[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, 17 insertions, 5 deletions

diff --git a/Documentation/DMA-mapping.txt b/Documentation/DMA-mapping.txt
index 10bf4deb96aa..7c717699032c 100644
--- a/Documentation/DMA-mapping.txt
+++ b/Documentation/DMA-mapping.txt
@@ -58,11 +58,15 @@ translating each of those pages back to a kernel address using
 something like __va().  [ EDIT: Update this when we integrate
 Gerd Knorr's generic code which does this. ]
 
-This rule also means that you may not use kernel image addresses
-(ie. items in the kernel's data/text/bss segment, or your driver's)
-nor may you use kernel stack addresses for DMA.  Both of these items
-might be mapped somewhere entirely different than the rest of physical
-memory.
+This rule also means that you may use neither kernel image addresses
+(items in data/text/bss segments), nor module image addresses, nor
+stack addresses for DMA.  These could all be mapped somewhere entirely
+different than the rest of physical memory.  Even if those classes of
+memory could physically work with DMA, you'd need to ensure the I/O
+buffers were cacheline-aligned.  Without that, you'd see cacheline
+sharing problems (data corruption) on CPUs with DMA-incoherent caches.
+(The CPU could write to one word, DMA would write to a different one
+in the same cache line, and one of them could be overwritten.)
 
 Also, this means that you cannot take the return of a kmap()
 call and DMA to/from that.  This is similar to vmalloc().
@@ -284,6 +288,11 @@ There are two types of DMA mappings:
 
              in order to get correct behavior on all platforms.
 
+             Also, on some platforms your driver may need to flush CPU write
+             buffers in much the same way as it needs to flush write buffers
+             found in PCI bridges (such as by reading a register's value
+             after writing it).
+
 - Streaming DMA mappings which are usually mapped for one DMA transfer,
   unmapped right after it (unless you use pci_dma_sync_* below) and for which
   hardware can optimize for sequential accesses.
@@ -303,6 +312,9 @@ There are two types of DMA mappings:
 
 Neither type of DMA mapping has alignment restrictions that come
 from PCI, although some devices may have such restrictions.
+Also, systems with caches that aren't DMA-coherent will work better
+when the underlying buffers don't share cache lines with other data.
+
 
                  Using Consistent DMA mappings.