linux-kernel - Re: [PATCH 1/4 v4] dma: document dma_{un}map_{single|sg}

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Message-ID: <20080314044742.GF24794@colo.lackof.org>
Date:	Thu, 13 Mar 2008 22:47:42 -0600
From:	Grant Grundler <grundler@...isc-linux.org>
To:	akepner@....com
Cc:	Tony Luck <tony.luck@...el.com>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Jes Sorensen <jes@....com>,
	Randy Dunlap <randy.dunlap@...cle.com>,
	Roland Dreier <rdreier@...co.com>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	David Miller <davem@...emloft.net>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Grant Grundler <grundler@...isc-linux.org>,
	Michael Ellerman <michael@...erman.id.au>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4 v4] dma: document dma_{un}map_{single|sg}_attrs()
	interface

On Wed, Mar 12, 2008 at 09:01:17PM -0700, akepner@....com wrote:
> 
> Document the new dma_{un}map_{single|sg}_attrs() functions.
> 
> Signed-off-by: Arthur Kepner <akepner@....com>
> Acked-by:  Jesse Barnes <jesse.barnes@...el.com>

Acked-by: Grant Grundler <grundler@...isc-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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