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    Subject[PATCH 1/3 v6] dma: document dma_*map*_attrs() interfaces

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

    Signed-off-by: Arthur Kepner <akepner@sgi.com>

    ---

    DMA-API.txt | 65 +++++++++++++++++++++++++++++++++++++++++++++++++++++
    DMA-attributes.txt | 24 +++++++++++++++++++
    2 files changed, 89 insertions(+)

    diff --git a/Documentation/DMA-API.txt b/Documentation/DMA-API.txt
    index b939ebb..fdb82b0 100644
    --- a/Documentation/DMA-API.txt
    +++ b/Documentation/DMA-API.txt
    @@ -395,6 +395,71 @@ Notes: You must do this:

    See also dma_map_single().

    +dma_addr_t
    +dma_map_single_attrs(struct device *dev, void *cpu_addr, size_t size,
    + enum dma_data_direction dir,
    + struct dma_attrs *attrs)
    +
    +void
    +dma_unmap_single_attrs(struct device *dev, dma_addr_t dma_addr,
    + size_t size, enum dma_data_direction dir,
    + struct dma_attrs *attrs)
    +
    +int
    +dma_map_sg_attrs(struct device *dev, struct scatterlist *sgl,
    + int nents, enum dma_data_direction dir,
    + struct dma_attrs *attrs)
    +
    +void
    +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 */
    +...
    +
    + DECLARE_DMA_ATTRS(attrs);
    + dma_set_attr(&attrs, DMA_ATTR_FOO);
    + ....
    + 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(attrs, DMA_ATTR_FOO);
    + ....
    + if (foo)
    + /* twizzle the frobnozzle */
    + ....
    +

    Part II - Advanced dma_ usage
    -----------------------------
    diff --git a/Documentation/DMA-attributes.txt b/Documentation/DMA-attributes.txt
    index e69de29..6d772f8 100644
    --- a/Documentation/DMA-attributes.txt
    +++ b/Documentation/DMA-attributes.txt
    @@ -0,0 +1,24 @@
    + DMA attributes
    + ==============
    +
    +This document describes the semantics of the DMA attributes that are
    +defined in linux/dma-attrs.h.
    +
    +DMA_ATTR_WRITE_BARRIER
    +----------------------
    +
    +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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