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tile PCI RC: add comment about "PCI hole" problem

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Explain the rationale of not overlapping the 64-bit DMA window
with the PA range.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>
This commit is contained in:
Chris Metcalf 2013-08-12 15:25:22 -04:00
parent bdb8205455
commit 5e7705df28
1 changed files with 5 additions and 0 deletions
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arch/tile/include/asm/pci.h
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@ -122,6 +122,11 @@ static inline void pci_iounmap(struct pci_dev *dev, void __iomem *addr) {}
* the CPA plus TILE_PCI_MEM_MAP_BASE_OFFSET. To support 32-bit
* devices, we create a separate map region that handles the low
* 4GB.
*
* This design lets us avoid the "PCI hole" problem where the host bridge
* won't pass DMA traffic with target addresses that happen to fall within the
* BAR space. This enables us to use all the physical memory for DMA, instead
* of wasting the same amount of physical memory as the BAR window size.
*/
#define TILE_PCI_MEM_MAP_BASE_OFFSET (1ULL << CHIP_PA_WIDTH())