[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>
diff --git a/Documentation/DMA-mapping.txt b/Documentation/DMA-mapping.txt
index 10bf4de..7c717699 100644
--- a/Documentation/DMA-mapping.txt
+++ b/Documentation/DMA-mapping.txt
@@ -58,11 +58,15 @@
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 @@
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 @@
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.