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().
int pci_set_dma_mask(struct pci_dev *pdev, u64 device_mask);
-The query for consistent allocations is performed via a a call to
+The query for consistent allocations is performed via a call to
pci_set_consistent_dma_mask():
int pci_set_consistent_dma_mask(struct pci_dev *pdev, u64 device_mask);
device supports. It returns zero if your card can perform DMA
properly on the machine given the address mask you provided.
-If it returns non-zero, your device can not perform DMA properly on
+If it returns non-zero, your device cannot perform DMA properly on
this platform, and attempting to do so will result in undefined
behavior. You must either use a different mask, or not use DMA.
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;
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:
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.
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.
always going to be SAC addressable.
The first thing your driver needs to do is query the PCI platform
-layer with your devices DAC addressing capabilities:
+layer if it is capable of handling your devices DAC addressing
+capabilities:
- int pci_dac_set_dma_mask(struct pci_dev *pdev, u64 mask);
+ int pci_dac_dma_supported(struct pci_dev *hwdev, u64 mask);
-This routine behaves identically to pci_set_dma_mask. You may not
-use the following interfaces if this routine fails.
+You may not use the following interfaces if this routine fails.
Next, DMA addresses using this API are kept track of using the
dma64_addr_t type. It is guaranteed to be big enough to hold any
git.openpandora.org / pandora-kernel.git / blobdiff