blob: d62d0c6a23677734fe079100a9f11752dcfc2059 [file] [log] [blame]
Arthur Kepnera75b0a22008-04-29 01:00:31 -07001 DMA attributes
2 ==============
3
4This document describes the semantics of the DMA attributes that are
5defined in linux/dma-attrs.h.
6
7DMA_ATTR_WRITE_BARRIER
8----------------------
9
10DMA_ATTR_WRITE_BARRIER is a (write) barrier attribute for DMA. DMA
11to a memory region with the DMA_ATTR_WRITE_BARRIER attribute forces
12all pending DMA writes to complete, and thus provides a mechanism to
13strictly order DMA from a device across all intervening busses and
14bridges. This barrier is not specific to a particular type of
15interconnect, it applies to the system as a whole, and so its
16implementation must account for the idiosyncracies of the system all
17the way from the DMA device to memory.
18
19As an example of a situation where DMA_ATTR_WRITE_BARRIER would be
20useful, suppose that a device does a DMA write to indicate that data is
21ready and available in memory. The DMA of the "completion indication"
22could race with data DMA. Mapping the memory used for completion
23indications with DMA_ATTR_WRITE_BARRIER would prevent the race.
24
Mark Nelson1ed6af72008-07-18 23:03:34 +100025DMA_ATTR_WEAK_ORDERING
26----------------------
27
28DMA_ATTR_WEAK_ORDERING specifies that reads and writes to the mapping
29may be weakly ordered, that is that reads and writes may pass each other.
30
31Since it is optional for platforms to implement DMA_ATTR_WEAK_ORDERING,
32those that do not will simply ignore the attribute and exhibit default
33behavior.
Marek Szyprowski8a413432011-12-23 09:30:47 +010034
35DMA_ATTR_WRITE_COMBINE
36----------------------
37
38DMA_ATTR_WRITE_COMBINE specifies that writes to the mapping may be
39buffered to improve performance.
40
41Since it is optional for platforms to implement DMA_ATTR_WRITE_COMBINE,
42those that do not will simply ignore the attribute and exhibit default
43behavior.
Marek Szyprowski64d70fe2012-03-28 07:55:56 +020044
45DMA_ATTR_NON_CONSISTENT
46-----------------------
47
48DMA_ATTR_NON_CONSISTENT lets the platform to choose to return either
49consistent or non-consistent memory as it sees fit. By using this API,
50you are guaranteeing to the platform that you have all the correct and
51necessary sync points for this memory in the driver.
Marek Szyprowskifdf1f8b2012-05-16 15:20:37 +020052
53DMA_ATTR_NO_KERNEL_MAPPING
54--------------------------
55
56DMA_ATTR_NO_KERNEL_MAPPING lets the platform to avoid creating a kernel
57virtual mapping for the allocated buffer. On some architectures creating
58such mapping is non-trivial task and consumes very limited resources
59(like kernel virtual address space or dma consistent address space).
60Buffers allocated with this attribute can be only passed to user space
61by calling dma_mmap_attrs(). By using this API, you are guaranteeing
62that you won't dereference the pointer returned by dma_alloc_attr(). You
63can threat it as a cookie that must be passed to dma_mmap_attrs() and
64dma_free_attrs(). Make sure that both of these also get this attribute
65set on each call.
66
67Since it is optional for platforms to implement
68DMA_ATTR_NO_KERNEL_MAPPING, those that do not will simply ignore the
69attribute and exhibit default behavior.
Laura Abbott09241a52012-10-29 13:08:03 -070070
71DMA_ATTR_STRONGLY_ORDERED
72-------------------------
73
74DMA_ATTR_STRONGLY_ORDERED allocates memory with a very restrictive type
75of mapping (no unaligned accesses, no re-ordering, no write merging, no
76buffering, no pre-fetching). This has severe performance penalties and
77should not be used for general purpose DMA allocations. It should only
78be used if one of the restrictions on strongly ordered memory is required.