[patch 2.6.13-rc2-git] Documentation/DMA-API.txt

[David Brownell <david-b@pacbell.net>]

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Imagine my surprise when I noticed that the reason someone didn't
know about using the sg_dma_len() and sg_dma_address() with the
dma_map_sg() calls was that they weren't documented except as
part of the old PCI-specific pci_map_sg() API.  Bleech!

So, this patch fixes that little problem, mostly by cut/paste.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>

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diff --git a/Documentation/DMA-API.txt b/Documentation/DMA-API.txt
--- a/Documentation/DMA-API.txt
+++ b/Documentation/DMA-API.txt
@@ -304,12 +304,12 @@ dma address with dma_mapping_error(). A 
 could not be created and the driver should take appropriate action (eg
 reduce current DMA mapping usage or delay and try again later).
 
-int
-dma_map_sg(struct device *dev, struct scatterlist *sg, int nents,
-	   enum dma_data_direction direction)
-int
-pci_map_sg(struct pci_dev *hwdev, struct scatterlist *sg,
-	   int nents, int direction)
+	int
+	dma_map_sg(struct device *dev, struct scatterlist *sg,
+		int nents, enum dma_data_direction direction)
+	int
+	pci_map_sg(struct pci_dev *hwdev, struct scatterlist *sg,
+		int nents, int direction)
 
 Maps a scatter gather list from the block layer.
 
@@ -327,12 +327,36 @@ critical that the driver do something, i
 aborting the request or even oopsing is better than doing nothing and
 corrupting the filesystem.
 
-void
-dma_unmap_sg(struct device *dev, struct scatterlist *sg, int nhwentries,
-	     enum dma_data_direction direction)
-void
-pci_unmap_sg(struct pci_dev *hwdev, struct scatterlist *sg,
-	     int nents, int direction)
+With scatterlists, you use the resulting mapping like this:
+
+	int i, count = dma_map_sg(dev, sglist, nents, direction);
+	struct scatterlist *sg;
+
+	for (i = 0, sg = sglist; i < count; i++, sg++) {
+		hw_address[i] = sg_dma_address(sg);
+		hw_len[i] = sg_dma_len(sg);
+	}
+
+where nents is the number of entries in the sglist.
+
+The implementation is free to merge several consecutive sglist entries
+into one (e.g. if DMA mapping is done with PAGE_SIZE granularity, any
+consecutive sglist entries can be merged into one provided the first one
+ends and the second one starts on a page boundary - in fact this is a huge
+advantage for cards which either cannot do scatter-gather or have very
+limited number of scatter-gather entries) and returns the actual number
+of sg entries it mapped them to. On failure 0 is returned.
+
+Then you should loop count times (note: this can be less than nents times)
+and use sg_dma_address() and sg_dma_len() macros where you previously
+accessed sg->address and sg->length as shown above.
+
+	void
+	dma_unmap_sg(struct device *dev, struct scatterlist *sg,
+		int nhwentries, enum dma_data_direction direction)
+	void
+	pci_unmap_sg(struct pci_dev *hwdev, struct scatterlist *sg,
+		int nents, int direction)
 
 unmap the previously mapped scatter/gather list.  All the parameters
 must be the same as those and passed in to the scatter/gather mapping