU cache for the given buffer assuming that it has been already
 90 transferred to 'device' domain. This attribute can be also used for
 91 dma_unmap_{single,page,sg} functions family to force buffer to stay in
 92 device domain after releasing a mapping for it. Use this attribute with
 93 care!
 96 -------------------------
 98 By default DMA-mapping subsystem is allowed to assemble the buffer
 99 allocated by dma_alloc_attrs() function from individual pages if it can
100 be mapped as contiguous chunk into device dma address space. By
101 specifying this attribute the allocated buffer is forced to be contiguous
102 also in physical memory.
105 ---------------------------
107 This is a hint to the DMA-mapping subsystem that it's probably not worth
108 the time to try to allocate memory to in a way that gives better TLB
109 efficiency (AKA it's not worth trying to build the mapping out of larger
110 pages).  You might want to specify this if:
111 - You know that the accesses to this memory won't thrash the TLB.
112   You might know that the accesses are likely to be sequential or
113   that they aren't sequential but it's unlikely you'll ping-pong
114   between many addresses that are likely to be in different physical
115   pages.
116 - You know that the penalty of TLB misses while accessing the
117   memory will be small enough to be inconsequential.  If you are
118   doing a heavy operation like decryption or decompression this
119   might be the case.
120 - You know that the DMA mapping is fairly transitory.  If you expect
121   the mapping to have a short lifetime then it may be worth it to
122   optimize allocation (avoid coming up with large pages) instead of
123   getting the slight performance win of larger pages.
124 Setting this hint doesn't guarantee that you won't get huge pages, but it
125 means that we won't try quite as hard to get them.
127 NOTE: At the moment DMA_ATTR_ALLOC_SINGLE_PAGES is only implemented on ARM,
128 though ARM64 patches will likely be posted soon.
131 ----------------
133 This tells the DMA-mapping subsystem to suppress allocation failure reports
134 (similarly to __GFP_NOWARN).
136 On some architectures allocation failures are reported with error messages
137 to the system logs.  Although this can help to identify and debug problems,
138 drivers which handle failures (eg, retry later) have no problems with them,
139 and can actually flood the system logs with error messages that aren't any
140 problem at all, depending on the implementation of the retry mechanism.
142 So, this provides a way for drivers to avoid those error messages on calls
143 where allocation failures are not a problem, and shouldn't bother the logs.
145 NOTE: At the moment DMA_ATTR_NO_WARN is only implemented on PowerPC.

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