dma: document dma_*map*_attrs() interfaces

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

[[email protected]: fix up for dma-add-dma_map_attrs-interfaces and update docs]
Signed-off-by: Arthur Kepner <[email protected]>
Acked-by: David S. Miller <[email protected]>
Cc: Tony Luck <[email protected]>
Cc: Jesse Barnes <[email protected]>
Cc: Jes Sorensen <[email protected]>
Cc: Randy Dunlap <[email protected]>
Cc: Roland Dreier <[email protected]>
Cc: James Bottomley <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
Cc: Grant Grundler <[email protected]>
Cc: Michael Ellerman <[email protected]>
Signed-off-by: Mark Nelson <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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 */
+...
+
+	DEFINE_DMA_ATTRS(attrs);
+	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
 -----------------------------

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.
+