Fedora kernel-2.6.17-1.2142_FC4 patched with stable patch-2.6.17.4-vs2.0.2-rc26.diff

[linux-2.6.git] / Documentation / DMA-mapping.txt

diff --git a/Documentation/DMA-mapping.txt b/Documentation/DMA-mapping.txt
index 6845574..7c71769 100644 (file)
--- 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. ]
 
 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().
 
 Also, this means that you cannot take the return of a kmap()
 call and DMA to/from that.  This is similar to vmalloc().


@@ -194,11 +198,13 @@ document for how to handle this case.
 Finally, if your device can only drive the low 24-bits of
 address during PCI bus mastering you might do something like:
 
 Finally, if your device can only drive the low 24-bits of
 address during PCI bus mastering you might do something like:
 

-       if (pci_set_dma_mask(pdev, 0x00ffffff)) {
+       if (pci_set_dma_mask(pdev, DMA_24BIT_MASK)) {

                printk(KERN_WARNING
                       "mydev: 24-bit DMA addressing not available.\n");
                goto ignore_this_device;
        }
                printk(KERN_WARNING
                       "mydev: 24-bit DMA addressing not available.\n");
                goto ignore_this_device;
        }

+[Better use DMA_24BIT_MASK instead of 0x00ffffff.
+See linux/include/dma-mapping.h for reference.]

 
 When pci_set_dma_mask() is successful, and returns zero, the PCI layer
 saves away this mask you have provided.  The PCI layer will use this
 
 When pci_set_dma_mask() is successful, and returns zero, the PCI layer
 saves away this mask you have provided.  The PCI layer will use this


@@ -210,7 +216,7 @@ functions (for example a sound card provides playback and record
 functions) and the various different functions have _different_
 DMA addressing limitations, you may wish to probe each mask and
 only provide the functionality which the machine can handle.  It
 functions) and the various different functions have _different_
 DMA addressing limitations, you may wish to probe each mask and
 only provide the functionality which the machine can handle.  It

-is important that the last call to pci_set_dma_mask() be for the 
+is important that the last call to pci_set_dma_mask() be for the

 most specific mask.
 
 Here is pseudo-code showing how this might be done:
 most specific mask.
 
 Here is pseudo-code showing how this might be done:


@@ -282,6 +288,11 @@ There are two types of DMA mappings:
 
              in order to get correct behavior on all platforms.
 
 
              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.
 - 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.


@@ -301,6 +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.
 
 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.
 
 
                 Using Consistent DMA mappings.