Commit a75b0a2f authored by Arthur Kepner's avatar Arthur Kepner Committed by Linus Torvalds
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dma: document dma_*map*_attrs() interfaces

Document the new dma_*map*_attrs() functions.

[ fix up for dma-add-dma_map_attrs-interfaces and update docs]
Signed-off-by: default avatarArthur Kepner <>
Acked-by: default avatarDavid S. Miller <>
Cc: Tony Luck <>
Cc: Jesse Barnes <>
Cc: Jes Sorensen <>
Cc: Randy Dunlap <>
Cc: Roland Dreier <>
Cc: James Bottomley <>
Cc: Benjamin Herrenschmidt <>
Cc: Grant Grundler <>
Cc: Michael Ellerman <>
Signed-off-by: default avatarMark Nelson <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent 74bc7cee
......@@ -395,6 +395,71 @@ Notes: You must do this:
See also dma_map_single().
dma_map_single_attrs(struct device *dev, void *cpu_addr, size_t size,
enum dma_data_direction dir,
struct dma_attrs *attrs)
dma_unmap_single_attrs(struct device *dev, dma_addr_t dma_addr,
size_t size, enum dma_data_direction dir,
struct dma_attrs *attrs)
dma_map_sg_attrs(struct device *dev, struct scatterlist *sgl,
int nents, enum dma_data_direction dir,
struct dma_attrs *attrs)
dma_unmap_sg_attrs(struct device *dev, struct scatterlist *sgl,
int nents, enum dma_data_direction dir,
struct dma_attrs *attrs)
The four functions above are just like the counterpart functions
without the _attrs suffixes, except that they pass an optional
struct dma_attrs*.
struct dma_attrs encapsulates a set of "dma attributes". For the
definition of struct dma_attrs see linux/dma-attrs.h.
The interpretation of dma attributes is architecture-specific, and
each attribute should be documented in Documentation/DMA-attributes.txt.
If struct dma_attrs* is NULL, the semantics of each of these
functions is identical to those of the corresponding function
without the _attrs suffix. As a result dma_map_single_attrs()
can generally replace dma_map_single(), etc.
As an example of the use of the *_attrs functions, here's how
you could pass an attribute DMA_ATTR_FOO when mapping memory
for DMA:
#include <linux/dma-attrs.h>
/* DMA_ATTR_FOO should be defined in linux/dma-attrs.h and
* documented in Documentation/DMA-attributes.txt */
dma_set_attr(DMA_ATTR_FOO, &attrs);
n = dma_map_sg_attrs(dev, sg, nents, DMA_TO_DEVICE, &attr);
Architectures that care about DMA_ATTR_FOO would check for its
presence in their implementations of the mapping and unmapping
routines, e.g.:
void whizco_dma_map_sg_attrs(struct device *dev, dma_addr_t dma_addr,
size_t size, enum dma_data_direction dir,
struct dma_attrs *attrs)
int foo = dma_get_attr(DMA_ATTR_FOO, attrs);
if (foo)
/* twizzle the frobnozzle */
Part II - Advanced dma_ usage
DMA attributes
This document describes the semantics of the DMA attributes that are
defined in linux/dma-attrs.h.
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 idiosyncracies 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.
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