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Based on kernel version 4.9. Page generated on 2016-12-21 14:33 EST.

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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