[PATCH] dma doc updates

This updates the DMA API documentation to address a few issues:

 - The dma_map_sg() call results are used like pci_map_sg() results:
   using sg_dma_address() and sg_dma_len().  That's not wholly obvious
   to folk reading _only_ the "new" DMA-API.txt writeup.

 - Buffers allocated by dma_alloc_coherent() may not be completely
   free of coherency concerns ... some CPUs also have write buffers
   that may need to be flushed.

 - Cacheline coherence issues are now mentioned as being among issues
   which affect dma buffers, and complicate/prevent using of static and
   (especially) stack based buffers with the DMA calls.

I don't think many drivers currently need to worry about flushing write
buffers, but I did hit it with one SOC using external SDRAM for DMA
descriptors:  without explicit writebuffer flushing, the on-chip DMA
controller accessed descriptors before the CPU completed the writes.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

1 files changed, 36 insertions, 13 deletions

diff --git a/Documentation/DMA-API.txt b/Documentation/DMA-API.txt
index 1af0f2d50220..2ffb0d62f0fe 100644
--- a/Documentation/DMA-API.txt
+++ b/Documentation/DMA-API.txt
@@ -33,7 +33,9 @@ pci_alloc_consistent(struct pci_dev *dev, size_t size,
 
 Consistent memory is memory for which a write by either the device or
 the processor can immediately be read by the processor or device
-without having to worry about caching effects.
+without having to worry about caching effects.  (You may however need
+to make sure to flush the processor's write buffers before telling
+devices to read that memory.)
 
 This routine allocates a region of <size> bytes of consistent memory.
 it also returns a <dma_handle> which may be cast to an unsigned
@@ -304,12 +306,12 @@ dma address with dma_mapping_error(). A non zero return value means the mapping
 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 +329,33 @@ critical that the driver do something, in the case of a block driver
 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. with an IOMMU, or if several pages just happen to be
+physically contiguous) 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