linux-kernel - [PATCH 1/3 v6] dma: document dma_*map*

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Message-ID: <20080401013558.GC29410@sgi.com>
Date:	Mon, 31 Mar 2008 18:35:58 -0700
From:	akepner@....com
To:	Andrew Morton <akpm@...ux-foundation.org>
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: [PATCH 1/3 v6] dma: document dma_*map*_attrs() interfaces


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

Signed-off-by: Arthur Kepner <akepner@....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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