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SubjectRe: [PATCH 1/4 v4] dma: document dma_{un}map_{single|sg}_attrs() interface
On Wed, Mar 12, 2008 at 09:01:17PM -0700, akepner@sgi.com wrote:
>
> Document the new dma_{un}map_{single|sg}_attrs() functions.
>
> Signed-off-by: Arthur Kepner <akepner@sgi.com>
> Acked-by: Jesse Barnes <jesse.barnes@intel.com>

Acked-by: Grant Grundler <grundler@parisc-linux.org>

thanks,
grant

> ---
>
> 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..a4106ec 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_BARRIER
> +----------------
> +
> +DMA_ATTR_BARRIER is a barrier attribute for DMA. DMA to a memory
> +region with the DMA_ATTR_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_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_BARRIER would prevent the race.
> +


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